The most preferred organosilicon compounds are those wherein R is methyl, ethyl or propyl, wherein R is a divalent lower alkylene radical of from 1 to 8 carbon atoms and wherein Z is 'S, NH, NCH or NC H The vulcanizable silicon terminated polymers of this invention can be prepared by reacting about a stoichiometric amount of an isocyanate terminated polyurethane prepolymer with about a stoichiomet