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Becky Kriger's List: Polymers and Plastics

"Elements of Chemistry: Polymers and Plastics"

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Polymers on 2008-12-04
Polymer Tutorial

A simply explained introduction to polymers.

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on 2008-12-02
- "Poly" means "many" and "mer"
  means "parts.
- The parts are usually the same part used repeatedly in a chain-like manner.
  Polymers are also referred to as plastics
- 16 more annotations...
- - Nature has many examples of polymers. Cotton fibers are made
    of sugar molecules that are repeated in a chain-like manner. Hair, wool,
    and other natural fibers are polymers.
  - Now be aware that one of the connecting sticks
    can spring open. To see animation of this, move cursor
    over the image. As long as cursor is over image, animation repeats
  - 1) We start with several pairs of balls each with two connecting
    sticks. 2) Something causes one of the connecting sticks to fling open. It then
    connects to another ball, but in doing so, causes one of its connecting sticks
    to fling open. 3) In moments all of the pairs of balls are now connected. Move cursor over image to see animation.
  - Sometimes heat, high energy light, or something else causes
    the double-bond to break and two of the middle 4 electrons split up and
    end up on the two outer ends of the molecule. These electrons are unpaired,
    which makes them eager to join with another electron.
  - (place
    cursor over the image to see animated version)
  - The unpaired electron
    triggers the ethylene molecule that bumps into it, to shift the inside electron
    to pair with it. That then causes the newly unpaired
    inner electron to move to the outside and it is now an unpaired
    electron ready to cause the next ethylene molecule to repeat the process.
  - The name of this polymer is appropriately
    called, polyethylene.
  - This is called high density polyethylene
    (HDPE).
  - High density polyethylene HDPE is used for bottles, buckets, jugs, containers,
    toys, even synthetic lumber, and many other things.
  - Sometimes the chains get up to 500,000 carbons long. Here
    they are tough enough for synthetic ice, replacement joints and bullet-proof
    vests. This is called Ultra High
    Molecular Weight
    PolyEthylene
    or UHMWPE.
  - low density polyethylene (LDPE).
  - It is made by causing the long chains of ethylene to branch.
    That way they cannot lie next each other, which reduces the density of
    the polyethylene. This makes the plastic lighter and more flexible.
  - Low density polyethylene is used to make plastic bags,
    plastic wrap, and squeeze bottles, plus many other things.
  - The favorite properties of plastics are that they are inert
    and won't react with what is stored in them. They also are durable and won't
    easily decay, dissolve, or break apart. These are great qualities for things
    you keep, but when you throw them away, they won't decompose.
  - The answer is to recycle the plastics. Here we see a bunch
    of CDs getting recycled.
  - Here are
    two recycle code drawings. You already know about HDPE and LDPE.
How are polymers made? : Scientific American

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polymers on 2008-12-03
- Synthetic polymers are produced by chemical reactions, termed "polymerizations."
- but such reactions consist of the repetitive chemical bonding of individual
  molecules, or monomers. Assorted combinations of heat, pressure and catalysis
  alter the chemical bonds that hold monomers together, causing them to bond with
  one another. Most often, they do so in a linear fashion, creating chains of
  monomers called polymers.
- 8 more annotations...
- - The monomer ethylene is composed of two carbon atoms, each bonded to two
    hydrogen atoms and sharing a double bond with one another. Polyethylene consists
    of a chain of single-bonded carbon atoms, each still carrying its two hydrogen
    atoms.
  - One way to produce polyethylene is called "free radical polymerization." As
    in other polymerizations, the process has three stages, known as initiation,
    propagation and termination. To begin, we need to add a catalyst to our supply
    of ethylene. A common catalyst is benzoyl peroxide, which when heated has the
    habit of splitting into two fragments, each with one unpaired electron, or free
    radical. These fragments are known as initiator fragments.
  - The unpaired electron naturally seeks another and finds a convenient target
    in the double bond between the carbon atoms in the ethylene molecule. Taking an
    electron from the carbon bond, the initiator fragment bonds itself to one of the
    monomer's carbon atoms.
  - The new radical also seeks a partner. And so ethylene monomers begin attaching
    themselves in a chain, creating new radicals each time and lengthening the
    chain. This stage is called propagation.
  - Eventually, free radical polymerization stops due to what are called termination
    reactions. For example, instead of stealing an electron from double-bonded
    carbons or a nearby propagating chain, the carbon atom with the free radical
    sometimes steals an entire hydrogen atom from another chain end. The polymer
    end--robbed of its hydrogen--easily forms a double bond with its adjacent carbon
    atom, and polymerization stops.
  - Because every part of the ethylene monomer is included in the finished polymer,
    the free radical polymerization of polyethylene is referred to as an addition
    polymerization
  - Polymerizations that use only portions of a monomer, however, are known as
    condensation polymerizations. The monomers that condense with each other must
    contain at least two reactive groups in order to form a chain.
  - For example, poly(ethyleneterepthalate), a polyester known as PET that is
    commonly found in soda bottles, forms from a reaction of two monomers: ethylene
    glycol and terephthoyl chloride. At the reaction's end, an atom of hydrogen and
    an atom of chlorine are left out of each PET molecular junction, resulting in a
    by-product of hydrogen chloride (HCl) gas.

Scientific Principles:Polymers

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on 2008-11-02 and saved by 2 people

The chemical reaction in which high molecular mass molecules are formed from
monomers is known as polymerization. There are two
basic types of polymerization, chain-reaction (or addition) and step-reaction (or condensation) polymerization.
One of the most common types of polymer reactions is chain-reaction
(addition) polymerization. This type of polymerization is a three step process
involving two chemical entities. The first, known simply as a monomer, can be regarded as one link in a polymer chain.
It initially exists as simple units. In nearly all cases, the monomers have at
least one carbon-carbon double bond. Ethylene is one example of a monomer used
to make a common polymer.
26 more annotations...

