Abstract
In recent years, there has been considerable interest in the synthesis of novel thionolactone (C(=S)O) monomers and inserting them into vinyl polymers to make degradable thioester (C(=O)S) units. This process is called Thiocarbonyl Addition Ring-Opening (TARO) polymerisation and has been driven by a first-generation thionolactone, dibenzo[c,e]oxepine-5(7H)-thione (DOT). Now, second-generation thionolactones are being synthesised at a rapid pace to find one which copolymerises with more challenging vinyl monomers such as methacrylates.
This work has shown that the thionolactone monomer, 3,3-dimethyl-2,3-dihydro-5H-benzo[e][1,4]dioxepine-5-thione (DBT), is able to homopolymerise, and for the first time, copolymerise with methacrylates. Polymerisation kinetics have been at the forefront of these studies and have helped understand how DBT copolymerises, especially in terms of composition and speed. The consumption of DBT in radical polymerisations with styrene has proved so rapid that a starve-fed semi-batch setup had to be employed, though further optimisation is required. Conversely, the consumption of DBT in copolymerisations with methacrylates was sluggish and led to few thioester linkages being introduced; this has led to the feed of DBT being increased to force thionolactone incorporation.
Whilst DBT monomer can be handled at room temperature, elevated temperatures (> 110 °C) or the presence of acid can compromise its stability. This is because DBT has a propensity to rearrange, transforming from a 7- to a 6-membered thionolactone. The rearrangement appears to be novel.