Unprecedented Biodegradable Cellulose-Derived Polyesters with Pendant Citronellol Moieties: From Monomer Synthesis to Enzymatic Degradation

Abstract

Levoglucosenone (LGO) is a cellulose-derived molecule that is present commercially on a multi-ton/year scale. Taking advantage of the α,β-conjugated ketone of LGO, a new citronellol-containing 5-membered lactone (HBO-citro) was synthesized through a one-pot two-step pathway involving oxa-Michael addition and Baeyer-Villiger oxidation. The solvent-free treatment of HBO-citro with NaBH4 at room temperature led to the full reduction of the lactone moiety which gave a novel fully renewable triol monomer having a citronellol side chain (Triol-citro). Noticeably, by simply changing the reducing agent, temperature and reaction duration, the partial reduction of HBO-citro can be achieved to yield a mixture of 5- and 6-membered Lactol-citro molecules. Triol-citro was chosen to prepare functional renewable polyesters having citronellol pendant chains via polycondensation reactions with diacyl chlorides having different chain lengths. Good thermal stability (Td5% up to 170 °C) and low glass transition temperatures (as low as −42 °C) were registered for the polyesters obtained. The polymers were then hydrolyzed using a commercial lipase from Thermomyces lanuginosus (Lipopan® 50 BG) to assess their biodegradability. A higher degradation profile was found for the polyesters prepared using co-monomers (acyl chlorides) having longer chain lengths. This is likely due to the decreased steric hindrance around the ester bonds which allowed enhanced accessibility of the enzyme.

Synthesis and Enzymatic Degradation of Sustainable Levoglucosenone-Derived Copolyesters with Renewable Citronellol Side Chains

“…The synthesis of 5- and 6-membered Lactol-citro (5ML and 6ML, respectively) was performed from HBO-citro according to a recently reported procedure [ 34 ]. It is worth mentioning that although the reduction of HBO-ctiro is easy to perform, reaction conditions must be strictly controlled to avoid the formation of Triol-citro ( Scheme 1 ).…”

“…Given our strong expertise in levoglucosenone (LGO), a wood-based functional molecule produced at the scale of several tons per year [ 28 , 29 , 30 ], we have recently become interested in developing not only renewable functional monomers and polymers from LGO [ 31 , 32 , 33 , 34 , 35 , 36 ] but also biodegradation protocols to evaluate the end-of-life of our in-house corresponding materials. In this context, we recently reported the first fully renewable citronellol-containing monomers from LGO [ 34 , 35 ]. More precisely, we took advantage of the α,β-conjugated double bond of LGO to perform the acid-catalyzed [ 37 ] oxa-Michael addition of citronellol ( Scheme 1 ).…”

“…Notably, when NaBH 4 was used as a reducing agent, it led to the formation of the tris-hydroxy monomer, Triol-citro. However, when diisobutylaluminum hydride (DIBAL-H) was adopted, a mixture of 5- and 6-membered cyclic forms of Lactol-citro (5ML and 6ML) were obtained ( Scheme 1 ) [ 34 ]. Indeed, the ring-chain tautomerism of furanoses (5-membered rings) and pyranoses (6-membered rings) is very common in sugars [ 38 ].…”

“…These furanose-pyranose interconversions exist in equilibrium which led, in our case, to a mixture of Lactol-citro molecules. In our previous work [ 34 ], Triol-citro was selected to prepare renewable polyesters (P1–P4) via its polycondensation reaction with diacyl chlorides having different chain lengths (m = 1–4, respectively). When P1–P4 were exposed to Lipopan ® 50 BG, a commercial lipase from Thermomyces lanuginosus , an 80% degradation of P2–P4 was observed after 80 h [ 34 ].…”

“…In our previous work [ 34 ], Triol-citro was selected to prepare renewable polyesters (P1–P4) via its polycondensation reaction with diacyl chlorides having different chain lengths (m = 1–4, respectively). When P1–P4 were exposed to Lipopan ® 50 BG, a commercial lipase from Thermomyces lanuginosus , an 80% degradation of P2–P4 was observed after 80 h [ 34 ]. Inspired by this work, we decided herein not only to prepare new fully renewable polyesters (P5–P8) that contain two types of lactol rings (5-membered and 6-membered) but also to study and compare the biodegradation of the newly synthesized lactol-based copolyesters (P5–P8) to that of the previously reported triol-based polymers (P1–P4).…”