Examples of the ??, ??-ethylenically unsaturated monomers which are copolymerizable with the alkyl methacrylate include alkyl acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, cyclohexyl acrylate and lauryl acrylate; hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate; hydroxyalkyl methacrylates such as 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 4-hydroxybutyl