A process as claimed in claim 1 in which the polymerisation initiator (B) is at least one compound of the formula (I) ##EQU3## wherein L is a divalent to heptavalent metal or non metal, Q is a halogen atom or one of the groups Q may be a hydroxyl group, q is an integer from 1 to 3, m is an integer corresponding to the valency of L+q, a is 1 or 2, n is an integer from 1 to 3 and R is an ion selecte