Linking in the Polymerization of Dienes

Cross-Linking in the Polymerization of Dienes.—The unsaturated structural units into which conjugated dienes are converted on polymerization are potentially capable of copolymerizing with monomer in subsequent stages of the polymerization (see Chap. VI). A polymer radical may conceivably add either to the 1,4 or to ie 1,2 unit as indicated below the greater susceptibility to be expected for the latter may be compensated to some extent by the usually smaller proportion of 1,2 units. 

Occurrence of either step causes the polymer molecule I to be incorporated in the growing chain. The unit of the polymer to which the radical adds becomes a tetrafunctional unit, which is equivalent to a pair of cross-linked units, i.e., to a cross-linkage. 

The kinetic treatment corresponds to that given above for the case of chain transfer with polymer. We have merely to write for the rate of generation of cross-linked units 

The latter equation, shown graphically in Fig. 73, expresses quantitatively the expected rapid increase in cross-linking with conversion. The rate constant ratio cor- 

The commonly observed, and technically troublesome, formation of gel in the polymerization of dienes testifies to the occurrence of cross-linking processes during their polymerization. According to Eq. (13), gel should begin to form when p reaches the critical value pc = l/y , where is the weight average degree of polymerization of the primary molecules. Thus 

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