The other chemical reactant is a catalyst. In chain-reaction polymerization, the
catalyst can be a free-radical peroxide added in
relatively low concentrations. A free-radical is a chemical component that
contains a free electron that forms a covalent bond with an electron on another
molecule. The formation of a free radical from an organic peroxide is shown
below:
The first step in the chain-reaction polymerization process, initiation, occurs
when the free-radical catalyst reacts with a double bonded carbon monomer,
beginning the polymer chain. The double carbon bond breaks apart, the monomer
bonds to the free radical, and the free electron is transferred to the outside
carbon atom in this reaction.
The next step in the process, propagation, is a repetitive operation in which
the physical chain of the polymer is formed. The double bond of successive
monomers is opened up when the monomer is reacted to the reactive polymer chain.
The free electron is successively passed down the line of the chain to the
outside carbon atom.
Thermodynamically speaking, the sum of the energies of the polymer is less than
the sum of the energies of the individual monomers. Simply put, the single
bounds in the polymeric chain are more stable than the double bonds of the
monomer.
Termination occurs when another free radical (R-O^.), left over
from the original splitting of the organic peroxide, meets the end of the
growing chain. This free-radical terminates the chain by linking with the last
CH₂^. component of the polymer chain. This reaction
produces a complete polymer chain. Termination can also occur when two
unfinished chains bond together. Both termination types are diagrammed below.
Other types of termination are also possible.
This exothermic reaction occurs extremely fast, forming individual chains of polyethylene often in less than 0.1 second.
Step-reaction (condensation) polymerization is another common type of
polymerization. This polymerization method typically produces polymers of lower
molecular weight than chain reactions and requires higher temperatures to occur.
Unlike addition polymerization, step-wise reactions involve two different types
of di-functional monomers or end groups that react
with one another, forming a chain. Condensation polymerization also produces a
small molecular by-product (water, HCl, etc.).
As indicated above, both addition and condensation polymers can be linear,
branched, or cross-linked. Linear polymers are made
up of one long continuous chain, without any excess appendages or attachments.
Branched polymers have a chain structure that
consists of one main chain of molecules with smaller molecular chains branching
from it. A branched chain-structure tends to lower the degree of crystallinity
and density of a polymer. Cross-linking in polymers
occurs when primary valence bonds are formed between separate polymer chain
molecules.
Chains with only one type of monomer are known as homopolymers. If two or more different type monomers are
involved, the resulting copolymer can have several
configurations or arrangements of the monomers
along the chain. The four main configurations are depicted below:
They can be found in either crystalline or amorphous forms. Crystalline polymers are only possible
if there is a regular chemical structure (e.g., homopolymers or alternating
copolymers), and the chains possess a highly ordered arrangement of their
segments. Crystallinity in polymers is favored in symmetrical polymer chains,
however, it is never 100%. These semi-crystalline polymers possess a rather
typical liquefaction pathway, retaining their solid state until they reach their
melting point at T_m.
Amorphous polymers do not show order. The molecular segments in amorphous
polymers or the amorphous domains of semi-crystalline
polymers are randomly arranged and entangled. Amorphous polymers do not have a
definable T_m due to their randomness
At low temperatures, below their glass transition temperature (T_g), the segments are immobile and the
sample is often brittle. As temperatures increase close to T_g,
the molecular segments can begin to move. Above T_g, the
mobility is sufficient (if no crystals are present) that the polymer can flow as
a highly viscous liquid.
Thermoplastics are generally carbon containing polymers synthesized by addition
or condensation polymerization. This process forms strong covalent bonds within
the chains and weaker secondary Van der Waals bonds
between the chains. Usually, these secondary forces can be easily overcome by
thermal energy, making thermoplastics moldable at high temperatures.
Thermosets have the same Van der Waals bonds that thermoplastics do. They also
have a stronger linkage to other chains. Strong covalent bonds chemically hold
different chains together in a thermoset material. The chains may be directly
bonded to each other or be bonded through other molecules. This "cross-linking"
between the chains allows the material to resist softening upon heating.
In 1989, a billion pounds of virgin PET were used to make beverage bottles of
which about 20% was recycled. Of the amount recycled, 50% was used for fiberfill
and strapping. The reprocessors claim to make a high quality, 99% pure,
granulated PET. It sells at 35 to 60% of virgin PET costs.
The major reuses of PET include sheet, fiber, film, and extrusions.
Of the plastics that have a potential for recycling, the rigid HDPE container is
the one most likely to be found in a landfill. Less than 5% of HDPE containers
are treated or processed in a manner that makes recycling easy.
There is a great potential for the use of recycled HDPE in base cups, drainage
pipes, flower pots, plastic lumber, trash cans, automotive mud flaps, kitchen
drain boards, beverage bottle crates, and pallets.
LDPE is recycled by giant resin suppliers and merchant processors either by
burning it as a fuel for energy or reusing it in trash bags. Recycling trash
bags is a big business.
There is much controversy concerning the recycling and reuse of PVC due to
health and safety issues. When PVC is burned, the effects on the incinerator and
quality of the air are often questioned. The Federal Food and Drug
Administration (FDA) has ordered its staff to prepare environmental impact
statements covering PVC's role in landfills and incineration. The burning of PVC
releases toxic dioxins, furans, and hydrogen chloride.
PVC is used in food and alcoholic beverage containers with FDA approval. The
future of PVC rests in the hands of the plastics industry to resolve the issue
of the toxic effects of the incineration of PVC. It is of interest to note that
PVC accounts for less than 1% of land fill waste.
PS and its manufacturers have been the target of environmentalists for several
years. The manufacturers and recyclers are working hard to make recycling of PS
as common as that of paper and metals. One company, Rubbermaid, is testing
reclaimed PS in service trays and other utility items.

Table 3: Major Plastic Resins and Their Uses

Resin Name	Common Uses	Examples of Recycled Products
Polyethylene Terephthalate (PET or PETE)	Soft drink bottles, peanut butter jars, salad dressing bottles, mouth wash jars	Liquid soap bottles, strapping, fiberfill for winter coats, surfboards, paint brushes, fuzz on tennis balls, soft drink bottles, film
High density Polyethylene (HDPE)	Milk, water, and juice containers, grocery bags, toys, liquid detergent bottles	Soft drink based cups, flower pots, drain pipes, signs, stadium seats, trash cans, re-cycling bins, traffic barrier cones, golf bag liners, toys
Polyvinyl Chloride or Vinyl (PVC-V)	Clear food packaging, shampoo bottles	Floor mats, pipes, hoses, mud flaps
Low density Polyethylene (LDPE)	Bread bags, frozen food bags, grocery bags	Garbage can liners, grocery bags, multi purpose bags
Polypropylene (PP)	Ketchup bottles, yogurt containers, margarine, tubs, medicine bottles	Manhole steps, paint buckets, videocassette storage cases, ice scrapers, fast food trays, lawn mower wheels, automobile battery parts.
Polystyrene (PS)	Video cassette cases, compact disk jackets, coffee cups, cutlery, cafeteria trays, grocery store meat trays, fast-food sandwich container	License plate holders, golf course and septic tank drainage systems, desk top accessories, hanging files, food service trays, flower pots, trash cans

