UV initiated formation of polymer monoliths in glass and polymer microreactors

Abstract

Polymer monoliths with good flow-through properties were prepared by UV initiated polymerisation to form a support for heterogeneous palladium catalysis in glass and polymer microchips. Preparation of homogeneous polymer monoliths required investigation of different light source/photoinitiator combinations and manipulation of the polymerisation mixture to accommodate different channel dimensions. A deep UV (DUV) flood exposure lamp, UV tubes with respective outputs at 255 and 365 nm and a UV LED array with output at 365 nm were used to initiate the polymerisation. The spectra of light source and initiator were matched; 2,2-dimethoxy-2-phenylacetophenone (DMPAP) was selected for polymerisation in the DUV and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) was used for polymerisation in the near UV (NUV). This is the first report of the use of a BAPO-type photoinitiator for the formation of organic polymer monoliths. Only the DUV lamp and 365 nm UV LED array resulted in the formation of homogenous and continuous monoliths which were subsequently used to create continuous-flow microreactors in fused silica capillaries, borosilicate chips and in cyclic olefin copolymer (COC) capillaries and chips. All microreactors gave high to quantitative yields for the Suzuki–Miyaura's coupling of iodobenzene with 4-tolyl boronic acid. In addition to demonstrating the first polymer chip for heterogeneous Suzuki–Miyaura's catalysis, the UV LED array in combination with BAPO was found to be a suitable budget alternative to a DUV exposure source for monolith formation in devices with internal diameter of up to 2 mm.