tailieunhanh - Plastics Materials 7E Episode 2

Tham khảo tài liệu 'plastics materials 7e episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Further Consideration of Addition Polymerisation 27 In the case of mechanism 6 there are materials available which completely prevent chain growth by reacting preferentially with free radicals formed to produce a stable product. These materials are known as inhibitors and include quinone hydroquinone and tertiary butylcatechol. These materials are of particular value in preventing the premature polymerisation of monomer whilst in storage or even during manufacture. It may be noted here that it is frequently possible to polymerise two monomers together so that residues from both monomers occur together in the same polymer chain. In addition polymerisation this normally occurs in a somewhat random fashion and the product is known as a binary copolymer . It is possible to copolymerise more than two monomers together and in the case of three monomers the product is referred to as a ternary copolymer or terpolymer. The term homopolymer is sometimes used to refer to a polymer made from a single monomer. Other copolymer forms are alternating copolymers block copolymers and graft polymers. Figure illustrates some possible ways in which two monomers A and B can be combined together in one chain. a AABAAABBABABBAAAB b -ABABABABAB c AAAAAAAAABBBBBBBAAA d AAAAAAAAAAAA X Figure . a Random copolymer b alternating copolymer c block copolymer d graft copolymer Polymerisation may be carried out in bulk in solution in a suitable solvent in suspension or emulsion. Detailed considerations with individual polymers are given in later chapters but a number of general points may be made here. Bulk polymerisation is in theory comparatively straightforward and will give products of as good a clarity and electrical insulation characteristics as can be expected of a given material. However because polymerisation reactions are exothermic and because of the very low thermal conductivity of polymers there are very real dangers of the reactants overheating and the reaction getting out of

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