Addition Polymers

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Addition Polymerization polymers on 2008-12-03
- Addition polymers are usually made from molecules that have the following
  general structure:
  
  Different W, X, Y, and Z groups distinguish one addition polymer from
  another.
- In the first stage, a substance is split into two identical parts, each with an
  unpaired electron. (Peroxides, which contain an O-O bond, are often used in this
  role.) A molecule with an unpaired electron is called a free radical. The free
  radical then initiates the reaction sequence by forming a bond to one of the
  carbon atoms in the double bond of the monomer. One electron for this new bond
  comes from the free radical, and the second electron for the new bond comes from
  one of the two bonds between the carbon atoms. The remaining electron from the
  broken bond shifts to the carbon atom on the far side of the molecule, away from
  the newly formed bond, forming a new free radical. Each half-headed arrow
  indicates the shift of one electron.
- 4 more annotations...
- - The chain begins to grow--propagate, stage two--when the new free radical formed
    in the initiation stage reacts with another monomer to add two more carbon
    atoms. This process repeats over and over again
  - It can be terminated--stage three--when any two free radicals combine, thus
    pairing their unpaired electrons and forming a covalent bond that links two
    chains together.
  - Polyethylene molecules made with the free radical initiation process tend to
    form branches that keep the molecules from fitting closely together. Techniques
    have been developed that use catalysts, like Cr₂O₃, to
    make polyethylene molecules with very few branches.
  - yielding a high-density polyethylene, HDPE, that is more opaque, harder, and
    stronger than the low-density polyethylene, LDPE, made with free radical
    initiation.
Condensation Polymerization

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condensation polymers polymers on 2008-12-02
- The monomers that are involved in condensation polymerization are not the
  same as those in addition polymerization. The monomers for condensation
  polymerization have two main characteristics:.
  Instead of double bonds, these monomers have functional groups (like
  alcohol, amine, or carboxylic acid groups).
  Each monomer has at least two reactive sites, which usually means two
  functional groups.
  Some monomers have more than two reactive sites, allowing for branching
  between chains, as well as increasing the molecular mass of the polymer.
- Let's look again at the functional groups on these monomers. We've seen
  three:
  
  The carboxylic acid group
  The amino group
  
  The alcohol group
- 4 more annotations...
- - You might have learned in chemistry or biology class that these groups can
    combine in such a way that a small molecule (often H₂O) is given off.
    
    The Amide Linkage:
    When a carboxylic acid and an amine react, a water
    molecule is removed, and an amide molecule is formed.
    
    Because of this amide formation, this bond is known as an amide
    linkage.
    The Ester Linkage:
    When a carboxylic acid and an alcohol react, a water
    molecule is removed, and an ester molecule is formed.
    
    Because of this ester formation, this bond is known as an ester
    linkage.
  - Example 1:
    A carboxylic acid monomer and an amine monomer can join in an
    amide linkage.
    
    As before, a water molecule is removed, and an amide linkage is formed.
    Notice that an acid group remains on one end of the chain, which can react with
    another amine monomer. Similarly, an amine group remains on the other end of the
    chain, which can react with another acid monomer.
    Thus, monomers can continue to join by amide linkages to form a long chain.
    Because of the type of bond that links the monomers, this polymer is called a
    polyamide.
  - Example 2:
    A carboxylic acid monomer and an alcohol monomer can join in an
    ester linkage.
    
    <form>A water molecule is removed as the ester linkage is formed. Notice the
    acid and the alcohol groups that are still available for bonding.</form>
  - Because the monomers above are all joined by ester linkages, the polymer
    chain is a polyester. This one is called PET, which stands for poly(ethylene
    terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many
    other plastic products.)
Polymer - Condensation polymers

