JEE Chemistry Polymers Notes Set 03

Polymers and Their Classification

A polymer is a very large molecule (molecular mass from about 10⁴ to 10⁶) built by joining many small repeating units called monomers; the joining process is polymerisation. Every polymer is a macromolecule, but not every macromolecule is a polymer: chlorophyll is large but has no repeating unit. A sample of a synthetic polymer holds chains of different lengths, so its mass is always an average, while natural polymers such as proteins have chains of identical length and a definite mass. Polymers can be classified in five ways.

Natural polymers that control life processes (polysaccharides, proteins, nucleic acids) are called biopolymers. Semisynthetic polymers are chemical modifications of natural ones: cellulose acetate, cellulose nitrate (gun cotton) and rayon. Viscose rayon is made by dissolving cellulose in cold NaOH, treating it with \( \ce{CS_2} \) to give a viscose solution and forcing it through fine holes into dilute \( \ce{H_2SO_4} \); it is chemically cellulose with the shine of silk, so it is called artificial silk. Cellophane is viscose rayon in sheet form. Natural silk contains nitrogen and burns with the smell of hair, artificial silk does not.

1. Linear polymers pack well, so they have high density, tensile strength and melting point. Branched polymers pack poorly and have lower density and melting point. Cross-linked polymers form a three-dimensional network and are hard, rigid and brittle.
2. Elastomers have weak forces between coiled chains, so they stretch and recover. Fibres have strong hydrogen bonding or dipole attraction, so they pack closely, have high tensile strength and sharp melting points. The order of intermolecular force is polythene < Buna-S < nylon-66.
3. Thermoplastics soften repeatedly on heating and can be remoulded; they are linear or slightly branched with no cross-links. Thermosetting plastics set permanently on heating because they become cross-linked, so they cannot be remoulded and do not dissolve.
4. A plasticiser (a high-boiling ester such as dioctyl phthalate, or chlorinated paraffin wax) lowers the softening point and makes a hard plastic such as PVC soft and pliable, but it cannot turn a thermosetting polymer into a thermoplastic one.

Addition and Condensation Polymers

Examples of addition polymers are polythene, polypropene, PVC, polystyrene, teflon and orlon; condensation polymers include nylon-66, terylene, bakelite, glyptal and melamine resin. A copolymer made from two monomers may be alternating, random, block (blocks of each monomer) or graft (branches of one monomer on a chain of the other). Copolymerisation can give properties quite different from either homopolymer: vinyl chloride with vinylidene chloride gives saran, and maleic anhydride, which does not polymerise alone, copolymerises with styrene.

Mechanisms of Chain Growth Polymerisation

1. Free radical: an organic peroxide (benzoyl or t-butyl peroxide), or \( \ce{FeSO_4 + H_2O_2} \), splits homolytically into radicals. The radical adds to the alkene to make a new radical, which adds to another alkene, and the chain grows. It stops when two growing radicals combine or disproportionate. Very pure monomer is needed, since impurities and benzoquinone act as chain-transfer agents or inhibitors; they give a stable, unreactive radical and stop the chain. Carbon tetrachloride is a chain-transfer agent and lowers the average molecular mass.
2. Cationic: initiated by acids such as \( \ce{H_2SO_4} \), HF or \( \ce{BF_3} \) with water; a proton adds to the alkene to give a carbocation, the chain grows through carbocations and ends by loss of a proton. Isobutene polymerises this way.
3. Anionic: initiated by a nucleophile such as \( \ce{KNH_2} \) or \( \ce{LiNH_2} \); a carbanion grows the chain, and groups like -CN or -COOR that stabilise a negative charge help.
4. Co-ordination (Ziegler-Natta): \( \ce{TiCl_4} \) with triethylaluminium, \( \ce{Al(C_2H_5)_3} \), polymerises ethene and propene at low pressure and about 333 to 343 K to give linear, stereoregular, high density polymers. Ethene by free radicals at high pressure instead gives branched LDPE.

Stereoregularity matters for a substituted chain {-CH2-CHY-}: in an isotactic polymer all Y groups lie on the same side of the chain, in a syndiotactic polymer they alternate, and in an atactic polymer they are random. The first two pack well, are crystalline and are stronger; atactic polymers are soft and amorphous. A telomer is a low molecular mass polymer with end groups supplied by a chain-transfer agent.

Rubber

Natural rubber is the coagulated latex of the rubber tree (coagulated with acetic or formic acid). It is cis-1,4-polyisoprene with head-to-tail joined isoprene units; destructive distillation gives isoprene, 2-methylbuta-1,3-diene. Raw rubber is soft, sticky, weak, absorbs water and works only over a narrow temperature range, and it dissolves in benzene, ether and petrol. The trans form is gutta-percha. Vulcanisation, introduced by Goodyear, heats rubber with sulphur (about 5 percent for tyres, about 30 percent for ebonite or battery cases) with accelerators such as ZnO. Sulphur forms cross-links at the double bonds and allylic carbons, which prevents the chains slipping, so vulcanised rubber is harder, stronger, more elastic, wears better, works from about -40 to 100°C and is insoluble in common solvents.

Important Plastics, Resins and Fibres

Nylon-66 is made by heating the two monomers at about 553 K under nitrogen, and the chains are held by hydrogen bonds between N-H and C=O. Celluloid is cellulose nitrate plasticised with camphor. Cellulose acetate gives safety film because cellulose nitrate burns readily. Dynel, a copolymer of acrylonitrile and vinyl chloride, is used for wigs.

Biodegradable Polymers and Molecular Mass

Biodegradable polymers are broken down by micro-organisms, mainly by hydrolysis, and are used for surgical sutures, drug-release devices, packaging and agricultural films. Aliphatic polyesters are the main class: PHB from 3-hydroxybutanoic acid, PHBV from 3-hydroxybutanoic and 3-hydroxypentanoic acids (the first adds stiffness and the second flexibility), polyglycolic acid (PGA), polylactic acid (PLA), and poly(caprolactone). PGA and PLA in a 90:10 ratio dissolve in the body within about 15 days to a month and replace catgut.

The number average molecular mass is \( \ce{M_n = \frac{\sum N_i M_i}{\sum N_i}} \) and is found from colligative properties such as osmotic pressure; the weight average is \( \ce{M_w = \frac{\sum N_i M_i^2}{\sum N_i M_i}} \) and is found by light scattering or ultracentrifugation. The polydispersity index is \( \ce{PDI = M_w / M_n} \). Natural polymers such as proteins are monodisperse with PDI equal to 1, and synthetic polymers are polydisperse with \( \ce{M_w > M_n} \) and PDI greater than 1. For example masses of 40000 and 30000 give a PDI of 1.33.