US 9,869,931 B2  36  35  -continued  -continued  (A-2)-61  (A-2)-62  (A-2)-63  CH3  I  -CH-0  (A-2)-64  (A-2)-65  (A-2)-66  CH3  I  -CH-0  (A-2)-67  (A-2)-68  (A-2)-70  CH3  CH3  I  I  -CH-O-CH2CH2-0-CH- The crosslinking acetal groups of formulae (A-2a) and  (A-2b) are exemplified by the following formulae (A-2)-70  through (A-2)-77.  Preferably, "A" is selected from divalent to tetravalent,  straight, branched or cyclic C$_{1 }$-C$_{20 }$alkylene, alkyltriyl and  alkyltetrayl groups, and  CcC$_{30 }$ arylene groups, which may  be separated by a heteroatom or in which some carbon› bonded hydrogen atoms may be substituted by hydroxyl,  carboxyl, acyl moieties or halogen atoms. The subscript cl  is preferably an integer of 1 to 3.  Herein R 40  and R 41  each are hydrogen or a straight,  branched or cyclic C$_{1 }$-Cg alkyl group, or R$^{40 }$and R 41 , taken  together, may form a $^{rin}$8 with the carbon atom to which  they are attached, and R 4  and R$^{41 }$are straight or branched  C$_{1 }$-Cg alkylene groups when they form a ring. R 42  is a  straight, branched or cyclic C$_{1 }$-C$_{10 }$alkylene group. Each of  bl and dl is O or an integer of 1 to 10, preferably O or an  integer of 1 to 5, and cl is an integer of 1 to 7. "A" is a  (cl+l)-valent aliphatic or alicyclic saturated hydrocarbon  group, aromatic hydrocarbon group or heterocyclic group  having 1 to 50 carbon atoms, which may be separated by a  heteroatom or in which some carbon-bonded hydrogen  atoms may be substituted by hydroxyl, carboxyl, carbonyl  moieties or fluorine atoms. "B" is ----CO-O-, -NHCO› O- or -NHCONH-.  Other examples of acid labile groups include those of the  following formula (A-2a) or (A-2b) while the polymer may  be crosslinked within the molecule or between molecules  with these acid labile groups.  Of the acid labile groups of formula (A-2), the cyclic ones  are, for example, tetrahydrofuran-2-yl, 2-methyltetrahydro› furan-2-yl, tetrahydropyran-2-yl, and 2-methyltetrahydropy› ran-2-yl.  (A-2b)  R4o  R4o  I  I  -C-O-R 42 -B-A-+B-+R 42 -0-t,;;-C-]c1  I  di  I  R41  R41  (A-2a)  R4o  R4o  I  I  -,-+O-R 42 -ni"O-A-+O-+ R 42 -0-t;ITl-]c!  R41  R41  (A-2)-69 