Examples of the monomer from which the recurring unit  (b2) is derived are shown below, but not limited thereto. R 5  and R  6  are as defined above.  -continued  18  US 10,162,262 B2  17  OR 6  In formula (A-1), RL 1  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). Al is an integer of Oto 6.  Exemplary tertiary alkyl groups are t-butyl, t-pentyl,  1,1-diethylpropyl, 1-ethylcyclopentyl, 1-butylcyclopentyl,  1-ethy lcyclohexy 1, 1-buty lcyclohexyl, 1-ethy 1-2-cyclopen› tenyl, 1-ethy 1-2-cyclohexeny 1, and 2-methy 1-2-adamantyl.  Exemplary trialkylsilyl groups are trimethylsilyl, triethylsi- lyl, and dimethyl-t-butylsilyl. Exemplary oxoalkyl groups  are  3-oxocyclohexyl,  4-methyl-2-oxooxan-4-yl,  and  5-methyl-2-oxooxolan-5-yl.  (A-1)  (A-2)  (A-3)  The acid labile groups represented by R 4  and R 6  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).  OR 6 