A copolymer containing repeat units formed from the monomers (I) and (II), wherein each R.sup.1, if present, is independently hydrogen or hydrocarbyl; r is 0, 1 or 2; s is either 2 or 3; R.sup.2 R.sup.4 are each independently hydrogen or hydrocarbyl; R.sup.5 and R.sup.6 are each independently hydrogen, halo or hydrocarbyl; P.sub.d.sup.- is the anion formed by loss of a proton from a proton donating group P.sub.d--H; Z is either a bond or hydrocarbyl; R.sup.7 and R.sup.8 are each independently hydrogen, halo or hydrocarbyl; and either (a) A is an electron accepting group and W is either hydrogen or hydrocarbyl, (b) W is an electron accepting group and A is either a bond or hydrocarbyl or (c) both A and W are electron accepting groups. The invention also provides a method for preparing the copolymer by polymerising a mixture of (I) and (II) or their non-ionised precursors, preferably by exposure to ultraviolet radiation. The invention also relates to the uses of the copolymer in adhesive, coating, sealant and stereolithographic compositions ##STR00001##