Sophie M. Guillaume

Beyond Stereoselectivity, Switchable Catalysis: Some of the Last Frontier Challenges in Ring-Opening Polymerization of Cyclic Esters

Metal-based catalysts and initiators have played a pivotal role in the ring-opening polymerization (ROP) of cyclic esters, thanks to their high activity and remarkable ability to control precisely the architectures of the resulting polyesters in terms of molar mass, dispersity, microstructure, or tacticity. Today, after two decades of extensive research, the field is slowly reaching maturity. However, several challenges remain, while original concepts have emerged around new types or new applications of catalysis. This Review is not intended to comprehensively cover all of these aspects. Rather, it provides a personal overview of the very recent progress achieved in some selected, important aspects of ROP catalysis--stereocontrol and switchable catalysis. Hence, the first part addresses the development of new metal-based catalysts for the isoselective ROP of racemic lactide towards stereoblock copolymers, and the use of syndioselective ROP metal catalysts to control the monomer sequence in copolymers. A second part covers the development of ROP catalysts--primarily metal-based catalysts, but also organocatalysts--that can be externally regulated by the use of chemical or photo stimuli to switch them between two states with different catalytic abilities. Current challenges and opportunities are highlighted.

Poly(carbonate-urethane): an isocyanate-free procedure from α,ω-di(cyclic carbonate) telechelic poly(trimethylene carbonate)s

A simple isocyanate-free method to synthesize poly(trimethylene carbonate hydroxy-urethane)s is described. The strategy first involves the synthesis of α,ω-di(cyclic carbonate) telechelic polycarbonate precursors upon ring-opening polymerization of trimethylene carbonate using a cyclic carbonate alcohol as chain transfer agent, followed by the ring-opening polyaddition of the terminal cyclic carbonate with a diamine. Such poly(carbonate-hydroxyurethane)s exhibit macromolecular carbonate segments of tunable length/molar mass.

α,ω-Di(glycerol carbonate) telechelic polyesters and polyolefins as precursors to polyhydroxyurethanes: an isocyanate-free approach

α,ω-Di(glycerol carbonate) telechelic poly(propylene glycol) (PPG), poly(ethylene glycol) (PEG), poly(ester ether) (PEE), and poly(butadiene) (PBD) have been synthesized through chemical modification of the corresponding α,ω-dihydroxy telechelic polymers (PPG-OH2, PEG-OH2, PEE-OH2 and PBD-OH2, respectively). Tosylation of the polymer diols with 4-tosylmethyl-1,3-dioxolan-2-one (GC-OTs) afforded, in high yields, the desired PPG, PEG, PEE and PBD end-capped at both termini with a five-membered ring cyclic glycerol carbonate (4-hydroxymethyl-1,3-dioxolan-2-one, GC). The GC-functionalization of the polymers at both chain-ends has been confirmed by NMR (1H, 13C, 1D and 2D) and FTIR spectroscopies. Using PPG-GC2 to demonstrate the concept, the corresponding polyhydroxyurethanes (PHUs/non-isocyanate polyurethanes (NIPUs)) have been subsequently prepared following a non-isocyanate method upon ring-opening catalyst-free polyaddition of the PPG-GC2 with JEFFAMINEs (Mn = 230–2000 g mol−1). The effect of various additives introduced during the polyaddition reaction has been studied at different temperatures. In particular, addition of LiBr (5 mol%) to the reaction medium was found to slightly promote the cyclocarbonate/amine reaction. The polymerization process was supported by FTIR and SEC analyses.

Recent advances in metallo/organo-catalyzed immortal ring-opening polymerization of cyclic carbonates

The ring-opening polymerization of cyclic carbonate monomers derived from biomass feedstocks constitutes an attractive way to prepare polycarbonates that are potentially useful as high-tech materials or commodity “bioplastics”. This process can be catalyzed by inherently different systems ranging from simple basic organocatalysts, simple Lewis acidic metallic salts such as triflates, or more sophisticated discrete metallo-organic complexes derived from oxophilic metals (zinc, yttrium). In this perspective, the most recent achievements in this chemistry are reviewed. The quite different performances within reach of those catalytic systems, which are assessed in terms of intrinsic reactivity, robustness and regioselectivity towards dissymmetric monomers, are highlighted.