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on 2008-11-03
- Polymers are made up of extremely large, chainlike molecules consisting of numerous, smaller, repeating units called monomers. Polymer chains, which could be compared to paper clips linked together to make a long strand, appear in varying lengths. They can have branches, become intertwined, and can have cross-links. In addition, polymers can be composed of one or more types of monomer units, they can be joined by various kinds of chemical bonds, and they can be oriented in different ways. Monomers can be joined together by addition, in which all the atoms in the monomer are present in the polymer, or by condensation, in which a small molecule byproduct is also formed.
- The importance of polymers is evident as they occur widely both in the natural world in such materials as wool, hair, silk and sand, and in the world of synthetic materials in nylon, rubber, plastics, Styrofoam, and many other materials.
- 22 more annotations...
- - Polymers are extremely large molecules composed of long chains, much like paper clips that are linked together to make a long strand. The individual subunits, which can range from as few as 50 to more than 20,000, are called monomers (from the Greek mono meaning one and meros meaning part). Because of their large size, polymers (from the Greek poly meaning many) are referred to as macromolecules.
  - Most synthetic polymers are made from the non-renewable resource, petroleum, and as such, the "age of plastics" is limited unless other ways are found to make them. Since most polymers have carbon atoms as the basis of their structure, in theory at least, there are numerous materials that could be used as starting points.
  - Disposing of plastics is also a serious problem, both because they contribute to the growing mounds of garbage accumulating everyday and because most are not biodegradable. Researchers are busy trying to find ways to speed-up the decomposition time which, if left to occur naturally, can take decades.
  - n order for monomers to chemically combine with each other and form long chains, there must be a mechanism by which the individual units can join or bond to each other. One method by which this happens is called addition because no atoms are gained or lost in the process. The monomers simply "add" together and the polymer is called an addition polymer.
  - The simplest chemical structure by which this can happen involves monomers that contain double bonds (sharing two pairs of electrons). When the double bond breaks and changes into a single bond, each of the other two electrons are free and available to join with another monomer that has a free electron. This process can continue on and on. Polyethylene is an example of an addition polymer.
  - The polymerization process can be started by using heat and pressure or ultraviolet light or by using another more reactive chemical such as a peroxide. Under these conditions the double bond breaks leaving extremely reactive unpaired electrons called free radicals. These free radicals react readily with other free radicals or with double bonds and the polymer chain starts to form.
  - ifferent catalysts yield polymers with different properties because the size of the molecule may vary and the chains may be linear, branched, or cross-linked. Long linear chains of 10,000 or more monomers can pack very close together and form a hard, rigid, tough plastic known as high-density polyethylene or HDPE
  - Shorter, branched chains of about 500 monomers of ethylene cannot pack as closely together and this kind of polymer is known as low-density polyethylene or LDPE.
  - The ethylene monomer has two hydrogen atoms bonded to each carbon for a total of four hydrogen atoms that are not involved in the formation of the polymer. Many other polymers can be formed when one or more of these hydrogen atoms are replaced by some other atom or group of atoms.
  - Natural and synthetic rubbers are both addition polymers. Natural rubber is obtained from the sap that oozes from rubber trees. It was named by Joseph Priestley who used it to rub out pencil marks, hence, its name, a rubber. Natural rubber can be decomposed to yield monomers of isoprene.
  - It was sticky and smelly when it got too hot and it got hard and brittle in cold weather. These undesirable properties were eliminated when, in 1839, Charles Goodyear accidentally spilled a mixture of rubber and sulfur onto a hot stove and found that it did not melt but rather formed a much stronger but still elastic product. The process, called vulcanization, led to a more stable rubber product that withstood heat (without getting sticky) and cold (without getting hard) as well as being able to recover its original shape after being stretched. The sulfur makes cross-links in the long polymer chain and helps give it strength and resiliency, that is, if stretched, it will spring back to its original shape when the stress is released.
  - A second method by which monomers bond together to form polymers is called condensation.
  - Unlike addition polymers, in which all the atoms of the monomers are present in the polymer, two products result from the formation of condensation polymers, the polymer itself and another small molecule which is often, but not always, water.
  - One of the simplest of the condensation polymers is a type of nylon called nylon 6.
  - All amino acids molecules have an amine group (NH₂) at one end and a carboxylic acid (COOH) group at the other end. A polymer forms when a hydrogen atom from the amine end of one molecule and an oxygen-hydrogen group (OH) from the carboxylic acid end of a second molecule split off and form a water molecule. The monomers join together as a new chemical bond forms between the nitrogen and carbon atoms. This new bond is called an amide linkage.
  - The new molecule, just like each of the monomers from which it formed, also has an amine group at one end (that can add to the carboxylic acid group of another monomer) and it has a carboxylic acid group at the other end (that can add to the amine end of another monomer). The chain can continue to grow and form very large polymers.
  - Polymers formed by this kind of condensation reaction are referred to as polyamides.
  - Nylon became a commercial product for Du Pont when their research scientists were able to draw it into long, thin, symmetrical filaments. As these polymer chains line up side-by-side, weak chemical bonds called hydrogen bonds form between adjacent chains. This makes the filaments very strong.
  - Another similar polymer of the polyamide type is the extremely light-weight but strong material known as Kevlar. It is used in bullet-proof vests, aircraft, and in recreational uses such as canoes. Like nylon, one of the monomers from which it is made is terephthalic acid. The other one is phenylenediamine.
  - Polyesters are another type of condensation polymer, so-called because the linkages formed when the monomers join together are called esters.
  - Probably the best known polyester is known by its trade name, Dacron.
  - Dacron is used primarily in fabrics and clear beverage bottles. Films of Dacron can be coated with metallic oxides, rolled into very thin sheets (only about one-thirtieth the thickness of a human hair), magnetized, and used to make audio and video tapes. When used in this way, it is extremely strong and goes by the trade name Mylar. Because it is not chemically reactive, and is not toxic, allergenic, or flammable, and because it does not promote blood-clotting, it can be used to replace human blood vessels when they are severely blocked and damaged or to replace the skin of burn victims.
Chemistry - Condensation polymer

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on 2008-12-02
- Condensation polymers are any class of polymer formed through a condensation reaction, as opposed to addition polymers which involve the reaction of unsaturated monomers. Types of condensation polymer include polyamides and polyesters.
- The carboxylic acids and amines link to form peptide bonds, also known as amide groups. Proteins are condensation polymers made from amino acid monomers. Carbohydrates are also condensation polymers made from sugar monomers such as glucose and galactose.
- 1 more annotations...
- - Condensation Polymers, unlike Addition polymers are bio-degradable. The peptide or ester bonds between monomers can be hydrolysed by acid catalysts or bacterial enzymes breaking the polymer chain into smaller pieces.
Polymer Structures

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on 2008-12-02 and saved by 6 people
- Add Sticky Note
- Although the fundamental property of bulk polymers is the degree of
  polymerization, the physical structure of the chain is also an important factor
  that determines the macroscopic properties.
- 13 more annotations...
- - Configuration
    refers to the order that is determined by chemical bonds. The configuration of a
    polymer cannot be altered unless chemical bonds are broken and reformed. Conformation
    refers to order that arises from the rotation of molecules about the single
    bonds. These two structures are studied below.
  - The two types of polymer configurations are cis and trans.
    These structures can not be changed by physical means (e.g. rotation). The
    cis configuration arises when substituent groups are on the same side of
    a carbon-carbon double bond. Trans refers to the substituents on opposite
    sides of the double bond.
  - Three distinct structures can be obtained. Isotactic
    is an arrangement where all substituents are on the same side of the polymer
    chain. A syndiotactic
    polymer chain is composed of alternating groups and atactic is
    a random combination of the groups. The following diagram shows two of the three
    stereoisomers
    of polymer chain.
  - The ability of an atom to rotate this way relative to the atoms which it joins
    is known as an adjustment of the torsional angle. If
    the two atoms have other atoms or groups attached to them then configurations
    which vary in torsional angle are known as conformations.
  - different conformation may represent different potential energies of the
    molecule. There several possible generalized conformations: Anti (Trans),
    Eclipsed (Cis), and Gauche (+ or -). The following animation illustrates the
    differences between them.
  - The geometric arrangement of the bonds is not the only way the structure of a
    polymer can vary. A branched
    polymer is formed when there are "side chains" attached to a main
    chain. A simple example of a branched polymer is shown in the following diagram.
  - One of these types is called "star-branching".
    Star branching results when a polymerization starts with a single monomer and
    has branches radially outward from this point. Polymers with a high degree of
    branching are called dendrimers Often
    in these molecules, branches themselves have branches.
  - A separate kind of chain structure arises when more that one type of monomer is
    involved in the synthesis reaction. These polymers that incorporate more than
    one kind of monomer into their chain are called copolymers. There
    are three important types of copolymers. A random
    copolymer contains a random arrangement of the multiple monomers. A block
    copolymer contains blocks of monomers of the same type. Finally, a graft
    copolymer contains a main chain polymer consisting of one type of
    monomer with branches made up of other monomers. The following diagram displays
    the different types of copolymers.
  - In addition to the bonds which hold monomers together in a polymer chain, many
    polymers form bonds between neighboring chains. These bonds can be formed
    directly between the neighboring chains, or two chains may bond to a third
    common molecule.
  - Polymers with a high enough degree of cross-linking have "memory." When the
    polymer is stretched, the cross-links prevent the individual chains from sliding
    past each other. The chains may straighten out, but once the stress is removed
    they return to their original position and the object returns to its original
    shape.
  - In vulcanization, a series of cross-links are introduced into an elastomer
    to give it strength. This technique is commonly used to strengthen rubber.
  - Elastomers,or
    rubbery materials, have a loose cross-linked structure. This type of chain
    structure causes elastomers to possess memory. Typically, about 1 in 100
    molecules are cross-linked
  - Natural and synthetic rubbers are both common examples of elastomers. Plastics are
    polymers which, under appropriate conditions of temperature and pressure, can be
    molded or shaped (such as blowing to form a film). In contrast to elastomers,
    plastics have a greater stiffness and lack reversible elasticity.
Copolymerization

