Structure abnormalities in poly vinyl chloride

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. 

The unsaturated end-groups arise from termination and/or chain transfer reactions. The unsaturation content varies from 1-3 double bonds per 1000 monomer units, depending on the polymerization conditions (Table 3.13) [271, 312]. Internal double bonds (—CHCl—CH=CH—CHj—) may grow to up to 30% of the total amount of unsaturation in the starting material [312]. The double bonds present in a polymer absorb only at below 220 nm and cannot be responsible for light absorption 250 nm. 

The a-chlorotrienes are probably the most important absorbing species [684, 685]. Formation of these structures can be observed both at 254 nm 

Unsaturated groups Long branches Abnormal internal structures 

In addition to these structural defects several oxygen-containing groups, such as ketonic carbonyl (CO) aldehyde (CHO), hydroperoxide (OOH), and ether links (—O—) can be formed during polymerization and storage of poly(vinyl chloride) in the presence of air. 

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