The unusual chemical properties of supercritical fluids (SCFs) make them an unique and interesting reaction medium. They are easily removed and recycled making them a more environmentally friendly solvent choice compared to traditional organic solvents. We will discuss how to improve the sustainability of radical polymerisations through the use of SCFs.

Polymers are essential for a range of materials including, plastics, paints, coatings and various electronic and biomedical devices. Traditionally, free radical polymerisation is initiated by the decomposition of a highly reactive and unstable species, such as an azo compound (e.g. azobisisobutyronitrile, AIBN) or a peroxide (e.g. di t butyl peroxide). Conventional initiators are frequently immobilised on silica supports to allow for easy removal and recycling.

In this contribution, we will show how radicals are generated from calcined silica (monitored by electron paramagnetic resonance) and employed as novel polymerisation initiators. These initiators are not only capable of polymerising both highly reactive monomers (e.g. methyl acrylate), and traditionally non reactive substrates (e.g. norbornadiene), but also facilitate the copolymerisation of different monomers. Gel permeation chromatography has confirmed the presence of polymeric materials with a low dispersity index, usually around 1.2. When examining the immobilisation of traditional radical initiators on silica the question arises whether (a) the silica surface is, in fact, innocent during the reaction and (b) whether it is necessary at all to use these initiators, as silica may be sufficient. The coupling of our all silica initiator with supercritical CO² represents a “green” approach to the production of polymers.