Branching graft polymerization

The chain transfer to polymer process that produces long-chain branching is also a graft polymerization process (Sec. 3-6d). 

Ionic routes such as formation of the polymer anion by reaction with a strong base or the direct reaction of a polyamide with sodium are less likely to be used in reactive processing than is free-radical initiation. The process of self-graft polymerization by chain transfer to polymer, when it occurs in a single monomer/polymer system during polymerization, is an example of chain branching that is discussed in the next section. 

A specific case of branched radical polymerization is the graft polymerization of styrene on rubber. This reaction is the basis for the manufacture of some industrially important composite plastics, such as shock-proof polystyrene, ABC plastic, and similar materials. 

The number of monomer units of the trunk polymer per polystyrene branch decreased from 43 to 23 as the reaction proceeded. Namely, one polystyrene branch exists in every 23 monomer units on an average in the most highly branched graft copolymer obtained. The degree of polymerization of the trunk polymer was 970, so the number of polystyrene branches per trunk polymer increased from 22 to 43. 

An alternative approach is the so-called hypergrafting that relies on the use of a linear block copolymer with a poly-ftmctional (usually relatively short) second block that acts as an initiator in the grafting polymerization of the branching monomer even for a step-growth mechanism. In this case the polyfimctionality of the initiator permits control over molecular weights and polydispersity and suppresses homopolymetization. 

Bottlebmsh copolymers are a class of shape-persistent macromolecules that possess a highly branched comblike architecture. Steric repulsion between densely grafted polymeric branches causes the backbone to adopt an extended conformation. By combining this feature with grafts that are... 

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