Defect groups unsaturated chain ends

Studies with model polymers have shown that the head-head linkage and the unsaturated chain end are less stable thermally than the saturated end. This work provides on explanation for previous observations that anionically polymerized methyl methacrylate is more stable thermally than tliat prepared by a free-radical mechanism. Tlie anionic polymer lacks the head-head linkage and the unsaturated end. It also provides yet another example of the importance of defect groups and their major influence on the properties of the polymer. 

Structural defects and/or irregularities are weak points. They may function as initiation sites for degradation, and thus usually decrease the TTOS. Such weak points include chain ends, branches, chain backbone unsaturation (such as double bonds involving at least one atom of the chain backbone), and carbonyl groups incorporated in polymers (such as polyethylene) where they are not a part of the normal structure. 

Commercially available poly(vinyl chloride) contains small amounts of different abnormal structures (defects) which may originate from synthesis. Such groups are random unsaturation (allylic chlorines) [316, 317,710,956,957] chain end groups [2, 3, 357, 710, 955, 1431, 2052] branch points (tertiary-bonded chlorine atoms) [2,3, 319, 357, 995,1514,2052-2054] head-to-head units [3,309,357,710] and oxidized structures [3,317,357,700]. It has been estimated that the number of defects per 1000 monomer units in commercial poly(vinyl chloride) samples are [357] 4-6 chloromethyl branches, 0.4-2.4 chloroethyl branches, 0.4-1.6 butyl branches (value uncertain), 0.18-2.4 long branches (value uncertain), 6-7 head-to-head structures (values uncertain), 1.4-3 total double bonds and 0.08-0.27 internal double bonds. Labile chloride atoms have been estimated at 0.6-2.5 per 1000 monomer units of which 0.5-2.5 are allylic chlorine or ketochloroallylic chlorine and 0.16-1.0 are chlorine at tertiary carbon (value uncertain). The possible structures of these defects are given in Table 3.12. 

In termination, unsaturated and saturated ends are formed when the propagating species undergo disproportionation, head-to-head linkages when they combine, and other functional groups may be introduced by reactions with inhibitors or transfer agents (Scheme 1.2). In-chain defect structures (within the polymer molecule) can also arise by copolymerization of the unsaturated byproducts of initiation or termination. 

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