During the free radical polymerization of vinyl chloride to form PVC, which of the following statements about the 'termination step' is generally considered correct under typical industrial conditions?
Correct: B
In free radical polymerization, termination typically occurs by two main mechanisms: combination (coupling) or disproportionation. For vinyl chloride (and many other vinyl monomers), termination by combination is often predominant, especially at lower temperatures. In combination, two growing polymer radicals react to form a single, longer polymer chain. Disproportionation involves one radical abstracting a hydrogen atom from another, resulting in one saturated and one unsaturated polymer chain. While both can occur, and the ratio depends on factors like monomer type and temperature, for many industrial processes forming PVC, combination is a significant pathway. Option C describes chain transfer to initiator, which is a type of chain transfer, not the primary termination step between two growing chains. Option D is incorrect as chain transfer agents are used to control molecular weight, not always for termination.