Q.Identify 'A' and 'B' in the following reaction ----
a. n HO-CH2-CH2-OH + n HOOC-C6H4-COOH (terephthalic acid) --[Delta, 533K]--> 'A'
b. n H2N-(CH2)6-NH2 + n HOOC-(CH2)4-COOH --[533K]--> 'B'

Beyond rubber, several individually-important polymers dominate everyday and industrial use, each defined by its own monomer and preparation route. Polythene, [-CH2-CH2-]n, comes in two forms: low density polythene (LDP), made at very high pressure/temperature with an oxygen/peroxide initiator via a free-radical mechanism that branches the chain (giving flexible, low-density films used for packaging, cable insulation and pipes), and high density polythene (HDP), made at lower pressure/temperature using a Ziegler-Natta catalyst, giving an unbranched, denser, stiffer chain (used for buckets, bottles, laboratory ware). Teflon, [-CF2-CF2]n, made by free-radical polymerization of tetrafluoroethylene, owes its chemical inertness and heat resistance to its very strong C-F bonds, and is used for non-stick cookware, seals and gaskets. Polyacrylonitrile, made from acrylonitrile (CH2=CHCN), resembles wool and is marketed as orlon/acrilan. The polyamides ('nylons') carry a -CO-NH- linkage: Nylon 6,6, from adipic acid + hexamethylenediamine (condensation, via an intermediate nylon salt), and Nylon 6, from epsilon-caprolactam (ring-opening) -- both high-tensile-strength fibres. The polyesters carry an ester linkage: terylene/dacron, from ethylene glycol + terephthalic acid (condensation), PET (essentially the same polyester, used for bottles/containers), and polycarbonates (from bisphenol + phosgene, a high-melting thermosetting polyester). Bakelite (phenol + formaldehyde, via the intermediate novolac) and melamine-formaldehyde resin (melamine + formaldehyde) are cross-linked thermosetting resins used respectively for insulators/kitchenware and decorative laminates/dinnerware. Viscose rayon, a semisynthetic regenerated-cellulose fibre, is made from wood-pulp cellulose via alkali cellulose and cellulose xanthate intermediates, extruded through spinnerets into an acid bath. Table 15.1 additionally lists Perspex, Buna N, PVC, polyacrylamide, urea-formaldehyde resin, glyptal, polycarbonate and thermocol as further commercially important polymers with their own monomers and uses.