Copolymerization of ß-Butyrolactones into Functionalized Polyhydroxyalkanoates Using Aluminum Catalysts: Influence of the Initiator in the Ring-Opening Polymerization Mechanism.

Within bioplastics, natural poly(3-hydroxybutyrate) (PHB) stands out as fully biocompatible and biodegradable, even in marine environments; however, its high isotacticity and crystallinity limits its mechanical properties and hence its applications. PHB can also be synthesized with different tacticities via a catalytic ring-opening polymerization (ROP) of rac-ß-butyrolactone (BBL), paving the way to PHB with better thermomechanical and processability properties. In this work, the catalyst family is extended based on aluminum phenoxy-imine methyl catalyst [AlMeL2], that reveals efficient in the ROP of BBL, to the halogeno analogous complex [AlClL2]. As well, the impact on the ROP mechanism of different initiators is further explored with a particular focus in dimethylaminopyridine (DMAP), a hardly studied initiator for the ROP of BBL. A thorough mechanistic study is performed that evidences the presence of two concomitant DMAP-mediated mechanisms, that lead to either a DMAP or a crotonate end-capping group. Besides, in order to increase the possibilities of PHB post-polymerization functionalization, the introduction of a side-chain functionality is explored, establishing the copolymerization of BBL with ß-allyloxymethylene propiolactone (BPLOAll), resulting in well-defined P(BBL-co-BPLOAll) copolymers.

Chemistry that allows plastic recycling.

Enhancing thermal stability of polyhydroxyalkanoates for their closed-loop chemistry.

Polythioesters Prepared by Ring-Opening Polymerization of Cyclic Thioesters and Related Monomers.

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyesters with a wide range of applications; in particular, they currently stand as promising alternatives to conventional polyolefin-based "plastics". The introduction of sulfur atoms within the PHAs backbone can endow the resulting polythioesters (PTEs) with differentiated, sometimes enhanced thermal, optical and mechanical properties, thereby widening their versatility and use. Hence, PTEs have been gaining increasing attention over the past half-decade. This review highlights recent advances towards the synthesis of well-defined PTEs by ring-opening polymerization (ROP) of cyclic thioesters - namely thiolactones - as well as of S-carboxyanhydrides and thionolactones; it also covers the ring-opening copolymerization (ROCOP) of cyclic thioanhydrides or thiolactones with epoxides or episulfides. Most of the ROP reactions described are of anionic type, mediated by inorganic, organic or organometallic initiators/catalysts, along with a few enzymatic reactions as well. Emphasis is placed on the reactivity of the thio monomers, in relation to their ring-size ranging from 4- to 5-, 6- and 7-membered cycles, the nature of the catalyst/initiating systems implemented and their efficiency in terms of activity and control over the PTE molar mass, dispersity, topology, and microstructure.

Tacticity Control of Cyclic Poly(3-Thiobutyrate) Prepared by Ring-Opening Polymerization of Racemic ß-Thiobutyrolactone.

We report here on the ring-opening polymerization (ROP) of racemic ß-thiobutyrolactone (rac-TBL), as the first chemical synthesis of poly(3-thiobutyrolactone) (P3TB), the thioester analogue of the ubiquitous poly(3-hydroxybutyrate) (P3HB). The ROP reactions proceed very fast (TOF >12 000 h-1 at r.t.) in the presence of various metal-based catalysts. Remarkably, catalyst systems based on non-chiral yttrium complexes stabilized by tetradentate amino alkoxy- or diamino-bis(phenolate) ligands {ONXOR1,R2 }2- (X=O, N) provide access to cyclic P3TB with either high isoselectivity (Pm up to 0.90) or high syndiotactic bias (Pr up to 0.70). The stereoselectivity can be controlled by manipulation of the substituents on the ligand platform and adequate choice of the reaction solvent and temperature as well. The cyclic polymer topology is evidenced by MALDI-ToF MS, NMR and TGA. Highly isotactic cyclic P3TB is a semi-crystalline material as revealed by DSC.

Recent Advances in Metal-Mediated Stereoselective Ring-Opening Polymerization of Functional Cyclic Esters towards Well-Defined Poly(hydroxy acid)s: From Stereoselectivity to Sequence-Control.

