Polymer formation chain initiation, hydrogen

Linear products result if the reactants are bifunctional, as in the case of Perlon U. Higher functionality leads to the formation of branched chain or crosslinked materials. This tendency is enhanced by the additional reaction of the isocyanate with the urethane, urea, or amide groups already introduced during initial polymer formation, reactions (a), (b), (c) above. The hydrogen on the urethane, urea or amide group is still active, and attacks a further isocyanate group. 

An intermolecular reaction is responsible for the formation of long-chain branching where a growing polymer chain abstracts hydrogen from another polymer chain anywhere along the polymer backbone, which initiates polymerization at this newly formed free-radical site. This is illustrated below. 

Addition of the strong Lewis acids, such as B(QF5)3, also initiates hydrogen release via the formation of boronium intermediates. These intermediates then further react with AB to form oligomeric or polymer aminoboranes with release of Hj. The relative concentration of Lewis add needs to be kept low (<0.5 mol%) to avoid chain termination resulting in aminodiborane, BjHjlji-NHj), and concentrated solutions can also produce large amounts of borazine. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

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