above to form a copolymer generally of the formula: ##STR4## wherein X, X', Y and Y' are selected from the group consisting of --O-- and N(R), provided that X and Y cannot both be --O-- and X' and Y' cannot both be --O--; S and S' are independently integers in the range of from about 1 to about 10,000 and are preferably integers in the range of from about 1 to about 1,000 and W is an integer in th