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copolymers polymerization on 2008-12-04
- The major practical problem, however, is that homopolymers
  blend together with difficulty and even where blends are possible, as in some
  thermoplastics, phase separation can occur readily.
  
  This problem is often overcome by polymerizing a mixture of
  monomers, a process known as copolymerization.
- It gives a much greater range of structures than is possible by mixing
  homopolymers because of the possibility of branching, structural isomerism
  within a single monomer, and the way in which the different repeat units can be
  added together.
- 9 more annotations...
- - suppose that two monomers, A and B, are copolymerized. The chain could start
    with either a molecule of A or a molecule of B, and at each successive addition
    there are always two possibilities as to which monomer molecule will be
    attached. As shown in Table 7, the number of possible
    chain structures grows rapidly as n increases. Since the number of
    possible structures is proportional to 2ⁿ, it is easy to see
    that even for low degrees of polymerization the number of possible copolymers is
    very large indeed. Some of these molecules are identical however
  - Table 7 shows that the composition varies from chains
    of only monomer A (homopolymer A) to chains containing only monomer B
    (homopolymer B).
  - In many of the structures no regularity can be detected,
    although there will be short sequences of one type of unit, and the copolymer
    can be regarded as completely random; such copolymers are usually said to be
    ideal copolymers. These possible copolymer structures are shown
    schematically in Table 7.
  - It can be shown that the rate of change of monomer
    concentration in any copolymerization is given by the equation
    
    where [M₁] and [M₂] are the
    concentrations of monomers 1 and 2 at any instant and r₁ and
    r₂., are reactivity ratios. The reactivity ratios represent
    the rate at which one type of growing chain end adds on to a monomer of the same
    structure relative to the rate at which it adds on to the alternative monomer.
    The copolymer equation can be used to predict chain structure in the
    three different ways, already mentioned.
  - An ideal copolymer will tend to form when each type of
    chain end shows an equal preference for adding on to either monomer. In this
    case,
    
    and the copolymer equation becomes
    
    Hence composition depends on the relative amounts of
    monomer present at any time and the relative reactivities of the two
    monomers.
  - Step growth copolymerizations produce ideal (random) copolymers since in this
    special case r ₁ = r ₂ = 1.
  - The main reason for copolymerizing different monomers is to adjust the physical
    properties of a given homopolymer to meet a specific demand. SBR elastomer, for
    example (Table 1), based on 24 wt%
    styrene monomer shows better mechanical properties and better resistance to
    degradation than polybutadiene alone
  - A second reason for copolymerization is to enhance the chemical reactivity of a
    polymer, particularly to aid crosslinking. Conventional vulcanization in rubbers
    is brought about by forming sulphur crosslinks at or near double bonds in the
    chain
  - To show the dramatic effect of copolymer structure on
    physical properties, consider the change from random SBR copolymer to a block
    copolymer of exactly the same chemical composition but where the styrene and
    butadiene parts are effectively homopolymer chains linked at two points:
    
    The material behaves like a vulcanized butadiene rubber
    without the need for chemical crosslinking since the styrene chains segregate
    together to form small islands or domains within the structure. Such so-called
    thermoplastic elastomers (TPEs) today form an important growth area for
    new polymers because of the process savings in manufacture that can be achieved
    with their use.
Emulsion polymerization - Wikipedia, the free encyclopedia

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Emulsion Polymerization polymers on 2008-12-03
- Emulsion polymerization is a type of radical polymerization that usually
  starts with an emulsion
  incorporating water, monomer, and surfactant. The most common type of
  emulsion polymerization is an oil-in-water emulsion, in which droplets of
  monomer (the oil) are emulsified (with surfactants) in a continuous phase of water.
- Typical monomers are those that
  undergo radical polymerization, are liquid or gaseous at reaction conditions,
  and are poorly soluble in water.
- 18 more annotations...
- - A dispersion resulting from emulsion polymerization is often called a latex (especially if derived from a synthetic rubber) or
    an emulsion (even though "emulsion" strictly speaking refers to a dispersion of
    a liquid in water). These emulsions find applications in adhesives, paints, paper coating and textile coatings.
  - Advantages of emulsion polymerization include:^[1]
    
    High molecular weight polymers can be made at fast
    polymerization rates. By contrast, in bulk and solution free radical polymerization, there
    is a tradeoff between molecular weight and polymerization rate.
    The continuous water phase is an excellent conductor of heat and allows the heat to be
    removed from the system, allowing many reaction methods to increase their rate.
    Since polymer molecules are contained within the
    particles, viscosity remains close
    to that of water and is not dependent on molecular weight.
    The final product can be used as is and does not generally need to be
    altered or processed.
    Surfactants and other polymerization adjuvants remain in the polymer or are difficult to
    remove
    For dry (isolated) polymers, water removal is an energy-intensive process
    Emulsion polymerizations are usually designed to operate at high conversion
    of monomer to polymer. This can result in significant chain transfer to
    polymer.
  - The first "true" emulsion polymerizations, which used a surface-active agent and polymerization initiator,
    were conducted in the 1920s to polymerize isoprene.^[6]^[7]
  - The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion
    polymerization is summarized by the following steps:
    
