Photopolymerizable composition comprising multi-photoinitiator system

DETAILED DESCRIPTION OF THE INVENTION Suitable substituents X in the general formula (I) above exhibiting a Hammett's sigma (.
sigma.
) constant ranging from about -0.
9 to about +0.
5 can include, e.
g.
, a hydrogen atom, an alkyl group, preferably an alkyl group having 1 to 4 carbon atoms (e.
g.
, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a tert-butyl group, etc.
), a phenyl group, a trifluoromethyl group, an acetyl group, an ethoxycarbonyl group, a carboxyl group, a carboxylato group (--COO.
sup.
-), an amino group, a methylamino group, a dimethylamino group, an ethylamino group, a diethylamino group, an acetylamino group, a --PO.
sub.
3 H group, a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, a pentyloxy group, a phenoxy group, a hydroxy group, an acetoxy group, a methylthio group, an ethylthio group, an isopropylthio group, a mercapto group, an acetylthio group, a thiocyano group (--SCN), a methylsulfinyl group, an ethylsulfinyl group, a methylsulfonyl group, an ethylsulfonyl group, an aminosulfonyl group, a dimethylsulfonio group (--S.
sup.
+ (CH.
sub.
3).
sub.
2), a sulfonato group (--SO.
sub.
3.
sup.
-), a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, an iodyl group, a trimethylsilyl group (--Si(CH.
sub.
3).
sub.
3), a triethylsilyl group, a trimethylstannyl group (--Sn(CH.
sub.
3).
sub.
3), etc.
Of these substituents, a hydrogen atom, a methyl group, an ethyl group, a methoxy group, an ethoxy group, a dimethylamino group, a diethylamino group, a chlorine atom and a bromine atom are preferred.
A --PO.
sub.
3 H group, a methylthio group, an ethylthio group, an isopropylthio group, a mercapto group, an acetylthio group, a methylsulfinyl group, an ethylsulfinyl group, a dimethylsulfonio group and a sulfonato group are less preferred because on polymerization a lower defree of polymerization results.
The Hammett's sigma (.
sigma.
) constant is well-known and the definition as used herein is as set forth in John E.
Leffler and Ernest Grunwald, Rates and Equilibria of Organic Reactions, John Wiley and Sons, Inc