Abstract
Radical ring-opening polymerisation (RROP) of cyclic ketene acetals (CKAs) has gained momentum as it yields polyesters as biodegradable polymers from a radical polymerisation. In order to advance the polymerisation, some of its major limitations were addressed in the research presented, focussing on the four mainly used CKAs in modern research on RROP. Monomer synthesis has been updated towards a cobalt/TMSCl-based system that was performed reliably on several monomers at room temperature. Calculations using the density functional theory(DFT) revealed that the ring-opening step is energetically hampered in comparison to a ring-retaining reaction, which explained the challenges faced to promote the ring-opening reaction. Higher molecular weights up to four times the values reached by thermally initiated polymerisation were obtained by exploiting UV light and ultrasound as alternative methods to facilitate the polymerisation. The reaction procedure also influenced thermal properties of the polymers, which in turn affected the enzymatic degradation of nanoparticles based on those polymers. Altogether, the present study offers a holistic update to enhance the RROP of CKAs.
Highlights
The continuous development of biodegradable polymers such as polyesters is of significant interest in modern polymer chemistry [1,2]
cyclic ketene acetals (CKAs) synthesis via the CoCl2/TMSCl catalysed new acetal route was expanded and the necessary cobalt loading was minimised for each monomer. 2 mol% was the minimal catalyst load to reach the acetal precursor of MDO, 4 mol% was the minimum required towards the precursors of DMMDO and BMDO and 8 mol% was the minimal loading to get the precursor of MTC (Fig. 2a)
The chlorinated acetal intermediate of BMDO had the highest yield of 97%, whereas those of MDO and DMMDO resulted in acceptable yields of 60% and 57%, respectively
Summary
The continuous development of biodegradable polymers such as polyesters is of significant interest in modern polymer chemistry [1,2]. Ionic ring-opening polymerisation (ROP), polycondensation (PCo) and polyaddition (PAd) are already well-established methods to generate biodegradable polyesters [3,4]. All of these methods have limitations either in the range of possible polymers or in the control of the polymerisation. RROP has been known for more than 35 years, it has only grasped increased attention over the past decade [5,7,8,9] It adheres to three principles of green chemistry [10], since starting diols like 1,4-butanediol and diethylene glycol can be bio-sourced (principle 7), no protection groups are used in monomer synthesis and polymerisation (principle 8) and the resulting polyesters are designed for degradation (principle 10). Polyesters from RROP have been applied as homopolymers [11,12,13,14], (block-)
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