Interpretation:
When polyethylene is made by free radical
Concept introduction:
Free radical polymerization is an example of chain-growth polymerization, one general class of polymerization reactions. In chain-growth polymerization, reaction occurs at the end of the growing chain, and each reaction lengthens the polymer chain by a single repeating unit. Branching can occur during chain-growth polymerization if the reactive site becomes located somewhere in the middle of a chain. The free radical at one end of one growing chain encounters a hydrogen atom in the middle of another chain. After hydrogen atom abstraction, the original free radical has become a terminated closed-shell species and the reactive radical appears somewhere in the middle of another chain. Further polymerization at the new radical creates a branch in the chain. Due to this reason, polyethylene has a significant amount of branching. But in polystyrene, phenyl groups are present which sterically hinder the radicals that may abstract
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