eterocyclic groups and combinations thereof having up to about 30 carbon atoms, except that both R8 and R9 cannot be hydrogen; or, when taken together in combination with the carbon atom to which they are attached, R8 and R9 represent a carbonyl group; (3) each of R10 and R11 is a methyl group; (4) a represents an integer from 1 to about 20; (5) b represents an integer from 1 to about 20; (6) Z is