Martine Slawinski

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).

Macromolecular engineering via ring-opening polymerization (2): L-lactide/trimethylene carbonate copolymerization – kinetic and microstructural control via catalytic tuning

Copolymerization of L-LA and TMC has been achieved with binary systems, associating either a metal-based (pre)catalyst or an organocatalyst with benzyl alcohol (BnOH) acting as a co-initiator and a chain transfer agent. Kinetic monitoring indicated that copolymerizations mediated by the zinc-β-diketiminate system [(BDI)Zn{N(SiMe3)2}]/BnOH proceed by preferential consumption of L-LA first and then of TMC, in contrast to homopolymerizations of the individual monomers that progress roughly at the same rate. In striking contrast, systems based on metal triflates, M(OTf)3/BnOH (M = Al, Bi, Yb), copolymerize TMC faster than L-LA, as established for the first time, while these systems also homopolymerize the individual monomers at about the same rate. Eventually, the [(BDI)Zn{N(SiMe3)2}]/BnOH and M(OTf)3/BnOH systems produce P(LLA-grad-TMC) gradient copolymers featuring PLLA and PTMC blocks of significant length (ca. 10 units with Al-, Yb-; ca. 6 units with Zn-, Bi-based systems), as confirmed by 13C NMR analyses. On the other hand, copolymerizations mediated by the guanidine-based organocatalyst system TBD/BnOH (TBD = 1,5,7-triazabicyclo[4.4.0]dec-5-ene) gave P(LLA-ran-TMC) random copolymers. Optimal control of molar masses and dispersities, along with minimized transesterifications, is achieved with the zinc and ytterbium systems. While metal triflates induce partial decarboxylation of TMC units, the zinc and TBD systems are perfectly chemoselective. Kinetic and microstructural control in random copolymerization of L-LA and TMC can thus be achieved via catalytic tuning.

Macromolecular engineering via ring-opening polymerization (3): trimethylene carbonate block copolymers derived from glycerol

Linear trimethylene carbonate (1,3-dioxane-2-one, TMC) thermoplastic copolymers derived from glycerol have been synthesized upon sequential copolymerization. The “immortal” ring-opening polymerization (iROP) of the benzyloxy-substituted TMC, 3-benzyloxytrimethylene carbonate (BTMC), with systems composed of the β-diketiminate discrete zinc complex [(BDIiPr)Zn(N(SiMe3)2)] or the phosphazene base 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP) as a catalyst, and benzyl alcohol (BnOH) as an initiator/chain transfer agent is first described. Next, diblock and triblock copolycarbonates featuring a TMC segment along with one or two adjacent BTMC, or 2,2-dimethoxypropane-1,3-diol carbonate (TMC(OMe)2), or racemic β-butyrolactone (BL) chains have been prepared. The copolymerization under “immortal” operating conditions was carried out in bulk or toluene solution and promoted by [(BDIiPr)Zn(N(SiMe3)2)] as a catalyst, and either BnOH, 1,3-propanediol, or the poly(trimethylene carbonate) (PTMC) derived macro-ols, H-PTMC-OBn or H-PTMC-H, as an initiator/chain transfer agent. In particular, the PTMC-b-[poly(TMC(OMe)2)]1,2 couple of copolymers provide a basis for the assessment of thermal and mechanical property differentiation in close relationship with the co-monomers content. Unlike PTMC which is elastomeric and P(TMC(OMe)2) which is rather rigid and brittle much like semi-crystalline poly(L-lactide), the diblock and triblock copolymers show characteristics lying in between those of each homopolymer, without any significant influence of the topology. Thus PTMC/PTMC(OMe)2 block copolymers mechanically behave similarly to PTMC/PLLA block copolymers.