    A monomer is dispersed or emulsified
    in a solution of surfactant and water forming relatively large droplets of
    monomer in water.
    Excess surfactant creates micelles
    in the water.
    Small amounts of monomer diffuse through the water to the micelle.
    A water-soluble initiator is introduced into the water phase where it reacts
    with monomer in the micelles. (This characteristic differs from suspension polymerization where an
    oil-soluble initiator dissolves in the monomer, followed by polymer formation in
    the monomer droplets themselves.) This is considered Smith-Ewart Interval 1.
    The total surface area of the micelles is much greater than the total
    surface area of the fewer, larger monomer droplets; therefore the initiator
    typically reacts in the micelle and not the monomer droplet.
    Monomer in the micelle quickly polymerizes and the growing chain terminates.
    At this point the monomer-swollen micelle has turned into a polymer particle.
    When both monomer droplets and polymer particles are present in the system, this
    is considered Smith-Ewart Interval 2.
    More monomer from the droplets diffuses to the growing particle, where more
    initiators will eventually react.
    Eventually the free monomer droplets disappear and all remaining monomer is
    located in the particles. This is considered Smith-Ewart Interval 3.
    Depending on the particular product and monomer, additional monomer and
    initiator may be continuously and slowly added to maintain their levels in the
    system as the particles grow.
    The final product is a dispersion of polymer particles
    in water. It can also be known as a polymer colloid, a latex, or commonly and inaccurately as an
    'emulsion'.
  - Both thermal and redox generation of free radicals have been used in
    emulsion polymerization. Persulfate salts are commonly used in both initiation modes. The persulfate ion
    readily breaks up into sulfate radical ions above about 50°C, providing a
    thermal source of initiation.
  - Emulsion polymerizations have been used in batch, semi-batch, and continuous
    processes. The choice depends on the properties desired in the final polymer or
    dispersion and on the economics of the product. Modern process control schemes
    have enabled the development of complex reaction processes, with ingredients
    such as initiator, monomer, and surfactant added at the beginning, during, or at
    the end of the reaction.
  - Colloidal stability is a factor in
    design of an emulsion polymerization process. For dry or isolated products, the
    polymer dispersion must be isolated, or converted into solid form. This can be
    accomplished by simple heating of the dispersion until all water evaporates. More
    commonly, the dispersion is destabilized (sometimes called "broken") by addition
    of a multivalent cation. Alternatively, acidification will
    destabilize a dispersion with a carboxylic acid surfactant. These techniques
    may be employed in combination with application of shear to increase the rate of
    destabilization. After isolation of the polymer, it is usually washed, dried,
    and packaged.
  - Ethylene and other simple olefins must be
    polymerized at very high pressures (up to 800 bar).
  - Copolymerization is common in emulsion
    polymerization. The same rules and comonomer pairs that exist in radical
    polymerization operate in emulsion polymerization.
  - Monomers with greater aqueous solubility will tend to partition in the aqueous phase and not
    in the polymer particle. They will not get incorporated as readily in the
    polymer chain as monomers with lower aqueous solubility.
  - Ethylene and other olefins are used as minor comonomers in emulsion
    polymerization, notably in vinyl acetate copolymers.
  - Redox initiation takes place when an oxidant such as a persulfate salt, a reducing agent such as
    glucose, Rongalite, or sulfite, and a redox catalyst such as an
    iron compound are all included in the polymerization recipe. Redox recipes are
    not limited by temperature and are used for polymerizations that take place
    below 50°C.
  - Selection of the correct surfactant is critical to the development of any
    emulsion polymerization process. The surfactant must enable a fast rate of
    polymerization, minimize coagulum or
    fouling in the reactor and other
    process equipment, prevent an unacceptably high viscosity during polymerization
    (which leads to poor heat transfer), and maintain or even improve properties in
    the final product such as tensile strength, gloss, and water absorption
  - Anionic, nonionic, and cationic surfactants
    have been used, although anionic surfactants are by far most prevalent.
  - Examples of surfactants commonly used in emulsion polymerization include fatty acids,
    sodium
    lauryl sulfate, and alpha olefin sulfonate.
  - Some grades of poly(vinyl alcohol) and other water soluble polymers can
    promote emulsion polymerization even though they do not typically form micelles
    and do not act as surfactants (for example, they do not lower surface tension). It is
    believed that these polymers graft onto growing polymer particles and stabilize
    them.^[12]
  - Other ingredients found in emulsion polymerization include chain
    transfer agents, buffering agents, and inert salts. Preservatives are added to
    products sold as liquid dispersions to retard bacterial growth. These are
    usually added after polymerization, however.
  - Polymers produced by emulsion polymerization can be divided into three rough
    categories.
    
    Synthetic rubber
    Some grades of styrene-butadiene
    (SBR)
    Some grades of Polybutadiene
    Polychloroprene (Neoprene)
    Nitrile rubber
    Acrylic
    rubber
    Fluoroelastomer
    (FKM)
    Plastics
    Some grades of PVC
    Some grades of polystyrene
    
    Some grades of PMMA
    Acrylonitrile-butadiene-styrene terpolymer (ABS)
    Polyvinylidene fluoride
    PTFE
    Dispersions (i.e. polymers sold as aqueous dispersions)
    polyvinyl
    acetate
    polyvinyl acetate copolymers
    latexacrylic paint
    Styrene-butadiene
    VAE (vinyl acetate - ethylene copolymers)
Special Properties of Polymers

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polymers on 2008-12-02
- why these polymers, these macromolecules, act differently from small
  molecules.
- Chain entanglement
  Summation of intermolecular forces
  Time scale of motion
- 5 more annotations...
- - Remember now that most polymers are linear polymers;
    that is, they are molecules whose atoms are joined in a long line to form a huge
    chain. Now most of the time, but not always, this chain is not stiff and
    straight, but is flexible. It twists and bends around to form a tangled mess.
  - when a polymer is molten, the chains will act like spaghetti tangled up on a
    plate. If you try to pull out any one strand of spaghetti, it slides right out
    with no problem. But when polymers are cold and in the solid state, they act
    more like a ball of string.
  - intermolecular forces affect polymers just like small molecules. But with
    polymers, these forces are greatly compounded. The bigger the molecule, the more
    molecule there is to exert an intermolecular force. Even when only weak Van der
    Waals forces are at play, they can be very strong in binding different polymer
    chains together. This is another reason why polymers can be very strong as
    materials. Polyethylene, for example is very nonpolar. It
    only has Van der Waals forces to play with, but it is so strong it's used to
    make bullet proof vests.
  - This is a fancy way of saying polymers move more slowly than small
    molecules do. Imagine you are a first grade teacher, and it's time to go to
    lunch. Your task is to get your kids from the classroom to the cafeteria,
    without losing any of them, and to do so with minimal damage to the territory
    you'll have to cover to get to the cafeteria. Keeping them in line is going to
    be difficult. Little kids love to run around every which way, jumping and
    hollering and bouncing this way and that. One way to put a stop to all this
    chaotic motion is to make all the kids join hands when you're walking them to
    lunch. This won't be easy rest assured, as there's always going to be a lot of
    little boys who are too macho to hold the hands of the girls next to them in
    line, and some who are too insecure in their manhood to hold anyone's hand. But
    once you get them to do this, their ability to run around is severely limited.
    Of course, their motion will still be chaotic. The chain of kids will curve and
    snake this way and that on its way to eat soybean patties disguised as who knows
    what. But the motion will be a lot slower. You see, if one kid gets a notion to
    just bolt off in one direction, he or she can't do it because he or she will be
    bogged down by the weight of all the other kids to which he or she is bound.
    Sure, the kid can deviate from the straight path, and make a few other kids do
    so, but the deviation will be far less than you'd bet if the kids weren't all
    linked together.
    It's the same way with molecules.
  - So then how does this make a polymeric material different from a material made
    of small molecules? This slow speed of motion makes polymers do some very
    unusual things. For one, if you dissolve a polymer in a solvent, the solution
    will be a lot more viscous than the pure solvent.
What are Ziegler-Natta Catalysts?

