Temperature-responsive biobased block copolymers by RAFT polymerisation

Most plastics and polymeric materials are derived from fossil fuels, a rapidly depleting feedstock. As the demand for plastics grows, it is crucial to explore renewable alternatives that are often plant- or biologically-based materials that can replenish indefinitely.

The renewable feedstock, lactic acid, can be accessed through microbial fermentation of abundantly available carbohydrates from lignocellulosic biomass. N,N-dimethyl lactamide acrylate (DMLA) and ethyl lactate acrylate (ELA) are monomers derived from lactic acid-based, industrially available, green solvents. Additionally, these monomers contain vinyl functionality polymerisable through sequential monomer addition. This work identifies suitable reaction conditions to synthesise renewably-derived polymers based on DMLA and ELA using reversible addition-fragmentation chain-transfer (RAFT) polymerisation. The switch to renewable feedstocks alone is insufficient to create truly sustainable polymers. Therefore, green chemistry principles have been enacted to concurrently address other environmentally impactful criteria, such as solvent and energy use.

RAFT solution polymerisation of DMLA in water and dimethyl sulfoxide (DMSO) prepared PDMLAx homopolymers with varying degrees of polymerisation (DPx = 25–400), with relatively narrow molecular weight dispersities (Ð <1.30). Kinetic studies, thermal analysis and temperature-responsive assessments were also conducted on these homopolymers. A PDMLAx macromolecular chain transfer agent (macro-CTA) (DPx = 50) was subsequently chain extended with ELA by RAFT aqueous emulsion polymerisation, which through polymerisation-induced self-assembly (PISA) formed spherical nanoparticles. Additionally, a shorter PDMLAx macro-CTA (DPx = 25) was chain extended with ELA in aqueous emulsion and DMSO solution conditions. Replica reactions also produced equivalent diblock copolymers containing a fluorescent marker.

The amphiphilic PDMLA64-PELAy diblock copolymers (DPy =10–400) obtained molecular weight dispersities from 1.18 to 1.54. By drying these dispersions, atomic force microscopy (AFM) confirmed spherical morphologies and also identified some rod and platelet structures likely formed through crystallisation-driven self-assembly (CDSA). Optically transparent films were also observed during AFM preparation. Thermal analysis of the diblock copolymers identified two distinct glass transition temperatures, Tg(1) (7-16°C) and Tg(2) (55-69°C), representative of the PELA and PDMLA, respectively.

Many of these polymers also exhibited a reversible lower critical solution temperature (LCST) in water with a concomitant increase in PDMLA-b-PELA particle size. The PDMLA32-PELAy diblock copolymers (DPy = 10) and fluorescently-tagged equivalents obtained molecular weight dispersities from 1.16 and 1.46. Characterisation of the fluorescent properties confirmed adequate attachment and the potential for these copolymers to be identified using fluorescent identification techniques. Future work could involve application development either through lipopolymersome incorporation with uses in medical research where the fluorescent component could assist in their design or by synthesising triblock copolymers for use as rheological modifiers in aqueous paint formulations.