Synthetic challenges of more sustainable RAFT polymers derived from plant oils

Polymeric materials based on fatty acids (FAs) have a combination of characteristics (alkene groups, hydrophobicity, tuneable Tg) that give them great potential as renewable, high value materials. The work presented in this thesis aimed to address the synthetic challenges of making plant oil based monomers (POBMs) via more sustainable and industrially aligned practices. Furthermore, this work aimed to facilitate their incorporation into polymeric materials of controlled size and structure via RAFT polymerisation. POBMs were synthesised via base catalysed transesterification of plant oils with N-(2- hydroxyethyl)acrylamide using two mono-unsaturated plant oils (high oleic sunflower oil and extra virgin olive oil), as well as with the novel use of two fully saturated oils (hydrogenated coconut and hydrogenated rapeseed). A much more thorough understanding of the monomer synthesis reaction was developed. The use of an industrially relevant brine washing process was found to result in mixtures of the product POBM and a variety of impurities (66 – 74%), identified through characterisation (oleochemical derivatives, radical inhibitors and co-monomers). The reaction of acrylic functional species, in a previously unidentified side-reaction, was established as a potential cause of the limited conversions achieved in transesterification reactions (55%). A more effective chromatographic purification method was developed (informed by green chemistry principles and industrial practice) which facilitated the work up of higher purity (> 90%) batches of the high-oleic sunflower oil based monomer.

The RAFT solution polymerisation of each of the POBMs was demonstrated, resulting in poly(POBM) materials with controlled Mn of 3,000 to 12,000 g mol−1 and low dispersities (Đ < 1.29). Batch kinetics experiments highlighted the differing polymerisation behaviour of differently structured POBMs. Higher apparent propagation rate constants found for the saturated monomers (HCM-A kp app = 2.32 h-1) than for the unsaturated monomers (HOSM-A kp app = 0.12 h-1), a result of allylic chain transfer. Improvements in the maximum Mn achievable were made via the use of higher purity monomer, increased initiator concentrations and RAFT agents with primary or secondary R groups. Early termination and limits to the maximum Mn achievable were further investigated by timesweep kinetics (in flow) and triple detection GPC. The observation of reaction orders of ~1.25 and differential gradients in Mark-Houwink plots could support a case for further investigation into secondary reactions in the RAFT polymerisation of POBMs. Finally, the chain extension of poly(γMeMBL) macro-CTAs with POBMs was demonstrated. Low conversions were initially achieved with unsaturated HOSM, due to poor reinitiation by the tertiary propagating poly(γMeMBL) radical, however this was overcome via the use of a faster reacting saturated monomer, HCM, or via use of a higher initiator concentration. The saturated poly(HCM-A) was found to display semi-crystalline behaviour (Tg = 22 to 34 °C, Tm = 48 to 71 °C), whereas unsaturated poly(HOSM-C) was fully amorphous (Tg = −1 to 12 °C), indicating the broad range of properties achievable dependent on the fatty acid structure of the repeat unit. This work demonstrates the first example of RAFT polymerisation of acrylamide monomers derived from plant oils by a one-step direct transesterification and the guidelines developed throughout this work open the door for the further development of novel well-defined, functional bio-based polymers.