23  US 9,152,050 B2  24  Note that R $^{12 }$and R $^{13 }$are as defined above.  The acid labile groups represented by R $^{11 }$and R $^{13 }$in for- mula (3) may be selected from a variety of such groups. The  acid labile groups may be the same or different and preferably  include groups of the following formulae (A-1) to (A-3).  In formula (A-1),  RL $^{30 }$is a tertiary alkyl group of  4  to 20  carbon atoms, preferably  4  to 15 carbon atoms, a trialkylsilyl  group in which each alkyl moiety has 1 to 6 carbon atoms, an  oxoalkyl group of  4  to 20 carbon atoms, or a group of formula  (A-3). Exemplary tertiary alkyl groups are tert-butyl, tert- amyl, 1,1-diethylpropyl, 1-ethylcyclopentyl, 1-butylcyclo› pentyl, 1-ethylcyclohexyl, 1-butylcyclohexyl, l-ethyl-2-cy› clopentenyl,  l-ethyl-2-cyclohexenyl,  and  2-methyl-2- adamantyl. Exemplary trialkylsilyl groups are trimethylsilyl,  triethylsilyl,  and  dimethyl-tert-butylsilyl.  Exemplary  oxoalkyl groups are 3-oxocyclohexyl, 4-methyl-2-oxooxan- 4-yl, and 5-methyl-2-oxooxolan-5-yl. Letter Al is an integer  ofO to 6.  Note that R $^{10 }$and R $^{11 }$are as defined above.  Examples of the monomer from which recurring units (a2)  are derived are given below.  (A-1)  (A-2)  (A-3)  Rl2  &12  ORl3  ~  I  ORl3  ~  0  0  0  OR 11  RIO  ~OR 11  0 