. . "Any suitable chain extending agent having active hydrogen containing groups reactive with isocyanate groups may be used such as, for example, diols including ethylene glycol, propylene glycol, butylene glycol, 1,4-butanediol, butanediol, butynediol, xylylene glycols, amylene glycols, 1,4-phenylene-bis-??-hydroxy ethyl ether, 1,3-phenylene-bis-??-hydroxy ethyl ether, bis-(hydroxy-methyl-cyclohexane" . . . . .