This invention relates to a resin having the general formula--A--B--C--wherein a plurality of each of components A, B and C occur in ordered or random sequence in the resin and wherein A is present in said resin at about 5% to about 20% by weight and comprises groups of the formula ##STR1## B is present in said resin at about 4% to about 30% by weight and comprises groups of the formula ##STR2## and C is present in said resin at about 50% to about 91% by weight and comprises acetal groups consisting of groups of the formulae ##STR3## where R is lower alkyl or hydrogen, and wherein said group I is present in component C from about 75% to about 85%; group II is present in component C from about 3% to about 5%; and group III is present in component C from about 10% to about 22%.