UV degradation of poly(lactic acid) materials through copolymerisation with a sugar-derived cyclic xanthate †

The copolymerisation of L -Lactide with a cyclic xanthate monomer derived from tri- O -acetyl- D -glucal has been used to incorporate thionocarbonate and thioester linkages into a polyester backbone. The poly(lactide- co -xanthate) copolymers show enhanced UV-degradability compared to PLA, with 40% mass loss within 6 hours of UV exposure (365 nm) for only 3% of sulfur-containing linkages.

Amidst the environmental persistence and reliance on fossil-based resources of the most common polymers, polymers based on renewable resources, and which can be degraded, have been investigated as alternatives. [1][2][3] Poly(lactic acid) (PLA) is a thermoplastic aliphatic polyester derived from lactic acid and is perhaps the most widely studied degradable and renewable polymer. While PLA degrades under industrial composting conditions (62 AE 4 1C, 460% relative humidity) within weeks, it lacks facile degradability in natural environments, e.g. in soil or in seawater. 4,5 One strategy to adjust the degradation of PLA has been the copolymerisation of lactide (LA) with other monomers resulting in linkages more susceptible to hydrolysis. 4,6 Once oligomeric, PLA indeed dissolves quicker and is attacked by enzymes more effectively. Thus, in biomedical applications, poly(lactic-co-glycolic acid) (PLGA) is a common material. 7 PLA hydrolytic degradation has also been enhanced recently by copolymerisation with cyclic phosphoester 8 and salicylates. 9 However, to the best of our knowledge, no modification of PLA has been directed at enhancing its degradability towards stimuli other than hydrolysis, such as light.
As part of our wider research programme on renewable polymers, we, amongst others, have been investigating the incorporation of monosaccharide units into polymer backbones. The natural abundance, structural diversity and functionalisation potential of sugars make them a renewable resource with tremendous potential for sustainable materials. 10,11 Of relevance here, we have thus reported the ring-opening copolymerisation of a xylose-based oxetane with CS 2 , which yield polythiocarbonates that rapidly degrade under UV radiation. 12 We also previously reported the Ring-Opening Polymerisation (ROP) of cyclic xanthates from C5 sugars (D-ribose and D-xylose). 13 However, their synthesis was challenging, with extensive purification needed for ROP. Herein, our hypothesis has been that the copolymerisation of lactide with a cyclic xanthate would allow the incorporation of UV-sensitive linkages into PLA. We report the synthesis of a novel cyclic xanthate monomer derived from D-glucal, its ring-opening polymerisation (ROP) and copolymerisation with L-lactide, which results in unprecedented UV-degradable poly(lactic acid).
To circumvent the difficulties met previously, we selected tri-Oacetyl-D-glucal as starting material, a commercial derivative of C6 sugar D-glucose, previously used for polycarbonate synthesis by ROP. One attractive feature of glucal-derived monomers is the incorporation in the polymer backbone of a cyclic alkene, primed for postpolymerisation modifications. Building on the strategy developed by Wooley and coworkers, 14 the synthesis of the desired cyclic xanthate involved four high-yielding synthetic steps (59% yield) from tri-Oacetyl-D-glucal: Ferrier rearrangement, deprotection under Zemplén conditions, tosylation at the C-6 position, then cyclisation using CS 2 (Scheme 1).
After recrystallisation in ethanol, cyclic xanthate 1, was isolated as pale-yellow crystals, and was characterized by multinuclear NMR and FT-IR spectroscopies. Unambiguously, 13 C{ 1 H} NMR analysis identified a signal at d C E 208 ppm, characteristic of the C(S)SO linkage of cyclic xanthates. 15 Molecular structure and stereochemistry were further corroborated by single crystal X-ray diffraction crystallography (Fig. S23, ESI †).
As expected from an equilibrium-limited ROP process, initial monomer concentration could be manipulated to reach higher monomer conversion, and 90% conversion was achieved for [1] 0 = 2.0 mol L À1 (Table 1, entry 8). Interestingly for polymer chemical recyclability, a near quantitative recovery of the monomer is observed when poly(1) is stirred in the presence of TBD in dilute conditions (0.1 mol L À1 ). The temperature dependence of the ROP equilibrium was also investigated in 1,2-dichloroethane (DCE) at fixed initial monomer concentration, catalyst and initiator loadings. Decreasing temperature from 60 1C to 0 1C resulted in an increase in monomer conversion from 2% up to 85%. Below 0 1C 1 was no longer soluble resulting in no reaction. A plot of ln([1] eq ) vs. 1/T gave an approximation of the polymerisation thermodynamic parameters (Fig. S46, ESI †): DH p = À25.6 kJ mol À1 and DS p = À80 J mol À1 . A similar study using THF instead of DCE also gave an estimation of these parameters in THF (Fig. S47, ESI †; DH p = À22.0 kJ mol À1 and DS p = À61 J mol À1 K À1 ; revealing a moderate increase in the enthalpy of polymerisation, but with little impact on polymerisability. Upon reaction completion, the polymer products could be isolated by precipitation in diethyl ether, and SEC was utilised to estimate number-average molar masses (M n ) and dispersities (Ð M ). Discrepancy between expected molar masses and those calculated by SEC was found (Table 1), in addition to limitations in obtaining high molar mass polymers, which both may be due to impurities in the monomer acting as chaintransfer agents. Nevertheless, polymers of up to 10 700 g mol À1 (Ð M 1.76) could be obtained, some control of the polymerisation was indicated by the linear relationship between monomer conversion and M n (Fig. S45, ESI †) and first-order kinetics with respect to monomer concentration ( Fig. S40-S44, ESI †). 