11 n5 u2014 o u2014 r 8 and r 9 are independently of each other h or a c 1 c 8 alkyl group r 10 and r 11 are independently of each other h or a c 1 c 8 alkyl group r 12 is h or a c 1 c 8 alkyl group or a c 6 c 10 aryl group n1 is 1 or 2 n2 is 0 or 1 n3 is 1 or 2 n4 is an integer of 1 to 4 and n5 is an integer of 1 to 4 in said embodiment y is preferably a group of formula u2014 y 1 n1 u2014 y 2 u2014 y 3 n3 u2014 or u2014 y 1 n1 u2014 y 3 n3 u2014 y 2 is preferably a group u2014 o u2014 u2014 so 2 u2014 u2014 cr 8 r 9 n4 u2014 u2014 o u2014 cr 10 r 11 n5 u2014 o u2014 more preferably a group o u2014 u2014 so 2 u2014 u2014 ch 2 u2014 u2014 c ch 3 2 u2014 or u2014 o u2014 ch 2 2 u2014 o u2014 y 1 and y 3 are preferably 1 3 phenylene 1 4 phenylene and 3 6 naphthylene which may optionally be substituted by one or more c 1 c 18 alkyl groups or 2 6 pyridinylene and 4 6 pyrimidinylene which may optionally be substituted by one or more c 1 c 18 alkyl groups or c 1 c 18 alkoxy groups examples of preferred groups of formula.