A dual organic/organometallic approach for catalytic ring-opening polymerization.

A dual catalytic system combining an original cationic zinc complex and a tertiary amine is shown to promote efficiently the polymerization of lactide under mild conditions.

Sustained delivery of doxorubicin from thermogelling poly(PEG/PPG/PTMC urethane)s for effective eradication of cancer cells

A series of multiblock poly(ether carbonate urethane)s comprising poly(trimethylene carbonate), poly(ethylene glycol), and poly(propylene glycol) segments, with a molecular weight of 60 000 g mol−1, were synthesized. Thermogelling behaviors of the aqueous copolymer solutions were observed at gelation concentrations as low as 2 wt%. Rheological characterizations on the thermogel were carried out as a function of temperature and strain. The gels showed good recovery characteristics after being subjected to high strain. A sustained and complete doxorubicin release over 50 days can be achieved with this system. The rate of release can be tuned by changing the gel concentration or by using a copolymer with a different composition. The doxorubicin-loaded gels were effective in controlling the growth of HeLa cells when compared with doxorubicin dissolved in solution. The results demonstrated that the copolymers could be potentially used in chemotherapeutic applications.

Bis(phosphinimino)methanide Borohydride Complexes of the Rare‐Earth Elements as Initiators for the Ring‐Opening Polymerization of ε‐Caprolactone: Combined Experimental and Computational Investigations

Rare-earth-metal borohydrides are known to be efficient catalysts for the polymerization of apolar and polar monomers. The bis-borohydrides [{CH(PPh2 NSiMe3)2}La(BH4)2(THF)] and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu) have been synthesized by two different synthetic routes. The lanthanum and the lutetium complexes were prepared from [Ln(BH4)3(THF)3] and K{CH(PPh2NSiMe3)2}, whereas the yttrium analogue was obtained from in situ prepared [{CH(PPh2NSiMe3)2}YCl2]2 and NaBH4. All new compounds were characterized by standard analytical/spectroscopic techniques, and the solid-state structures were established by single-crystal X-ray diffraction. The ring-opening polymerization (ROP) of ε-caprolactone initiated by [{CH(PPh2NSiMe3)2}La(BH4)2(THF)] and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu) was studied. At 0 °C the molar mass distributions determined were the narrowest values (M(w)/M(n) = 1.06-1.11) ever obtained for the ROP of ε-caprolactone initiated by rare-earth-metal borohydride species. DFT investigations of the reaction mechanism indicate that this type of complex reacts in an unprecedented manner with the first B-H activation being achieved within two steps. This particularity has been attributed to the metallic fragment based on the natural bond order analysis.

