This paper reports the molecular weight control of polymers from bis-thyminyl monomers via confined photo-polymerisation inside different sized nanoreactors. Bis-thyminyl monomers were hosted inside nanoreactors (oil-in-water microemulsion, water-in-oil microemulsion, and miniemulsion) and topochemically polymerised using UV-light. UV-VIS and NMR spectroscopy and gel permeation chromatography (GPC) were used to confirm polymer formation and the molecular weight of formed polymers. The molecular weight of the formed polymer could be increased by increasing the diameter of the nanoreactors, which demonstrates that it is possible to control polymer molecular weight by modifying the nanoscopic reaction vessel size. This confined polymerisation provides polymers with a narrow molecular weight distribution when compared with conventional topochemical polymerisation.