a. at least one base monomer M1 of the following formula 1:

where

PG=a polymerizable group selected from OOC—CH═CH₂ and —OOC—C(CH₃)—CH₂;

S=a spacer group selected from unbranched and branched alkylene with C1-C10 carbon atoms that may additionally contain oxygen or —OOC—in the carbon chain,

or S is absent;

A=an aliphatic polycyclic group in which one or more hydrogen atoms may be replaced each independently of one another by C1-C4 alkyl radicals, C1-C4 alkoxy radicals, fluorine atoms, chlorine atoms or trifluoromethyl groups;

K=an aliphatic acyclic or cyclic, saturated or unsaturated unit with C1-C10 hydrocarbon atoms;

n=1-9;

b. at least one base monomer M2 of the following formula 2:

where

PG′=a polymerizable group selected from OOC—CH═CH₂ and —OOC—C(CH₃)—CH₂;

S′=a spacer group selected from unbranched and branched alkylene with C1-C10 carbon atoms that may additionally contain oxygen or —OOC—in the carbon chain,

or S′ is absent;

A′=an aliphatic polycyclic group in which one or more hydrogen atoms may be replaced each independently of one another by C1-C4 alkyl radicals, C1-C4 alkoxy radicals, fluorine atoms, chlorine atoms or trifluoromethyl groups;

where a mass ratio Y=m(M2)/m(M1) of the base monomers M2 to M1 is 0.9≤Y≤20.