Poly(hydroxy acid)s are a family of biocompatible and (bio)degradable polyesters with various outcomes in different domains of application. To date, poly(hydroxy acid)s are best prepared by ring-opening polymerization (ROP) of the corresponding cyclic esters. Using racemic chiral monomers featuring side-chain groups enables to access, providing a stereoselective catalyst/initiator system is implemented, stereoregular functional polymers, thereby improving their physico-chemical properties, and ultimately, widening their range of uses. Here, we highlight a few important advances in metal-mediated stereoselective ROP of cyclic esters towards the synthesis of (functional) stereoregular poly(hydroxy acid)s that have recently been disclosed, emphasizing on (functional) ß- and γ-lactones, diolide and O-carboxyanhydride (OCA) monomers and yttrium-based catalysis. Fine-tuning of the substituents flanked on the catalyst ligand enables reaching poly(hydroxy acid)s with syndiotactic and also isotactic microstructures. The stereocontrol mechanisms at work and their probable origin, relying on steric but also electronic factors imparted in particular by the ligand substituents, are discussed. Taking advantage of such stereoselective ROPs, original copoly(hydroxy acid)s with gradient or alternated patterns then become accessible from the use of mixtures of chemically different, oppositely configured enantiopure monomers.

Yttrium-Mediated Ring-Opening Copolymerization of Oppositely Configurated 4-Alkoxymethylene-ß-Propiolactones: Effective Access to Highly Alternated Isotactic Functional PHAs.

The ring-opening copolymerization (ROCOP) of functional 4-alkoxymethylene-ß-propiolactones (BPLOR s) by yttrium-bisphenolate complexes was investigated. The ROCOP of equimolar mixtures of BPLOR s of opposite configurations, namely (R)-BPLOR1 /(S)-BPLOR2 [R1 , R2 =OMe, OAllyl, OCH2 Ph (=OBn), OSiMe2 tBu (=OTBDMS)], by the syndioselective Y{ONOOcum }/iPrOH catalyst/initiator system affords P(HBOR1 -alt-HBOR2 ) copolymers with high alternation degrees (altern.=89-94 %), as determined by comprehensive kinetic, 13 C{1 H} NMR spectroscopy, MALDI-ToF MS and ESI MS/MS fragmentation studies. The ROCOP of the (R)-BPLOMe /(S)-BPLOTBDMS combination, featuring a large difference in the substituents' bulkiness, constitutes the only observed exception to this trend (altern.=64 %). On the other hand, the isoselectivity of the Y{ONNOCl }/iPrOH catalyst/initiator system has been exploited to generate, in a one-pot/one-step procedure, original mixtures of isotactic poly(hydroxyalkanoate)s (PHAs). This system efficiently transforms equimolar mixtures of (R)-BPLOAll /(S)-BPLOMe into a 1:1 mixture of the corresponding isotactic iso-(R)-PHBOAll and iso-(S)-PHBOMe homopolymers; almost no copolymerization defects are observed. This new approach has been extended successfully to the ROCOP of equimolar mixtures of racemic monomers, rac-BPLOAll /rac-BPLOMe .

Steric vs. electronic stereocontrol in syndio- or iso-selective ROP of functional chiral ß-lactones mediated by achiral yttrium-bisphenolate complexes.

Origins of stereoselectivity in ROP of racemic chiral cyclic esters promoted by achiral yttrium complexes, resulting in the formation of highly heterotactic polylactide, and highly syndiotactic or, more uniquely, highly isotactic poly(3-hydroxybutyrate)s, are discussed. A close interplay between the nature of the cyclic ester, most particularly of the exocyclic functional chain installed on the chiral center of ß-lactones, and the ortho-substituents installed on the phenolate rings of the ligand, results in various determining secondary interactions of steric and also electronic nature.

α,ω-Epoxide, Oxetane, and Dithiocarbonate Telechelic Copolyolefins: Access by Ring-Opening Metathesis/Cross-Metathesis Polymerization (ROMP/CM) of Cycloolefins in the Presence of Functional Symmetric Chain-Transfer Agents.

Epoxide- and oxetane-α,ω-telechelic (co)polyolefins have been successfully synthesized by the tandem ring-opening metathesis polymerization (ROMP)/cross-metathesis (CM) of cyclic olefins using Grubbs' second-generation catalyst (G2) in the presence of a bifunctional symmetric alkene epoxide- or oxetane-functionalized chain-transfer agent (CTA). From cyclooctene (COE), trans,trans,cis-1,5,9-cyclododecatriene (CDT), norbornene (NB), and methyl 5-norbornene-2-carboxylate (NBCOOMe), with bis(oxiran-2-ylmethyl) maleate (CTA 1), bis(oxetane-2-ylmethyl) maleate (CTA 2), or bis(oxetane-2-ylmethyl) (E)-hex-3-enedioate (CTA 3), well-defined α,ω-di(epoxide or oxetane) telechelic PCOEs, P(COE-co-NB or -NBCOOMe)s, and P(NB-co-CDT)s were isolated under mild operating conditions (40 or 60 °C, 24 h). The oxetane CTA 3 and the epoxide CTA 1 were revealed to be significantly more efficient in the CM step than CTA 2, which apparently inhibits the reaction. Quantitative dithiocarbonatation (CS2/LiBr, 40 °C, THF) of an α,ω-di(epoxide) telechelic P(NB-co-CDT) afforded a convenient approach to the analogous α,ω-bis(dithiocarbonate) telechelic P(NB-co-CDT). The nature of the end-capping function of the epoxide/oxetane/dithiocarbonate telechelic P(NB-co-CDT)s did not impact their thermal signature, as measured by DSC. These copolymers also displayed a low viscosity liquid-like behavior and a shear thinning rheological behavior.

