A compound of formula I P-Sp-Ar-R I wherein Ar is a group selected from the following formulae which is substituted by at least one group L a and optionally substituted by one or more groups L, L a is OCH 3, OC 2 H 5, O(CH 2) 2 CH 3, OC(CH 3) 3, OCF 3, OCF 2 H or OCFH 2, L is on each occurrence identically or differently F, Cl, -CN, P-Sp-, or straight chain, branched or cyclic alkyl having 1 to 25 C atoms, wherein one or more non-adjacent CH 2 -groups are optionally replaced by -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O- in such a manner that O- and/or S-atoms are not directly connected with each other, and wherein one or more H atoms are each optionally replaced by F or Cl, or L has the meaning of L a, P is a polymerisable group, selected from the group consisting of vinyloxy, acrylate, methacrylate, fluoroacrylate, chloroacrylate, oxetane and epoxide, wherin the compound contains exactly two or three polymerisable groups represented by the groups P, Sp is a spacer group that is optionally substituted by one or more groups P, or a single bond, wherein Sp, when being different from a single bond, is selected from -(CH 2) p2 -, -(CH 2) p2 -O-, -(CH 2) p2 -CO-O-, -(CH 2) p2 -O-CO-, wherein p2 is 2, 3, 4, 5 or 6, and the O-atom or the CO-group, respectively, is connected to the aromatic ring, or Sp-P denotes Sp 2 that is selected from the following formulae: -X-alkyl-CHPP S1 -X-alkyl-CH((CH 2) aa P)((CH 2) bb P) S2 -CO-NH((CH 2) aa P)((CH 2) bb P) S3 -X-alkyl-CHP-CH 2 -CH 2 P S4 -X-alkyl-C(CH 2 P)(CH 2 P)-C aa H 2aa+1 S5 -X-alkyl-CHP-CH 2 P S6 -X-alkyl-CPP-C aa H 2aa+1 S7 -X-alkyl-CHPCHP-C aa H 2aa+1 S8 in which P is as defined above, alkyl denotes a single bond or straight-chain or branched alkylene having 1 to 12 C atoms which is unsubstituted or mono- or poly-substituted by F, Cl or CN and in which one or more non-adjacent CH 2 groups may each, independently of one another, be replaced by -C(R 0)=C(R 0)-, -C≡C-, -N(R 0)-, -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O- in such a way that O and/or S atoms are not linked directly to one another, where R 0 has the meaning given in claim 1, aa, bb each, independently of one another, denote 0, 1, 2, 3, 4, 5 or 6, X is O, O-CO-, CO-O or a single bond, R is P-Sp-, H or has one of the meanings given for L, wherein, in case Ar is selected from formulae Ar1-Ar4, all groups P that are present in the compound have the same meaning. L a is OCH 3, OC 2 H 5, O(CH 2) 2 CH 3, OC(CH 3) 3, OCF 3, OCF 2 H or OCFH 2, L is on each occurrence identically or differently F, Cl, -CN, P-Sp-, or straight chain, branched or cyclic alkyl having 1 to 25 C atoms, wherein one or more non-adjacent CH 2 -groups are optionally replaced by -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O- in such a manner that O- and/or S-atoms are not directly connected with each other, and wherein one or more H atoms are each optionally replaced by F or Cl, or L has the meaning of L a, P is a polymerisable group, selected from the group consisting of vinyloxy, acrylate, methacrylate, fluoroacrylate, chloroacrylate, oxetane and epoxide, wherin the compound contains exactly two or three polymerisable groups represented by the groups P, Sp is a spacer group that is optionally substituted by one or more groups P, or a single bond, wherein Sp, when being different from a single bond, is selected from -(CH 2) p2 -, -(CH 2) p2 -O-, -(CH 2) p2 -CO-O-, -(CH 2) p2 -O-CO-, wherein p2 is 2, 3, 4, 5 or 6, and the O-atom or the CO-group, respectively, is connected to the aromatic ring, or Sp-P denotes Sp 2 that is selected from the following formulae: -X-alkyl-CHPP S1 -X-alkyl-CH((CH 2) aa P)((CH 2) bb P) S2 -CO-NH((CH 2) aa P)((CH 2) bb P) S3 -X-alkyl-CHP-CH 2 -CH 2 P S4 -X-alkyl-C(CH 2 P)(CH 2 P)-C aa H 2aa+1 S5 -X-alkyl-CHP-CH 2 P S6 -X-alkyl-CPP-C aa H 2aa+1 S7 -X-alkyl-CHPCHP-C aa H 2aa+1 S8 alkyl denotes a single bond or straight-chain or branched alkylene having 1 to 12 C atoms which is unsubstituted or mono- or poly-substituted by F, Cl or CN and in which one or more non-adjacent CH 2 groups may each, independently of one another, be replaced by -C(R 0)=C(R 0)-, -C≡C-, -N(R 0)-, -O-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O- in such a way that O and/or S atoms are not linked directly to one another, where R 0 has the meaning given in claim 1, aa, bb each, independently of one another, denote 0, 1, 2, 3, 4, 5 or 6, X is O, O-CO-, CO-O or a single bond, R is P-Sp-, H or has one of the meanings given for L,