Syndioselective ring-opening polymerization and copolymerization of trans-1,4-cyclohexadiene carbonate mediated by achiral metal- and organo-catalysts

The ring-opening polymerization (ROP) of trans-1,4-cyclohexadiene carbonate (CHDC) has been investigated computationally and experimentally. DFT computations indicate that ring-opening of CHDC is thermodynamically possible, yet to a lesser extent than that of trans-cyclohexene carbonate (CHC). Effective homopolymerizations of rac-CHDC and simultaneous or sequential copolymerizations of rac-CHDC with rac-CHC and L-LA were achieved with a diaminophenolate zinc-based complex ([(NNO)ZnEt]) or a guanidine (TBD) associated with an alcohol. These ROP reactions, which confirmed the lower reactivity of rac-CHDC vs. rac-CHC, especially in homopolymerization, proceeded without any decarboxylation. Quite uniquely, highly syndiotactic PCHDC was obtained from ROP of rac-CHDC with both the zinc- and TBD-based catalysts, as revealed by 13C{1H} NMR studies. The prepared homopolymers and block or random copolymers were characterized by 1H, 13C{1H} NMR, MALDI-ToF MS, SEC and DSC techniques.

Ethylene carbonate/cyclic ester random copolymers synthesized by ring-opening polymerization

The diaminophenolate and β-diketiminate zinc complexes [(NNO)ZnEt] ((NNO)− = 2,4-di-tert-butyl-6-{[(2′-dimethylaminoethyl)-methylamino]methyl}phenolate)) and [(BDIiPr)Zn{N(SiMe3)2}] (BDIiPr = CH(CMeNC6H3-2,6-iPr2)2), respectively, the Lewis acidic triflate salt Al(OTf)3, and the guanidine TBD (= 1,5,7-triazabicyclo[4.4.0]dec-5-ene), combined to a protic source as initiator, typically benzyl alcohol (BnOH), enabled the successful copolymerization of ethylene carbonate (EC) with various cyclic esters such as β-butyrolactone (BL), δ-valerolactone (VL), e-caprolactone (CL) or L-lactide (LLA). The random copolymerizations proceeded smoothly under mild operating conditions, preferentially from [(NNO)ZnEt]/BnOH at 60 °C in toluene within a few hours, affording the corresponding copolymers void of ether units, with Mn,SEC values in the range ca. 6000–93 350 g mol−1 and with unimodal, moderately broad dispersity values (ĐM = 1.3–2.1). Under the same experimental conditions, the homopolymerization of EC did not proceed. The first EC/BL random copolymers were thus synthesized with up to 26 mol% of EC inserted within the polyester, while the second example of P(EC-co-VL) was isolated. P(EC-co-VL), P(EC-co-CL), and P(EC-co-LLA) copolymers were prepared with higher than previously reported EC content, namely 23, 37, and 17 mol% vs. 10, 31, and 4 mol%, respectively. In contrast to other catalyst systems, the Al(OTf)3/BnOH system promoted CO2 elimination from the copolymers, thereby leading to ether defects. Microstructural analysis of the copolymers by 13C{1H} NMR spectroscopy revealed the presence of signals previously never described and possibly arising from consecutive EC units within the random copolymers. Thermal transition temperatures measured by DSC further supported the random nature of these copolymers.

Syndiotactic Specific Structures, Symmetry Considerations, Mechanistic Aspects

The mechanism of syndiospecific polymerization with the catalyst Systems isopropylidene(cyclopentadienyl-fluorenyl)MCl2; M = Zr, Hf / MAO is discussed by taking into account the structural characteristics of the metallocene molecules and the optical particularities of their cationic species within the framework of a chain migratory insertion mechanism. A generally accepted transition State structure that respects the relative importance of different steric interactions of the active participants in the polymerization process, ligand, growing polymer chain and the coordinating monomer is discussed. It is shown that the substitution-free two top quadrants left and right to the cyclopentadienyl and the free Space in the central position of the fluorenyl are, among others, essential factors to syndiospecificity of the catalyst. The model is examined on a new syndiospecific catalyst system, η1,η5-tert-butyl(3,6-bis-tert-butylfluorenyl-dimethylsilyl)amidodichloro-titanium / MAO and its validity is confirmed.