Evaluation of Band-Selective HSQC and HMBC: Methodological Validation on the Cyclosporin Cyclic Peptide and Application for Poly(3-hydroxyalkanoate)s Stereoregularity Determination.

Band-selective (bs) HSQC, improving spectral resolution by restriction of the heteronuclear dimension without inducing spectral folding, has been recently used for polymer tacticity determination. Herein is reported an evaluation of various bs-HSQC and bs-HMBC sequences, first from a methodological point of view (selectivity, dependence to INEPT interpulse delay or relaxation delay), using the cyclic peptide cyclosporin selected as a model compound, and then from an applicative approach, comparing tacticity determined from bs-HSQC and bs-HMBC experiments to the one obtained from 1D 13C{¹H} on poly(3-hydroxyalkanoate)s samples. For HSQC sequences, the 13C selectivity scheme consisting in substituting a 13C broadband refocalization by a selective one revealed itself problematic, with unwanted aliased signals, whereas the insertion of double pulsed field gradients spin-echo (DPFGSE) or the use of opposite sign gradients bracketing a selective refocalization gave satisfactory results. Determination of the probability of syndiotactic enchainments, Ps, by bs-HSQC is fully consistent and no precision loss was observed when decreasing acquisition time (37 min vs. 106 min for 1D 13C{¹H}). Bs-HMBC, although not straightforwardly applicable for tacticity determination, could provide (after a calibration step) an alternative for compounds of which only 13C carbonyl signals are resolved enough for discriminating between syndiotactic and isotactic configurations.

Highly Syndiotactic or Isotactic Polyhydroxyalkanoates by Ligand-Controlled Yttrium-Catalyzed Stereoselective Ring-Opening Polymerization of Functional Racemic ß-Lactones.

Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional ß-lactones, namely 4-alkoxymethylene-ß-propiolactones (BPLOR s). This process is catalyzed by an yttrium complex stabilized by a nonchiral tetradentate amino alkoxy bisphenolate ligand {ONOOR'2 }2- , which features both a good activity and a high degree of control over the molar masses of the resulting functional poly(3-hydroxyalkanoate)s. A simple modification of the R' substituents in ortho and para position on the ligand platform allows for a complete reversal from virtually pure syndioselectivity (Ps up to 0.91 with R'=cumyl) to very high isoselectivity (Pi up to 0.93 with R'=Cl), as supported by DFT insights. This is the first example of a highly isoselective ROP of a racemic chiral ß-lactone.

Opsonisation of nanoparticles prepared from poly(ß-hydroxybutyrate) and poly(trimethylene carbonate)-b-poly(malic acid) amphiphilic diblock copolymers: Impact on the in vitro cell uptake by primary human macrophages and HepaRG hepatoma cells.

The present work reports the investigation of the biocompatibility, opsonisation and cell uptake by human primary macrophages and HepaRG cells of nanoparticles (NPs) formulated from poly(ß-malic acid)-b-poly(ß-hydroxybutyrate) (PMLA-b-PHB) and poly(ß-malic acid)-b-poly(trimethylene carbonate) (PMLA-b-PTMC) diblock copolymers, namely PMLA800-b-PHB7300, PMLA4500-b-PHB4400, PMLA2500-b-PTMC2800 and PMLA4300-b-PTMC1400. NPs derived from PMLA-b-PHB and PMLA-b-PTMC do not trigger lactate dehydrogenase release and do not activate the secretion of pro-inflammatory cytokines demonstrating the excellent biocompatibility of these copolymers derived nano-objects. Using a protein adsorption assay, we demonstrate that the binding of plasma proteins is very low for PMLA-b-PHB-based nano-objects, and higher for those prepared from PMLA-b-PTMC copolymers. Moreover, a more efficient uptake by macrophages and HepaRG cells is observed for NPs formulated from PMLA-b-PHB copolymers compared to that of PMLA-b-PTMC-based NPs. Interestingly, the uptake in HepaRG cells of NPs formulated from PMLA800-b-PHB7300 is much higher than that of NPs based on PMLA4500-b-PHB4400. In addition, the cell internalization of PMLA800-b-PHB7300 based-NPs, probably through endocytosis, is strongly increased by serum pre-coating in HepaRG cells but not in macrophages. Together, these data strongly suggest that the binding of a specific subset of plasmatic proteins onto the PMLA800-b-PHB7300-based NPs favors the HepaRG cell uptake while reducing that of macrophages.

