Carbon-13 NMR studies of the structures and polymerization mechanisms of poly (vinyl chloride) and related copolymers

Carbon-13 NMR analysis after chemical reduction has established the following facts with regard to the structures and formation mechanisms of some chlorine-containing polymers that are prepared by free-radical routes. Poly(vinyl chloride) (PVC) contains chloromethyl, 2-chioroethyl, and 2,4-dichloro-n -butyl branches, as well as long branch structures. Tertiary halogen is present at all of the branch points except those bonded to chioromethyl. Abstraction of hydrogen from the polymer, followed by conventional chain propagation, leads to the 2-chloroethyl, 2,4-dichloro-n-butyl, and long branches; and in the case of the 2-chloroethyl and 2,4-dichloro-n-butyl branches, the abstraction is intramolecular. The chioromethyl branch arises via a head-to-head emplacement of monomer, followed by a very rapid 1,2 shift of a chioro substituent. This shift creates a radical that either propagates in the normal way (and thus produces the branch) or transfers a chlorine atom to vinyl chloride. As a result of this transfer process, -CH2-CH—CH-CH2C1 and -CHC1-CH2-CHC1-CH2C1 are the principal long-chain ends in PVC. When PVC is prepared by polymerization at a low concentration of monomer, it also has a detectable number of internal double bonds. In poly(vinyl chloride-co-carbon monoxide), the carbon monoxide is contained in chiorocarbonyl branches. These branches are formed by the addition of monomer to -CHCOC1 intermediates that result from the facile rearrangement of -CHC1CO radicals. In poly(vinyl chloride-co-1,3-butadiene), the diene comonomer is introduced as a backbone 2-butene unit or as a part of a structural segment in which a 2,4-dichlorocyclohexyl branch occurs. This branch arises from the addition of vinyl chloride to the radical that ensues when a -CHC1-CH2CH—.CH-CH2-CH2-CHC1-CH2-CHC1 chain end experiences 1,6 cyclization. If the polymerization of vinyl chloride is initiated thermally by azobis(isobutyronitrile) (AIBN), the resultant polymer contains copolymerized methacrylonitrile units as well as CN(CH3)2Cend groups derived from initiator radicals. The methacrylonitrile is formed from the initiator in situ, and it also can be introduced as an impurity in the starting AIBN.