The invention provides a method of preparing a crosslinked polymer, which method comprises polymerising branched polyunsaturated monomers by a metathesis polymerisation reaction, wherein the branched polyunsaturated monomers contain acyclic ethylenically unsaturated groups that are capable of undergoing polymerization by a metathesis reaction such that the metathesis polymerisation produces a crosslinked polymer and substantially no non-volatile ethylenically unsaturated by-products.