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.

Highly Stereocontrolled Ring-Opening Polymerization of Racemic Alkyl ß-Malolactonates Mediated by Yttrium [Amino-alkoxy-bis(phenolate)] Complexes.

Yttrium [amino-alkoxy-bis(phenolate)]amido complexes have been used for the ring-opening polymerization (ROP) of racemic alkyl ß-malolactonates (4-alkoxycarbonyl-2-oxetanones, rac-MLA(R) s) bearing an allyl (All), benzyl (Bz) or methyl (Me) lateral ester function. The nature of the ortho-substituent on the phenolate rings in the metal ancillary dictated the stereocontrol of the ROP, and consequently the syndiotactic enrichment of the resulting polyesters. ROP promoted by catalysts with halogen (Cl, Br)-disubstituted ligands allowed the first reported synthesis of highly syndiotactic PMLA(R) s (Pr ≥ 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho-substituted ligands proved largely ineffective. All polymers have been characterized by (1) H and (13) C{(1) H} NMR spectroscopy, MALDI-ToF mass spectrometry and DSC analyses. Statistical and thermal analyses enabled the rationalization of the chain-end control mechanism. Whereas the stereocontrol of the polymerization obeyed a Markov first-order (Mk1) model for the ROP of rac-MLA(Bz) and rac-MLA(All) , the ROP of rac-MLA(Me) led to a chain end-control of Markov second-order type (Mk2). DFT computations suggest that the high stereocontrol ability featured by catalysts bearing Cl- and Br-substituted ligands does not likely originate from halogen bonding between the halogen substituent and the growing polyester chain.

Poly(trimethylene carbonate)/Poly(malic acid) Amphiphilic Diblock Copolymers as Biocompatible Nanoparticles.

Amphiphilic polycarbonate-poly(hydroxyalkanoate) diblock copolymers, namely, poly(trimethylene carbonate) (PTMC)-b-poly(ß-malic acid) (PMLA), are reported for the first time. The synthetic strategy relies on commercially available catalysts and initiator. The controlled ring-opening polymerization (ROP) of trimethylene carbonate (TMC) catalyzed by the organic guanidine base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), associated with iPrOH as an initiator, provided iPrO-PTMC-OH, which served as a macroinitiator in the controlled ROP of benzyl ß-malolactonate (MLABe) catalyzed by the neodymium triflate salt (Nd(OTf)3). The resulting hydrophobic iPrO-PTMC-b-PMLABe-OH copolymers were then hydrogenolyzed into the parent iPrO-PTMC-b-PMLA-OH copolymers. A range of well-defined copolymers, featuring different sizes of segments (Mn,NMR up to 9300 g mol(-1) ; ÐM =1.28-1.40), were thus isolated in gram quantities, as evidenced by NMR spectroscopy, size exclusion chromatography, thermogravimetric analysis, differential scanning calorimetry, and contact angle analyses. Subsequently, PTMC-b-PMLA copolymers with different hydrophilic weight fractions (11-75 %) self-assembled in phosphate-buffered saline upon nanoprecipitation into well-defined nano-objects with Dh =61-176 nm, a polydispersity index <0.25, and a negative surface charge, as characterized by dynamic light scattering and zeta-potential analyses. In addition, these nanoparticles demonstrated no significant effect on cell viability at low concentrations, and a very low cytotoxicity at high concentrations only for PTMC-b-PMLA copolymers exhibiting hydrophilic fractions over 47 %, thus illustrating the potential of these copolymers as promising nanoparticles.

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.

(Iminophosphoranyl)(thiophosphoranyl)methane rare-earth borohydride complexes: synthesis, structures and polymerization catalysis.