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Ziegler-Natta Catalysts Ziegler-Natta Catalysts polymerization on 2008-12-03
- It was discovered that Group IV metals, especially titanium, were effective
  polymerization catalysts for ethylene. Following Ziegler’s successful
  preparation of linear polyethylene in 1953, Giulio Natta prepared and isolated
  isotactic (crystalline) polypropylene at the Milan Polytechnic Institute. This
  was immediately recognized for its practical importance. Ziegler and Natta
  shared the Nobel Prize in Chemistry in 1963.
- A Ziegler-Natta catalyst is composed of at least two parts: a transition metal
  component and a main group metal alkyl compound. The transition metal component
  is usually either titanium or vanadium. The main group metal alkyl compound is
  usually an aluminum alkyl. In common practice, the titanium component is called
  "the catalyst’ and the aluminum alkyl is called "the co-catalyst".
- 2 more annotations...
- - In some instances, especially for catalyzing the polymerization of propylene, a
    third component is used. This component is used to control stereoregularity
  - Today, Ziegler-Natta catalysts are used worldwide to produce the following
    classes of polymers from alpha olefins:
    
    High density polyethylene (HDPE)
    Linear low density polyethylene (LLDPE)
    Ultra-high molecular weight polyethylene (UHMWPE)
    Polypropylene (PP)--homopolymer, random copolymer and high impact copolymers
    
    Thermoplastic polyolefins (TPO’s)
    Ethylene propylene diene monomer polymers (EPDM)
    Polybutene (PB)
Polymerization catalysts past, present, and future

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Polymers catalysts polymerization on 2008-12-03
- The most common polymers are polyolefins, especially polyethylene (better
  known as Polythene, although this is a trade name owned by DuPont) and
  polypropylene. However, efficient ways of producing these vital materials are
  only the result of recent discoveries and have been dependant on the chemistry
  of catalysts.
- Since the 1950s, the production of polyolefins has depended on the use of
  Ziegler-Natta catalysts.
- 8 more annotations...
- - Ziegler-Natta catalysts are based on a mixture of a transition metal, commonly a
    titanium compound, and an alkali metal, most commonly aluminium oxide.
  - their products have variable physical properties. To this day, the systems are
    little understood, but the monomers (polymer starting materials) react through a
    number of reaction sites on the catalyst. Unfortunately, this means the polymer
    can grow from many sites and at different rates, leading to a very wide
    distribution in the molecular weight, based on the polymer chain length.
  - Modern life has demanded more of the humble polymer. We want polymers that are
    stronger than steel, lighter than aluminium, and can be dyed any colour
    imaginable
  - Metallocenes are positively charged metal ions, most commonly Titanium or
    Zirconium, sandwiched between two negatively charged cyclopentadienyl rings (see
    fig). Their big advantage over the Ziegler-Natta systems is that they catalyse
    the reaction of olefins through only one reactive site. Due to this “single
    site” reaction, the polymerization continues in a far more controllable fashion,
    leading to polymers with narrow ranges of molecular weight and, more
    importantly, predictable and desirable properties.
  - it has been found that changing the ligands (functional groups attached to the
    metal) upon the metallocene molecule can controllably affect the properties of
    the polymer.
  - The drawback of metallocene catalysts is that they are unable to polymerize
    polar molecules, such as common acrylics or vinyl chloride. This is due to the
    metallocenes’ oxophilicity – their propensity for binding to oxygen.
  - Catalysts using late transition metals – those metals from groups 6 and higher
    in the Periodic Table – have become increasingly utilized. These compounds have
    good polymerization activity, although slightly less than metallocenes. However,
    crucially they can polymerize reactions with polar monomers.
  - The most commercially advanced of this type of catalysts are the Brookhart
    catalysts [6], which are diimine complexes of palladium or nickel (see fig).
Effects of Temperature on Polymers

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polymerization on 2008-12-04
- Many polymers have a mixture of ordered (crystalline) regions and random
  (amorphous) regions. In the glassy state the tangled chains in the
  amorphous region are frozen so movement of chains is not possible. The
  polymer is brittle.
- If the glassy material is heated, the chains reach a temperature at which they
  can move. This temperature is called the glass transition temperature
  T_g. Above this temperature the polymer is flexible.
- 1 more annotations...
- - The glass transition temperature of a
    polymer can be changed by two different ways:
    
    Copolymerisation. Ethene can be
    polymerised with propene to give a new polymer with different properties.
    
    Plasticisers. PVC is quite
    brittle. Its T_g can be lowered, making it less brittle, by
    introducing a substance between the polymer chains, allowing the chains to slide
    over each other more easily. Such a substance is called a plasticiser.
Is Glass a Polymer?

