Preference is given to ethylenically unsaturated and hence copolymerizable silicon compounds of the general formula R.sup.3SiR.sup.2.sub.0-2(OR.sup.4).sub.1-3, whereR.sup.2 is a C.sub.1 to C.sub.3 alkyl radical, C.sub.1 to C.sub.3 alkoxy radical, or halogen (e.g., Cl or Br), R.sup.3 is CH.sub.2.dbd.CR.sup.3--(CH.sub.2).sub.0-1 or CH.sub.2.dbd.CR.sup.5CO.sub.2(CH.sub.2).sub.1-3, R.sup.4 is a branch