The (iminophosphoranyl)(thiophosphoranyl)methanide {CH(PPh2=NSiMe3)(PPh2=S)}(-) ligand has been used for the synthesis of divalent and trivalent rare-earth borohydride complexes. The salt metathesis of the potassium reagent [K{CH(PPh2=NSiMe3)(PPh2=S)}]2 with [Yb(BH4)2(THF)2] resulted in the divalent monoborohydride ytterbium complex [{CH(PPh2=NSiMe3)(PPh2=S)}Yb(BH4)(THF)2]. The 2D (31)P/(171)Yb HMQC-NMR spectrum clearly showed the coupling between both nuclei. The trivalent bisborohydrides [{CH(PPh2=NSiMe3)(PPh2=S)}Ln(BH4)2(THF)] (Ln = Y, Sm, Tb, Dy, Er, Yb and Lu) were obtained by reaction of [K{CH(PPh2=NSiMe3)(PPh2=S)}]2 with [Ln(BH4)3(THF)3]. All new compounds were characterized by single X-ray diffraction. The divalent and trivalent compounds were next used as initiators in the ring-opening polymerization (ROP) of ε-caprolactone (CL) and trimethylene carbonate (TMC). All complexes afforded a generally well-controlled ROP of both of these cyclic esters. High molar mass poly(ε-caprolactone) diols (Mn,NMR < 101,300 g mol(-1), DM = 1.44), and α,ω-dihydroxy and α-hydroxy,ω-formate telechelic poly(trimethylene carbonate)s (Mn,NMR < 20,000 g mol(-1), DM = 1.61) were thus synthesized under mild operating conditions.

PMLABe diol synthesized by ring-opening polymerization of racemic benzyl ß-malolactonate initiated by rare-earth trisborohydride complexes: an experimental and DFT study.

Polymer diols are a class of polymeric building blocks of high interest for the synthesis of complex macromolecular edifices. Rare-earth borohydride complexes are known as efficient initiators for the ring-opening polymerization (ROP) of cyclic esters, directly affording α,ω-dihydroxy-telechelic polyesters. Here, were report the direct synthesis of poly(benzyl ß-malolactonate) (PMLABe) diols, from the ROP of racemic (benzyl ß-malolactonate) (rac-MLABe), a valuable and renewable monomer, initiated by the homoleptic [Ln(BH4 )3 (thf)3 ] (Ln=La, Nd, and Sm) complexes. These initiators enabled the controlled ROP of this ß-lactone, affording well-defined syndiotactic-enriched (Pr ≈0.83) PMLABes (Mn up to 21 300 g mol(-1) , ÐM ≈1.5) as evidenced by size exclusion chromatography, (1) H and (13) C NMR spectroscopy, and MALDI-ToF mass spectrometry analyses. The first and second insertions of rac-MLABe, as assessed by DFT calculations, revealed more favorable stationary front-side than migratory back-side insertions, the thermodynamically and kinetically competitive ROP on two distinct arms with that on a one arm-only, and the thermodynamically slightly favored formation of syndiotactic-enriched PMLABes.

From syndiotactic homopolymers to chemically tunable alternating copolymers: highly active yttrium complexes for stereoselective ring-opening polymerization of ß-malolactonates.

Alternating copolymers constitute an attractive class of materials. It was shown previously that highly alternated poly(ß-hydroxyalkanoate)s (PHAs) can be prepared by ring-opening polymerization (ROP) of mixtures of two different enantiomerically pure 4-alkyl-ß-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 ß-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.

Organometallic calcium and strontium borohydrides as initiators for the polymerization of ε-caprolactone and L-lactide: combined experimental and computational investigations.

[Ca(BH4)2(THF)2] (1a), a known compound, was easily prepared following a convenient new procedure from [Ca(OMe)2] and BH3·THF in THF. Reaction of 1a with KCp* (Cp* = (η(5)-C5Me5)) and K{(Me3SiNPPh2)2CH} in a 1 : 1 ratio in THF resulted in the corresponding dimeric heteroleptic mono-borohydride derivatives [Cp*Ca(BH4)(THF)n]2 (2a) and [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2] (3a), respectively. Both compounds were fully characterized and the solid-state structure of 3a was established by single crystal X-ray diffraction. Compounds 1a, 2a, and 3a, together with the earlier reported compounds [Sr(BH4)2(THF)2] (1b), [Cp*Sr(BH4)(THF)2]2 (2b), and [{(Me3SiNPPh2)2CH}Sr(BH4)(THF)2] (3b), were used as initiators for the ROP of polar monomers. The general performances of the complexes in the ROP of ε-caprolactone and l-lactide demonstrate a relatively good control of the polymerization under the operating conditions established. α,ω-Dihydroxytelechelic poly(ε-caprolactone)s (PCLs) and poly(lactide)s (PLAs) were thus synthesized. DFT calculations on the initiation step of the ROP of ε-CL were carried out. Gibbs free energy profiles were determined for the three calcium complexes highlighting slightly more active calcium complexes as compared to strontium analogues, in agreement with experimental findings.