. . . "Such materials are preferably substantially linear so as to have the following general structure E-(L-E)u -L-E in which each E is the residue of the diprimary amine-terminated material and each L is a linking group that is the residue of the difunctional primary amine-reactive material. (By residue is meant those portions of the diprimary amine-terminated material and the difunctional primary am" .