Organocatalyzed controlled ROP of b-lactones towards poly(hydroxyalkanoate)s: from b-butyrolactone to benzyl b-malolactone polymers† C´ edric G. Jaffredo, Jean-François Carpentier * and Sophie M. Guillaume * Basic organocatalysts of the guanidine (1,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD), amidine (1,8-diazabicyclo- [5.4.0]-undec-7-ene, DBU) and phosphazene (2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2 diazaphosphorine, BEMP) type effectively polymerize benzyl b-malolactone (MLABe) under mild operating conditions. Poly(3-benzyloxybutyrate)s (PMLABe)s with controlled molecular features, i.e. controlled molar masses, narrow dispersities (1.12 < Ð M < 1.39) and well-defined end-groups, are thus formed at 60  C from bulk monomers, with M n,NMR up to 80 500 g mol 1 . The formation of a-guanidine/amidine/ phosphazene,u-benzyloxycarbonyl-crotonate telechelic polymers, [base{MLABe} n C(O)CH]CH(CO 2 Be)], was demonstrated from hydrolysis and methanolysis experiments along with detailed NMR and MALDI-ToF mass spectrometry analyses. A similar mechanism to that suggested for the ROP of b-butyrolactone (BL) is proposed. Introduction Poly(b-hydroxyalkanoate)s (PHAs) are natural aliphatic poly- esters which feature the same three-carbon backbone structure yet differing by their substituent at the b-position. Poly(3- hydroxybutyrate) (PHB), the most common member of this family was rst isolated from bacterial cells and characterized as an optically active ((R)-congurated) isotactic PHB in the early twenties. 1 Alternatively, PHB can be synthetically produced either by the alternating copolymerization of carbon monoxide and propylene oxide, 2 by direct polycondensation of b-hydroxy- butyric acid, 3a,b or by the ring-opening polymerization (ROP) of the corresponding four-membered ring b-butyrolactone (BL). 4 The ROP route, in which the relief of the ring-strain is the driving force for polymerization, remains the most attractive method (Fig. 1). The presence of chiral centers in the repeat unit of PHB arising from the methyl group in position 3 opens up the possibility to tune the conguration from one carbon to another. 4 Thus, this latter ROP approach is of topical interest since it allows – provided a suitable catalytic system is imple- mented – the preparation of PHBs with a high degree of control over molecular and microstructural characteristics. 4 Therefore, ROP offers a variety of PHAs which exhibit a range of thermal and mechanical properties larger than those of the parent natural isotactic PHB. The related benzyl b-malolactone (MLABe), which features a benzyloxycarbonyl (–CO 2 CH 2 Ph ¼ –CO 2 Be) substituent in the b-position to the carbonyl group (Fig. 1), has comparatively been much less studied than BL towards its polymerization into poly(benzyl b-malolactonate) (PMLABe), most likely due to the non-commercial availability of the monomer. 5 The benzyl group of MLABe/PMLABe constitutes a signicant opportunity as compared to BL/PHB since hydrogenolysis of the hydrophobic polymer generates carboxylic acid functions along the polyester backbone. These pendant –CO 2 H groups then provide hydro- philicity to the resulting poly(b-malic acid) (PMLA), a feature mainly exploited in the design of amphiphilic self-assembling PMLA based copolymers. 5a,6 Furthermore, such –CO 2 H func- tions can subsequently serve as anchoring sites for relevant Fig. 1 Structure of b-butyrolactone (BL), b-malolactone (MLABe) and of their corresponding polymers, poly(3-hydroxybutyrate) (PHB) and poly(3-benzyloxy- butyrate) (PMLABe), respectively. Institut des Sciences Chimiques de Rennes, Organometallics, Materials and Catalysis, UMR 6226 CNRS-Universit´ e de Rennes 1, Campus de Beaulieu, F-35042 Rennes Cedex, France. E-mail: sophie.guillaume@univ-rennes1.fr; jcarpent@univ-rennes1.fr † Electronic supplementary information (ESI) available: SEC traces of PMLABes produced from the ROP of MLABe mediated by TBD, DBU, and BEMP, kinetics of the ROP of MLABe promoted by TBD, DBU, and BEMP at 60  C, additional 1 H, 13 C and 31 P NMR, and MALDI-ToF mass spectra of PMLABes produced from DBU, TBD and BEMP, 1 H NMR spectra of the methanolysis product of PHB and of the monitoring of the TBD : BL adduct at VT, and the SEC traces of a double BL addition experiment. See DOI: 10.1039/c3py00401e Cite this: Polym. Chem., 2013, 4, 3837 Received 27th March 2013 Accepted 23rd April 2013 DOI: 10.1039/c3py00401e www.rsc.org/polymers This journal is ª The Royal Society of Chemistry 2013 Polym. Chem., 2013, 4, 3837–3850 | 3837 Polymer Chemistry PAPER