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1.What are polymers?
2.What is the difference between natural and synthetic polymers?
3.What is the origin of the word «polymer»?
4.How are polymers formed?
5.Where can polymers be applied?
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Unit 2
POLYMERIZATION
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Polymerization1 is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of polymerization and different systems exist to categorize them. The main categories are addition polymerization (also known as «chain-growth polymerization» or «chain polymerization») and condensation polymerization (also known as «step-growth polymerization» or «step polymerization»).
Addition polymerization involves the linking together of molecules incorporating double or triple chemical bonds. These unsaturated monomers (the identical molecules which make up the polymers) have extra internal bonds which are able to break and link up with other monomers to form the repeating chain. Addition polymerization is involved in the manufacture of polymers such as polyethylene, polypropylene and polyvinyl chloride (PVC). Basic addition polymerization steps are as follows:
I → 2 R· |
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CH2=CH2 + R· → RCH2-CH2· |
chain initiation |
RCH2-CH2· + CH2=CH2 → RCH2-CH2-CH2-CH2· |
chain propagation |
2 RCH2-CH2· → RCH2-CH2-CH2-CH2R |
chain termination |
2 RCH2-CH2· → RCH2-CH3 + CH2=CHR |
|
Condensation polymerization occurs when monomers bond together through condensation reactions. Generally, a condensation reaction is a chemical reaction in which two molecules combine to form one single molecule, together with the loss of a small molecule. Typically, condensation polymerization reactions can be achieved through reacting1 molecules incorporating alcohol, amine or carboxylic acid functional groups:
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HO-R-OH + HOOC-R’-COOH |
→ HO-R-O-CO-R’-COOH + H2O |
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glycol |
carboxylic acid |
ester group |
When an amine reacts with a carboxylic acid, the amide (or peptide) bond is formed with the release of water (hence, condensation polymerization). This is the process through which amino acids link up to form proteins, as well as how kevlar is formed:
2 H2N-CHR-COOH → H2N-CHR-CO-NH-CHR-COOH + H2O
ά-amino acid |
peptide bond |
Most step-growth polymers are classified as condensation polymers, but some step-growth polymers (like polyurethanes formed from isocyanate and alcohol bifunctional monomers) release no condensates, as in the case of addition polymers:
OCN-R-NCO + HO-R’-OH → OCN-R-NH-CO-O-R’-OH diisocyanate polyol urethane group
To alleviate inconsistencies in these naming methods, adjusted definitions for condensation and addition polymers have been developed. A condensation polymer is defined as a polymer that involves loss of small molecules during its synthesis, or contains heteroatoms as part of its backbone chain, or its repeat unit does not contain all the atoms present in the hypothetical monomer to which it can be degraded.
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Condensation polymers are any class of polymers formed through a condensation reaction, releasing (or condensing) a small molecule byproduct such as water or methanol, as opposed to addition polymers which involve the reaction of unsaturated monomers. Types of condensation polymers include polyamides, polyacetals, and polyesters.
Condensation polymerization, a form of step-growth polymerization, is a process by which two molecules join together, with the loss of a small molecule which is often water. The type of end product resulting from a condensation polymerization is dependent on the number of functional end groups of the monomer which can react.
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Monomers with only one reactive group terminate a growing chain, and thus give end products with a lower molecular weight. Linear polymers are created using monomers with two reactive end groups and monomers with more than two end groups give three dimensional polymers which are crosslinked.
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In polymer chemistry, a series of condensation reactions take place whereby monomers or monomer chains add to each other to form longer chains. This may also be termed as «condensation polymerization» or «step-growth polymerization». It occurs either as a homopolymerization of an A-B monomer or a polymerization of two co-monomers A-A and B-B. Small molecule condensates are usually liberated, unlike in addition polymerization where there is no liberation of small molecules. A high conversion rate is required to achieve high molecular weights.
In general, condensation polymers form more slowly than addition polymers, often requiring heat. They are generally lower in molecular weight. Monomers are consumed early in the reaction; the terminal functional groups remain active throughout and short chains combine to form longer chains.
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1.In the synthesis of polymers, monomers react and become covalently bonded with one another through the loss of a water molecule.
2.A condensation reaction may be considered as the opposite of a hydrolysis reaction.
3.Most commonly known condensation polymers are proteins, and fabrics such as nylon, silk, or polyester.
4.Addition polymerization involves the breaking of double or triple bonds, which are used to link monomers into chains.
5.Chain growth polymers grow to high molecular weight at a very fast rate.
6.Condensation polymers, unlike addition polymers are biodegradable.
7.The first useful addition polymer was made by accident in 1933 by chemists Reginald Gibson and Eric Fawcett.
8.Addition polymers are widely used today in the form of rubber.
9.Addition polymers are non-biodegradable and hard to recycle.
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10.An addition polymer is a polymer which is formed by an addition reaction, where many monomers bond together via rearrangement of bonds without the loss of any atom or molecule.
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1.What is addition polymerization?
2.What is condensation polymerization?
3.What are the examples of addition polymers?
4.What are the examples of condensation polymers?
5.Why do scientists qualify polyaddition reactions as a particular case of polycondensation?
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Unit 3
SUPRAMOLECULAR STRUCTURE OF POLYMERS
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Polymer molecules come in many shapes and sizes. A polymer may be a long chain of a single monomer repeated over and over again or a complex network containing dozens of different types of monomers. The identity, variety, and arrangement of monomers in a polymer molecule affect the chemical and physical properties of the polymer molecule. The arrangement of macromolecules defines bulk properties of the polymer material.
A synthetic polymer may be described as crystalline if it contains regions of three-dimensional ordering on atomic (rather than macromolecular) length scales, usually arising from intramolecular folding and/or stacking of adjacent chains. Synthetic polymers may consist of both crystalline and amorphous regions; the degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material. Few synthetic polymers are entirely crystalline.
Polymer crystals frequently do not display the perfection that is usual for low-molar-mass substances. A crystallite may have irregular boundaries and parts of its constituent macromolecules may extend beyond its boundaries. The smallest, regularly repeating material portion contained in a parallelepiped from which a crystal is formed by parallel displacements in three dimensions is called a unit cell.
Unlike in the case of low-molar-mass substances, the unit cell of polymer crystals usually comprises only parts of the polymer molecule and the regularity of the periodic repetition may be imperfect.
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