Photoinitiators

A photoinitiator of the formula (I):

• in which:
• Pi is a benzophenone photoinitiator moiety of formula (V):
• wherein Ar1 and Ar2 are optionally substituted phenyl, and Za may be present at any position on Ar2;
• Za and Zb together form a single bond, or a linker in which
• Za is selected from optionally substituted (—(O—(C1-C12 alkylene)))n-, optionally substituted —(NR1—(C1-C12 alkylene)nwherein R1 is H or optionally substituted C1-C12alkyl and optionally substituted —(S—(C1-C12 alkylene))n-;
• wherein n is 1;
• wherein Za is joined to Pi via the O, N or S atom in Za;
• and
• Zb is a single bond;
• X1 and X2 are independently selected from optionally substituted C1-C12 alkylene, optionally substituted C1-C12 alkenylene, optionally substituted heterocyclyl, —O—, —S—, —NR2—,—C(═O)—, —C(═NR2)—, —Si(R2)2—O—, optionally substituted aryl, and combinations thereof, wherein R2 is H or optionally substituted C1-C12 alkyl;
• wherein X1 and X2 may be linked to one another or to Za to form one or more ring structures;
• wherein Za, X1 and X2 are selected such that N is a tertiary amine;
• W1 and W2 are independently selected from alcohol, primary amine, secondary amine, or thiol groups.