Addition polymers general mechanism

Polymers are formed via two general mechanisms, namely chain or step polymerisation, originally called addition and condensation, respectively, although some polymerisations can yield polymers by both routes (see Chapter 2). For example, ring opening of cyclic compounds (e.g., cyclic lactides and lactams, cyclic siloxanes) yield polymers either with added catalyst (chain) or by hydrolysis followed by condensation (step). Many polymers are made via vinyl polymerisation, e.g., PE, PP, PVC, poly(methyl methacrylate) (PMMA). It could be argued that the ethylenic double bond is a strained cyclic system. 

Polymers are often classified as addition polymers or condensation polymers according to the general mechanism by which they are prepared. Most addition polymers are prepared by the reaction of an alkene monomer as illustrated in the following equation ... 

Polyurethane is a condensation polymer generally formed by the reaction between a di-isocyanate and a hydroxylated-terminated resin known as polyol in the presence of a catalyst and a foaming agent The urethane foam formed as a result of this reaction is a cellular polymer that derives its mechanical properties in part from the cell matrix formed during its manufacture and in part from the intrinsic polymer properties. Choice of the di-isocyanate and polyol dictates the inherent polymer properties in addition filler materials may be added to the polymer to improve its mechanical properties. 

Scheme 4.1 General mechanism for the thermal degradation of addition polymers.

Scheme 4.1 shows a general mechanism proposed by Stivala et al.15 for the thermal degradation of addition polymers with the following steps ... 

In general, addition polym isations tend to proceed by a Free Radical Mechanism, while condensation... 

A general mechanism for living anionic polymerization of a vinyl monomer is illustrated in Scheme 7.1, encompassing only initiation and propagation steps chains are terminated only by the deliberate addition of a Bronsted acid or an electrophile. Important aspects of this mechanism, and that of any living polymerization, are that one initiator generates one polymer chain and that the product after all of the monomer has been consumed is a polymer... 

Addition polymers are produced by reactions in which monomers are added one after another to a rapidly growing chain. The growing polymer in addition polymerization proceeds via a chain mechanism. Like all chain reactions, three fundamental steps are involved initiation, propagation, and termination. Monomers generally employed in addition polymerization are unsaturated (usually with carbon-carbon... 

Later, in 1953, P.J. Flory divided the polymers by their reaction mechanism into chain-reaction and step-reaction, rather than by comparing the polymer s constitutional unit and the monomer. The addition polymers are generally produced by a chain reaction mechanism, and the condensation polymers produced by a step-reaction mechanism. Currently it is customary, though not scientifically correct, to refer to addition or chain-reaction polymerization and to condensation or step-reaction polymerization. Some have suggested that the classification of polymers... 

The first step in a step reaction mechanism is the formation of esters or amides from the diols and diacids or diacids and diamines, respectively. From these intermediates, the polymerization reaction (second step) proceeds. Because the first step is a faster reaction than the second, the monomer is used up quickly. During the two steps of the reaction, small molecules such as H2O or CH3OH are eliminated. Water is the most frequent byproduct molecule, for example from the reaction between a diacid and a dialcohol. Unlike addition polymers, condensation polymers, because they incorporate functional groups, generally have noncarbon atoms (heteroatoms) as part of the main backbone chain. Examples are shown in Fig. 3.30. 

Novak et al., 1992) of cyclooctene to form polyoctenamer. Quite surprisingly, the double bond is maintained in the polymer, i.e., it is not a normal addition polymer. The generally accepted mechanism for these... 

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