Methacrylate homo- and copolymers containing photosensitive abietate group: Their high thermal stability, unique photocrosslinking behavior, transparency, and photolithographic application

The monomer methacryloyloxyethyl abietate (2) was synthesized by a simple two-step reaction starting from abietic acid. The homopolymerization of 2 and its copolymerization with MMA using AIBN as an initiator provided high-molecular-weight polymers 3-6 (10 × 10³ < M_n < 70 × 10³) in high yields. These polymers were well-dissolved in organic solvents. The glass-transition temperatures, T _g, of all polymers were higher than 130°C, much higher than that of PMMA. No melting points, T_m, were observed. Their decomposition temperatures were very high, as T_{5 wt % loss} > 300°C. These results indicate that these polymers are thermally stable and glassy at room temperature. The absorption bands due to the conjugated carbon-carbon double bonds within the abietate group significantly decreased with an increase in the exposure doses. Normalized absorbance as a function of UV exposure doses nonlinearly changed because of the occurrence of a certain threshold at 1.0 J/cm² in all polymers. Solvent extraction experiment revealed that interchain crosslinking effectively occurred even in copolymers with relatively low amounts of the abietate group. The polymer films showed a clear transparency of more than 90%. Photolithography of the polymer films provided high-resolution pattern images.