The photoinduced ring-opening or ring-closing of photochromic functionalities like spiropyrans and diarylethenes 77 result in both steric and electronic effects. Hecht and coworkers reported a diarylethene-based catalytic system and its activity in ring-opening polymerization (FIG. ). The ring-opened phenol catalyst with an exposed -OH group can activate lactide and thus exhibits a high polymerization rate. Upon UV irradiation (λ = 300 nm) for 200 s, a photostationary state can be reached, leading to 98% ring-closed ketone isomer, which shows a diminished polymerization rate (kfast/kslow = 6.6). The system can be further turned back on to the active state by irradiation with visible light. The catalytic activity of the catalyst in different states towards the copolymerization of valerolactone (VL) and trimethylenecarbonate (TMC) was also studied. In the kinetically controlled copolymerization process, the disparity in rates can be harnessed to control the microstructure of the polymer. The ring-opened phenol catalyst, which has a higher activity in VL polymerization, can incorporate more VL to synthesize statistical copolymers with higher VL content. On the other hand, the ring-closed ketone isomer showed a lower activity for VL polymerization, thus resulting in a polymer with lower VL content and higher TMC content.