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The present study provides a state of the art on methods for evaluating active chain density of rubbers from swelling experiments. This review focuses more particularly on the effects of components that are known to limit the swelling, and which typically results in an overestimation of the active chain density. These components are commonly considered as insoluble. Different corrections dealing with the two principal insoluble components, namely zinc oxide (Zn0) and carbon black (CB) filler, are thoroughly investigated. A comparative study on the same natural rubber (NR), either filled and unfilled, vulcanized with three different vulcanization systems, is performed to evaluate the relevance of each predictive approach. The results obtained highlight their respective limitations. In particular, it is shown that the swelling ratio of unfilled and filled natural rubbers are linearly related, suggesting that fillers have no significant effect on the overall number of cross‐links formed during vulcanization. As a result, we propose a unified approach for correcting the ZnO content for unfilled NR and both the ZnO and CB contents for filled NR. By comparing with results issued from uniaxial tensile tests, it is shown that the use of a mechanical response is a relevant alternative for determining the active chain density. [ABSTRACT FROM AUTHOR]

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‐β‐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 β‐allyloxymethylene propiolactone (BPLOAll), resulting in well‐defined P(BBL‐co‐BPLOAll) copolymers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report here on the ring‐opening polymerization (ROP) of racemic β‐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. [ABSTRACT FROM AUTHOR]

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) β‐ 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. [ABSTRACT FROM AUTHOR]

The article discusses a synthetic strategy based on designing polyhydroxyalkanoates (PHAs) with enhanced mechanical toughness and thermal stability, enabling PHAs to be recycled. It mentions how polymers can be improved by revisiting some fundamental chemistry concepts rather than establishing complex chemistry models. The method enables the introduction of other chemical groups at the alpha position, which opens the possibility of pushing PHAs toward more diversified functional materials.

The ring‐opening copolymerization (ROCOP) of functional 4‐alkoxymethylene‐β‐propiolactones (BPLORs) by yttrium‐bisphenolate complexes was investigated. The ROCOP of equimolar mixtures of BPLORs of opposite configurations, namely (R)‐BPLOR1/(S)‐BPLOR2 [R1, R2=OMe, OAllyl, OCH2Ph (=OBn), OSiMe2tBu (=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, 13C{1H} 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. [ABSTRACT FROM AUTHOR]

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 β-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. [ABSTRACT FROM AUTHOR]

Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional β-lactones, namely 4-alkoxymethylene-β-propiolactones (BPLORs). 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 β-lactone. [ABSTRACT FROM AUTHOR]

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 1H and 13C 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. [ABSTRACT FROM AUTHOR]

Yttrium [amino-alkoxy-bis(phenolate)]amido complexes have been used for the ring-opening polymerization (ROP) of racemic alkyl β-malolactonates (4-alkoxycarbonyl-2-oxetanones, rac-MLARs) 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 PMLARs ( Pr ≥ 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho-substituted ligands proved largely ineffective. All polymers have been characterized by 1H and 13C{1H} 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-MLABz and rac-MLAAll, the ROP of rac-MLAMe 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. [ABSTRACT FROM AUTHOR]

Amphiphilic polycarbonate-poly(hydroxyalkanoate) diblock copolymers, namely, poly(trimethylene carbonate) (PTMC)- b-poly(β-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 β-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. [ABSTRACT FROM AUTHOR]

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 31P/171Yb 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, ĐM = 1.44), and α,̉-dihydroxy and α-hydroxy,̉-formate telechelic poly(trimethylene carbonate)s (Mn,NMR ＜ 20 000 g mol−1, ĐM = 1.61) were thus synthesized under mild operating conditions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 β-malolactonate) (PMLABe) diols, from the ROP of racemic (benzyl β-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 β-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, 1H and 13C 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. [ABSTRACT FROM AUTHOR]

The synthesis of a series of divalentand trivalent β-diketiminateborohydrides [(dipp)2NacNacLn(BH4)(THF)2] ((dipp)2NacNac = (2,6-C6H3iPr2)NC(Me)CHC(Me)N(2,6-C6H3iPr2); Ln = Sm, Eu,Yb) and [(dipp)2NacNacLn(BH4)2(THF)] (Ln = Sc, Sm, Dy, Yb, Lu) is reported. All compoundswere obtained by salt metathesis in THF from [(dipp)2NacNacK]and the corresponding homoleptic divalent and trivalent borohydrides[Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb),[Sc(BH4)3(THF)2], and [Ln(BH4)3(THF)3] (Ln = Sm, Dy, Yb, Lu), respectively.The complexes were fully characterized, and their solid-state structureswere established by single-crystal X-ray diffraction. In both thedivalent and trivalent compounds, the BH4–groups coordinate in a κ3(H) mode to the metal.Only in the lutetium complex [(dipp)2NacNacLu(BH4)2(THF)] does one BH4–groupcoordinate in a κ3(H) mode, whereas the other onecoordinates as κ2(H). This kind of mixed κ2/κ3(H) coordination mode is rare. The applicationof the divalent and trivalent compounds as initiators in the ring-openingpolymerization (ROP) of ε-caprolactone (CL) and trimethylenecarbonate (TMC) was investigated. All complexes afforded a generallywell-controlled ROP of both of these cyclic esters. High molar masspoly(ε-caprolactone) diols (Mn,NMR< 92 700 g mol–1, ĐM= 1.51) and α-hydroxy,ω-formatetelechelic poly(trimethylene carbonate)s (Mn,NMR< 16 000 g mol–1, ĐM= 1.59) were thus synthesized under mild operatingconditions. [ABSTRACT FROM AUTHOR]

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/nonisocyanate 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. [ABSTRACT FROM AUTHOR]

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 (MLAR; 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 MLAAllyl and MLABenzyl. Hydrogenolysis of the benzyloxycarbonyl or functionalization of the allyl pendant groups opens a route towards a new class of functional alternating copolymers. [ABSTRACT FROM AUTHOR]