Materials and methods for photodirected synthesis of oligonucleotide arrays on a solid substrate by photodirected synthesis are disclosed which employ a film formed from (i) a photoacid generator that on photolysis generates acid that is capable of directly removing the protecting group of the linker molecules or oligonucleotides and (ii) a polymer substantially lacking electronegative heteroatoms that are capable of hydrogen bonding with photogenerated acid. Methods of synthesizing an oligomer arrays are also described that use a film that restricts diffusion of reactants and products on the substrate during synthesis of the array and which includes a precursor of a deprotecting reagent. The method involves removing one or more of the non-required products of the deprotection reaction from the reactive array elements at which they are produced during the reaction, in order to displace the deprotection reaction towards completion.