. . "Preferred chain extender is a compound having a molecular weight of not more than 400 and having two or moreactive hydrogen atoms capable of reacting with isocyanate group.The chain extender can include, for example, diols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 2-methyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-methyl-1,4-b" . . .