Chain Extension Studies Relevant to the Completeness of End-Linking in Elastomeric Polyurethane Networks

Polyurethane elastomers may be prepared by end-linking hydroxyl-terminated (atactic) poly(propylene oxide) chains with an aromatic triisocyanate. As is invariably the case, networks thus formed are insoluble and very difficult to analyze with regard to the completeness of the end-linking reaction. Relevant information can, however, be obtained by studying the products of the analogous reaction, between a structurally similar diisocyanate and the same polymer chains, under identical conditions. Since both reactants now have functionalities less than three, the reaction gives chain extension rather than crosslinking (gelation), and it is now possible to measure molecular weights before and after the reaction. Such experiments were carried out using diphenylmethane-4,4′-diisocyanate with poly(propylene oxide) chains which had viscosity-average molecular weights ranging from 10−3Mv=1.15−4.50 g mol−1. Values of Mv were found to increase by factors of 20–30, which indicates high extents of reaction. These results are relevant with regard to the probable efficiency of polyurethane curing techniques, and also for the characterization of “model” networks used in the quantitative evaluation of the molecular theories of rubberlike elasticity.