3,5-trimethylberzoyltriphenylphosphine oxide in amounts of in particular at most 1% by weight, based on the polymeric composition IV and the polymer IX, respectively, and the crosslinking may generally be carried out within from 0.5 to 15 minutes, advantageously under an inert gas, such as nitrogen or argon; by thermal free-radical polymerization, preferably at temperatures above 60??? C., it bein