Extension, Branching, and Cross-Linking Reactions

Other examples include the transannulation of poly(vinyl isocyanate) and poly(acrylonitrile). 

In these reactions, polymerizations initiated by free radicals or anions are to be preferred over those initiated by cations, since the latter tend to give frequent transfer reactions. For example, in the cyclization of natural rubber with the aid of concentrated acids of Lewis acids, only three trans-annular rings on the average are obtained [see reaction (25-20)]. 

Complete sequences of transannular rings give what are known as ladder polymers, because they look like ladders with steps. Of course, ladder polymers can also be produced by one-step transannular reactions. An example of this is the Diels-Alder polymerization of 2-vinyl butadiene and p-quinone  

Chain Extension, Branching, and Cross-Linking Reactions 

The degree of polymerization of the macromolecule increases in chain extension, branching, and cross-linking reactions. According to the chain structure of the newly produced macromolecule, distinction is made 

---

Sec. 23.5 Chain Extension, Branching, and Cross-Linking Reactions... 

An extensive treatment of this subject has been ven very recently by Lichti el a . (1980), and a brief summary was given in an earlier paper (Lichti el ai, 1978). The model assumed for this treatment is a three-state model in which i is 0, I, or 2. An earlier paper (Lichti et al., 1977) applied a similar treatment to a two-state model in which i is 0 or 1. The treatment allows for the possibility that mutual termination may result in either combination or disproportionation. It also allows for the possibility of transfer to monomer. It has not, however, been possible to make allowance for branching and cross-linking. Prediction of the full distribution of molecular sizes, and not merely of particular moments of the distribution, has been achieved. The conclusion has been reached that compartmen-talization of the reaction leads to a broadening of the molecular-weight distribution. 

Whea there are reactants with three or more functionahties participating ia the polymerization, branching and the formation of iatermolecular linkages, ie, cross-linking of the polymer chains, become definite possibiUties. If extensive cross-linking occurs in a polymer system to form network stmctures, the mobiUty of the polymer chains is greatiy restricted. Then the system loses its fluidity and transforms from a moderately viscous Hquid to a gelled material with infinite viscosity. The experimental results of several such reaction systems are collected in Table 6. 

By introducing branch points into the polymer chains, for example by incorporating about 2% of 1,2,3,-trichloropropane into the polymerisation recipe, chain extension may proceed in more than two directions and this leads to the formation of networks by chemical cross-links. However, with these structures interchange reactions occur at elevated temperatures and these cause stress relief of stressed parts and in turn a high compression set. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

---