New Linear and Star‐Shaped Thermogelling Poly([R]‐3‐hydroxybutyrate) Copolymers

The synthesis of multi-arm poly([R]-3-hydroxybutyrate) (PHB)-based triblock copolymers (poly([R]-3-hydroxybutyrate)-b-poly(N-isopropylacrylamide)-b-[[poly(methyl ether methacrylate)-g-poly(ethylene glycol)]-co-[poly(methacrylate)-g-poly(propylene glycol)]], PHB-b-PNIPAAM-b-(PPEGMEMA-co-PPPGMA), and their subsequent self-assembly into thermo-responsive hydrogels is described. Atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAAM) followed by poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) and poly(propylene glycol) methacrylate (PPGMA) was achieved from bromoesterified multi-arm PHB macroinitiators. The composition of the resulting copolymers was investigated by (1) H and (13) C J-MOD NMR spectroscopy as well as size-exclusion chromatography (SEC), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The copolymers featuring different architectures and distinct hydrophilic/hydrophobic contents were found to self-assemble into thermo-responsive gels in aqueous solution. Rheological studies indicated that the linear one-arm PHB-based copolymer tend to form a micellar solution, whereas the two- and four-arm PHB-based copolymers afforded gels with enhanced mechanical properties and solid-like behavior. These investigations are the first to correlate the gelation properties to the arm number of a PHB-based copolymer. All copolymers revealed a double thermo-responsive behavior due to the NIPAAM and PPGMA blocks, thus allowing first the copolymer self-assembly at room temperature, and then the delivery of a drug at body temperature (37 °C). The non-significant toxic response of the gels, as assessed by the cell viability of the CCD-112CoN human fibroblast cell line with different concentrations of the triblock copolymers ranging from 0.03 to 1 mg mL(-1) , suggest that these PHB-based thermo-responsive gels are promising candidate biomaterials for drug-delivery applications.

Macromolecular engineering via ring-opening polymerization (1): L-lactide/trimethylene carbonate block copolymers as thermoplastic elastomers†

Poly(L-lactide) (PLLA) is often regarded as tough and brittle while poly(1,3-trimethylene carbonate) (PTMC) is rather considered as a rubbery polymer. In an effort to improve the mechanical properties – especially ductility – of PLLA and thus to widen its field of applications, PLLA–PTMC diblock and triblock copolymers were synthesized through the sequential copolymerization of both L-lactide (L-LA) and trimethylene carbonate (TMC) using several catalytic systems. This process can be effectively catalyzed by inherently different systems ranging from a simple basic organocatalyst such as an amine (i.e., 4-N,N-dimethylaminopyridine, DMAP) or a phosphazene (i.e., 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine, BEMP), a simple Lewis acidic metallic salt such as aluminum triflate, or a more sophisticated discrete metallo-organic complex derived from a biofriendly metal, namely the β-diiminate zinc complex [(BDIiPr)Zn(N(SiMe3)2)] (BDI = CH(CMeNC6H3-2,6-iPr2)2), [(BDIiPr)Zn(N(SiMe3)2)]. Well-defined diblock PLLA-b-PTMC, triblock PLLA-b-PTMC-b-PLLA and 3-arm star GLY(PTMC-b-PLLA)3 copolymers with controlled molecular features, i.e. controlled functional end-groups and molar masses, rather narrow dispersity values, were thus prepared. The thermo-mechanical properties of the resulting copolymers revealed that a minimal block size of the PTMC and of the PLLA segments within the copolymer of Mn,PTMC = ca. 10000 g mol−1 and Mn,PLLA = ca. 23000 g mol−1 enables significant improvement of the elongation at break (eb) of PLLA up to 328%, while maintaining the Youngs modulus (E = 2781 MPa) close to that of PLLA (E = 3427 MPa).

From Syndiotactic Homopolymers to Chemically Tunable Alternating Copolymers: Highly Active Yttrium Complexes for Stereoselective Ring‐Opening Polymerization of β‐Malolactonates

Alternating copolymers constitute an attractive class of materials. It was shown previously that highly alternated poly(β-hydroxyalkanoate)s (PHAs) can be prepared by ring-opening polymerization (ROP) of mixtures of two different enantiomerically pure 4-alkyl-β-propiolactones. However, the approach could not be extended to PHAs with chemically tunable functional groups, which is highly desirable to access original advanced materials. Reported herein is the first highly syndioselective and controlled ROP of racemic allyl and benzyl β-malolactonates (MLA(R); R=allyl, benzyl) using an yttrium complex supported by a tetradentate dichloro-substituted bis(phenolate) ligand. This highly active catalyst allows the nearly perfect alternating copolymerization of MLA(Allyl) and MLA(Benzyl). Hydrogenolysis of the benzyloxycarbonyl or functionalization of the allyl pendant groups opens a route towards a new class of functional alternating copolymers.