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glass polymer on 2008-12-02
- The highest quality glass has the chemical formula SiO₂. But this is
  misleading. That formula conjures up ideas of little silicon dioxide molecules,
  analogous to carbon dioxide molecules. But little silicon dioxide molecules
  don't exist.
- Instead, in nature SiO₂ is often found as a crystalline solid, with a
  structure like you see on your right. Every silicon atom is bonded four oxygen
  atoms, tetrahedrally, of course; and every oxygen atom is bonded to two silicon
  atoms. When SiO₂ is in this crystalline form we call it
  silica.
- 4 more annotations...
- - But this silica isn't glass. We have to do something to it first to make it into
    glass. We have to heat it up until it melts, and then cool it down really fast.
    When it melts, the silicon and oxygen atoms break out of their crystal
    structure. If we cooled it down slowly, the atoms would slowly line back up into
    their crystalline arrangement as they slowed down.
  - As you can see, there is no order to the arrangement of the atoms. We call
    materials like this amorphous. This is the glass that is used for
    telescope lenses and such things. It has very good optical properties, but it's
    brittle. For everyday uses, we need something tougher. Most glass is made from
    sand, and when we melt down the sand, we usually add some sodium carbonate. This
    gives us a tougher glass with a structure that looks like this:
  - So is this a polymer or not? Usually it isn't considered as such. Why? Some may
    say it's inorganic, and polymers are usually organic. But there are many inorganic polymers out there. For example, what about polysiloxanes? These linear, and yes, inorganic
    materials have a structure very similar to glass, and they're considered
    polymers. Take a look at a polysiloxane:
  - So glass could be considered a highly crosslinked
    polysiloxane. But we usually don't think of it that way. Why not? Probably
    because even in a highly crosslinked system, you could still trace a polymer
    chain and see where the crosslinks are. But with glass, it'd be tough to do
    that.
Rubber Polymers

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Rubber polymers on 2008-12-03
- Rubber is an example of an elastomer type polymer, where the polymer has the
  ability to return to its original shape after being stretched or deformed.
- The elastic properties arise from the its ability to stretch the chains apart,
  but when the tension is released the chains snap back to the original position.
- 7 more annotations...
- - Natural rubber is an addition polymer
  - Natural rubber is from the monomer isoprene (2-methyl-1,3-butadiene). Since
    isoprene has two double bonds, it still retains one of them after the
    polymerization reaction. Natural rubber has the cis configuration for the methyl
    groups.
  - Charles Goodyear accidentally discovered that by mixing sulfur and rubber, the
    properties of the rubber improved in being tougher, resistant to heat and cold,
    and increased in elasticity. This process was later called vulcanization
  - Vulcanization causes shorter chains to cross link through the sulfur to longer
    chains.
  - Some of the most commercially important addition polymers are the copolymers.
    These are polymers made by polymerizing a
    mixture of two or more monomers. An
    example is styrene-butadiene rubber (SBR) - which is a copolymer of
    1,3-butadiene and styrene which is mixed in a 3 to 1 ratio, respectively.
  - More than 40% of the synthetic rubber production is SBR and is used in tire
    production. A tiny amount is used for bubble-gum in the unvulcanized form.
  - . At the nipple end of the balloon, there is lots of rubber and therefore many,
    many polymer chains - still loosely coiled. These chains can be pierced without
    popping the balloon because the the chains can still be stretched. This is
    because they allow the skewer in between the chains without breaking the chains
    or the bonds that connect them. But on the sides of the balloon, these chains
    are stretched almost to their limit and very far apart. The piercing is too much
    for the stretched chains and they break apart., and the balloon pops.
How Is Nylon Made?

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Polymers Nylon Polyamides on 2008-12-04
- nylon was used to make parachutes, clothes, military uniforms, tires,
  machine parts and other necessary items
- Nylon is made through a complex chemical reaction known as ring opening polymerization. In
  this reaction, a molecule with a ring shape such as hydrocarbons found in
  petroleum are submitted to various types of acids and bases. The ensuing
  chemical reactions cause the ring-shape molecular structure to flatten and
  lengthen. These molecules are caused to connect with one another to form
  molecular chains by being heated well above 600 degrees Fahrenheit. When done,
  what you have is a liquid with a high surface tension. If it cools down it will
  harden into a solid useless mass, so while it's still a liquid it is extruded
  through a hole with a diameter slightly greater than that of a human hair.
- 1 more annotations...
- - There is one problem, however, with this is process called hydrolysis. It's a
    chemical reaction during which the oxygen and hydrogen molecules in nylon's
    molecular chain can be broken away from the chain to produce water. This is the
    primary means by which nylon decays. It does not happen over time, but is
    instead a reaction to contact with certain caustic materials such as sulfuric or
    hydrochloric acid
OChem- Biopolymers

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on 2008-11-03
- The red lines connecting the 4th carbon atom
  of one isoprene unit with the 1st carbon of the next indicate that
  latex is an addition polymer that results from the 1,4-addition of
  one isoprene unit to the next. Note the head-to-tail pattern in
  which the isoprene units are connected. Note, too, that the
  stereochemistry is the same at each double bond, namely
  cis.
- Synthetic
  Rubber
  
  In 1839 Charles Goodyear discovered,
  literally by accident, that heating natural rubber with elemental
  sulfur altered the properties of the polymer, most notably making
  it tougher and more elastic. Goodyear's discovery led to the
  development of synthetic rubber, a material that found its most
  profitable application in the manufacture of automobile tires.
  Investigation of the structure of synthetic rubber revealed that
  the sulfur had formed disulfide bonds that linked one polyisoprene
  chain to the next. As Figure 2 demonstrates, these cross-links
  serve to restore the polymer to its original shape after it has
  been deformed by the application of a force.
  
  Figure
  2
  
  Bouncing
  Back
- 1 more annotations...
- - Not surprisingly, the desireable properties
    of natural as well as synthetic rubber led to investigations of
    the polymerization of structural analogs of isoprene. One notable
    success came from the polymerization of 2-chloro-1,3-butadiene,
    sometimes called chloroprene. Polychloroprene is known
    commercially as neoprene rubber. It is widely used in the
    automotive industry for the manufacture of oil-resistant hoses.
    Neoprene that contains entrapped air has good insulating
    properties and is used in the production of wet suits.
Polypeptides

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polymers biopolymers protein on 2008-12-02
- Polypeptides are chains of amino acids. Proteins are made up of one or more polypeptide molecules.
- One end of every polypeptide, called the amino terminal or N-terminal, has a free amino group. The other end, with its free carboxyl group, is called the carboxyl terminal or C-terminal.
- 1 more annotations...
- - he sequence of amino acids in a polypeptide is dictated by the codons in the messenger RNA (mRNA) molecules from which the polypeptide was translated. The sequence of codons in the mRNA was, in turn, dictated by the sequence of codons in the DNA from which the mRNA was transcribed.
    
    The schematic below shows the N-terminal at the upper left and the C-terminal at the lower right.

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A list of websites explaining the chemistry of plastics we use in our everyday lives. Topics included in the list are: Definition of a polymer, Addition polymerization, Condensation polymerization, Copolymers, Polyamides, Biopolymers, and Recycling polymers.

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Created on 2008-12-02

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