Classification based on mode of formation- chain growth and step growth polymerization

Polymers are large molecules made up of repeating units called Monomers The synthetic process is Polymerization. E.g.

A polymer prepared from a single monomer is a homopolymer If two or more monomers are employed, the product is a copolymer Linear polymer has no branching Graft copolymer is an example of a branched network

Two main classifications of Polymerization Addition reaction or Chain growth Molecular weight increases by successively adding monomers to a reactive polymer chain end resulting in high molecular weights at low conversions. STEP reaction or growth Polymers are formed by linking monomer molecules to form dimers, trimers and higher species in a step-wise fashion. The most abundant species react, and thus high molecular weight formed only beyond 99% conversion.

Polymerizations carried out at very low temperatures Addition reaction or Chain growth Ionic Chain (addition)-Growth Polymerization The choice of ionic procedure depends greatly on the electronic nature of the monomers to be polymerized Vinyl monomers with electron-accepting groups Anionic Polymerization Vinyl monomers with electron-donating groups Cationic Polymerization Monomers and reagents should be purified; water and oxygen should be removed. Polymerizations carried out at very low temperatures

Anionic Polymerizations Initiators include alkyl lithiums and sodium amide

Cationic Polymerization -- the formed carbocation must be quite stable Stable tertiary carbocation stable oxonium ion E.g. proton initiates polymerization of isobutane (2-methylpropene) Adhesive, sealant, insulating oil, lubricating oil

Chain Reaction: Free Radical Polymerization Initiation Propagation Random Termination Dead chains

Examples of Polymers Prepared by Radical Polymerization Monomer Polymer n Poly(styrene) n Poly(acrylonitrile) n Poly(methylacrylate) n Poly(vinylacetate)

Recent Example of a Graft Copolymer Synthesis + Copolymerization macromonomer monomer Graft copolymer Poly(AA) NIPAM + N-Isopropylacrylamide NIPAM monomer (excess) Poly(acrylic acid) macromonomer Insoluble in water above the lower critical solution temperature (LCST) McHale, Aldabbagh, Carroll, Yamada, J. Polym. Sci. Part A: Polym. Chem. 2007, 45, 4394-4400

Step-growth Polymerization Step-polymers are made by allowing difunctional monomers with complementary functional groups to react with one another Condensation between two molecules Poly(ethylene terephthalate) terephthalic acid ethylene glycol PET This is an example of a poly(ester) The reaction is a transesterification Recyclable plastic bottles and textile fabrics Using a condensation reaction

Time for litter to biodegrade Product Time to biodegrade Paper 2-5 months Wool socks 1 to 5 years Plastic coated paper milk cartons 5 years Plastic bags 10 to 20 years Nylon fabric 30 to 40 years Aluminum cans 80 to 100 years Plastic 6-pack holder rings 450 years Glass bottles 1 million years Plastic bottles Forever