109  US 9,250,518 B2  110  In the case of a monomer having a hydroxyl group, the  hydroxyl group may be replaced by an acetal group suscep› tible to deprotection with acid, typically ethoxyethoxy, prior  to polymerization, and the polymerization be followed by  deprotection with weak acid and water. Alternatively, the  hydroxyl group may be replaced by an acetyl, formyl, piv› aloyl or similar group prior to polymerization, and the poly› merization be followed by alkaline hydrolysis.  In another preferred embodiment, the copolymer may have  further copolymerized therein recurring units ( d) selected  from units of indene, benzofuran, benzothiophene, acenaph› thylene, chromone, coumarin, and norbomadiene, or deriva› tives thereof. Suitable monomers are exemplified below.  )yo  )yo  CD~  X  0  ~  ~  ’  o"C  ’  o"C  HO)---v  HO~  0  0  X  H:  ~O  X  \OH~~  0  OH  OH  ~cc, ~OH  OH  OH  0  CDyo  OH  OH  -continued 