13 C{ 1 H} NMR spectroscopy was primarily used to investigate the microstructure of poly(1) (Fig. 1). Two major thiocarbonyl resonances at d C E 194 and 223 ppm, of similar intensity, could be seen and were assigned to a mono-thionocarbonate (C(S)O 2 ) environment and a tri-thiocarbonate environment (C(S)S 2 ), respectively. 12,13 Only a small amount of xanthate linkages (SC(S)O; 1-2%) could be identified from a minor resonance at d C E 214 ppm. 19 1 H NMR spectra also featured a CH-O (d H = 5.68-5.75) and a shielded CH 2 -S (d H = 3.68-3.58) proton environments (Fig. S24, ESI †). Collectively, this data suggests the ROP of 1 proceeds via the alternating opening of 1  on either side of the xanthate thiocarbonyl, and subsequent selective chain propagation, yielding regioregular polymers with alternating C(S)S 2 and C(S)O 2 linkages, similar to what we observed for a xylose-based cyclic xanthate. While MALDI-ToF mass spectrometry proved unsuccessful to probe further the macromolecular structure of poly(1), a good agreement between M n,NMR (estimated by end-group analysis) and M n,SEC values suggested the absence of major cyclic polymer species. When the ROP of 1 was carried out above 50 1C, especially in THF, a vibration at 1709 cm À1 in the FTIR spectra, characteristic of a carbon-oxygen double bond (CQO) vibrational mode (Fig. S52, ESI †), emerged. Two new carbonyl resonances could also be observed by 13 C{ 1 H} NMR spectroscopy, at 169 and 154 ppm, assigned to mono-thiocarbonate (C(O)SO) and carbonate (C(O)O 2 ) environments, respectively (Fig. S48, ESI †). This was indicative of some S-O exchange reactions, which in situ IR spectroscopy showed to occur after maximum monomer conversion is reached.
Copolymerisation of 1 with L-lactide (L-LA) was first investigated through sequential monomer addition. 25  One-pot copolymerisation of 1 with L-LA was also performed ( Table 2). L-LA was quantitatively converted while as before, 1 reached a conversion plateau between 71-78%. 1 H and DOSY NMR analysis revealed the formation of a copolymer and its composition, which diverged from the monomers feed ratio as the proportion of 1 increased. In the carbonyl region of the 13 C{ 1 H} NMR spectra, alongside a major PLA ester signal at d C E 169 ppm, all copolymers exhibited multiple thiocarbonyl resonances at d C E 194 ppm, consistent with thionocarbonate (C(S)O 2 ) and thioester (C(O)S) environments (Fig. 2). As no xanthate or trithiocarbonate signals were observed, the probability of consecutively opening two or more molecules of 1 was deemed low. We propose that the incorporation of 1 in the copolymer mainly proceeds via the attack of 1 by a growing PLA chain and the cleavage of its C-S bond, producing a primary thiol which next opens a LA molecule. Copolymers with consecutive thionocarbonate and thioester linkages inserted within a polyester backbone are thus formed (poly(1-co-LLA)).
Building upon previous studies surrounding photodegradation, 20,21 as well as our experience with similar poly(thiocarbonates), poly(1) could be readily degraded under UV radiation (l = 365 nm) in THF, with or without tris-(trimethylsilyl)silane (TTMSS) to accelerate the reaction ( Fig. S85 and S86, ESI †). 12,22 Analysis of the degradation products by 1    spectroscopy proved difficult, although the absence of the H-6' proton signals suggested the complete breakdown of the trithiocarbonate linkages. The degradation of statistical copolymers poly(1-co-LLA)s under UV irradiation in THF, in the absence of TTMSS, was also investigated (Fig. 3, polymers from entries 2 and 4, Table 2). Satisfyingly, a gradual decrease in molar mass was observed, which correlated well with the quantity of thionocarbonate/thioester linkages in the polymer. Analysis of the degradation products by 1 H NMR spectroscopy revealed a breakdown of the thionocarbonate/thioester linkages leaving predominantly PLLA species ( Fig. S87 and S88, ESI †). Under identical conditions, no change was observed for PLLA, supporting the hypothesis that degradation is due to sulfur-containing linkages. UV-Vis spectra of the polymers (Fig. S82 and S83, ESI †) displayed a singular absorbance within the ultraviolet region (l max E 306 nm) confirming that UV irradiation is the stimulus promoting photodegradation. Photodegradation of PLA has been relatively unexplored, and often requires the incorporation of light responsive nanoparticles or the use of a nucleation agent, as well as extended periods of time (days/weeks) to reach significant levels of molar mass reduction (420%). [23][24][25] Here, over 40% mass loss within 6 hours of UV exposure was seen, including for copolymers with as little as 3% thionocarbonate/ thioester linkages.

H NMR
In summary, we have synthesised a novel, bio-derived, unsaturated xanthate cyclic monomer, reported its ring-opening polymerisation and characterised the resulting poly(thiocarbonate)s. The sugar-based monomer was also copolymerised with L-lactide towards block and statistical copolymers. Even at low content, introducing UV-cleavable sulfur-containing linkages into PLA enabled its effective photodegradation into biodegradable oligomeric species. The mechanism of degradation requires to be investigated in detail, and this strategy remains to be translated to real-life plastics objects, but it has the potential to increase the environmental degradability of PLA without significantly affecting its material properties.

Conflicts of interest
There are no conflicts of interest to declare.