1. 10 to 85 percent by weight of a polymer comprising untis of the formula ##STR3## wherein R is hydrogen or methyl and R4 is an alkyl group of 1 to 4 carbon atoms, said polymer having no more than 50 percent by weight of said unis having R4 groups of more than 2 carbon atoms, and preferably no more than 40 percent by weight and most preferably no more than 30 percent by weight of said units havin