By way of examples of ethylenically unsaturated monomers, there may be mentioned vinyl esters as vinyl acetate, acrylic acid, acrylic esters such as methyl acrylate, methacrylic acid, methacrylic esters such as methyl methacrylate, nitriles and acrylic or methacrylic amides, styrene monomers such as styrene, olefin monomers such as butadiene and halogenated vinyl monomers.