Facile Synthesis of Hydroxyl-Ended, Highly Stereoregular, Star-Shaped Poly(lactide) from Immortal ROP of rac-Lactide and Kinetics Study

Enhancing the stereoselectivity of Me2GaOR(NHC) species in the ring-opening polymerization of rac-lactide, with the help of the chelation effect

Using dialkylgallium alkoxides with N-hetrocyclic carbenes (Me2GaOR(NHC)) for the ring-opening polymerization of rac-lactide, we have demonstrated the effect of the chelate interaction between the growing PLA chain and gallium on the stereoselectivity of dialkylgallium alkoxide propagating species - Me2Ga(OPLA)(NHC). In order to do so, we have conducted the structure-activity studies of both Me2Ga(OCH2CH2OMe)(NHC) (NHC = SIMes (1) and IMes (2)) and Me2Ga(OCH(Me)CO2Me)(NHC) (NHC = SIMes (3) and IMes (4)), the latter mimicking active species in the ROP of lactide with growing PLA chain. Based on VT NMR and FTIR spectroscopy, the effect of toluene, CH2Cl2 and THF on the structure of 3 and 4 have been demonstrated, especially with regard to the interaction of methyl lactate ligand with gallium. In a combination with the latter, the studies on the activity of 1 and 2 in the ROP of rac-LA, in different solvents, and at temperatures between -40 °C and 40 °C, have shown the extent of the chelation effect on the isoselectivity of Me2Ga(OPLA)(NHC) in the ROP of rac-LA, which varied between P m of 0.75 and 0.89 depending on the polymerization conditions. Both the latter, and the contribution resulting from the structure of Me2Ga(OPLA)(NHC) (P m = 0.75) have been decisive for the total isoselectivity observed under specific conditions. Our finding represents the first evidence demonstrating that the chelation effect, resulting from the weak interaction between the growing PLA chain and the metal centre, can be responsible for the enhancement of stereoselectivity in the ROP of rac-LA with metal alkoxide propagating species. It should remain of interest, especially in the case of metal based catalysts, which are able to carry out the stereoselective polymerization of rac-LA at mild conditions, under which the chelation effect can manifest itself.

Synthesis of Poly(δ-caprolactone) via Bis(phenolate) Rare-Earth Metal Complexes Mediated Ring-Opening Polymerization and Its Chemical Recycling

New amine-amino-bis(phenolate) ligands (H2LtBu and H2LCl) with a cyclic tertiary amine (pyrrolidine) as a side arm and tBu or Cl group on the phenolate ring have been prepared. The alkane elimination reaction between these free ligands and rare-earth tris(alkyl)s Ln(CH2SiMe3)3(THF)2 afforded the corresponding silylalkyl complexes LtBuLnCH2SiMe3(THF) (Ln = Y (1), Lu (2)) and LClYCH2SiMe3(THF) (3), where the solid-state structure of complex 1 was unambiguously confirmed by X-ray diffraction (XRD) analysis. These rare-earth metal complexes have been utilized as catalysts for the ring-opening polymerization (ROP) of biobased δ-caprolactone (δCL), either in the absence or presence of alcohols, to give poly(δ-caprolactone) (PδCL) with controlled molecular weight and narrow distribution (Đ < 1.2). The polymerization kinetics of δCL in toluene with yttrium complexes 1 and 3 were investigated. Oligomers prepared with complex 3 alone and the 3/PhCHMeOH binary catalyst system were well characterized with 1H NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). Moreover, chemical recycling of the resultant PδCL was achieved with high yield in a solution at ambient temperature (>92%) or in bulk at 130 °C (>82%) by using commercial KOtBu as a promotor.

Immortal Polymerization of LA: the Influence of Steric Effect, Electron Effect and pKa for Chain Transfer Agents

The “immortal” ring-opening polymerization (iROP) of L-LA, catalyzed by ligand-free Ca[N(SiMe3)2]2(THF)2 in combination with different chain transfer agents (alcohols, phenols and PhCH2NH2) was systematically investigated for the first time. When...

Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-Lactide and rac-β-Butyrolactone

A series of new phosphasalalen pro-ligands, analogues of salalen but with an iminophosphorane replacing the imine functionality, and their corresponding rare-earth alkoxide and siloxide complexes were synthesized. The multinuclear NMR spectra and X-ray diffraction analyses revealed that, for the tert-butoxide and ethoxide complexes, the resulting phosphasalalen rare-earth product was composed of a mononuclear alkoxide and a binuclear complex containing bridged alkoxo and hydroxo groups, while an analogous binuclear complex was isolated as the sole product for the siloxide complex. All the complexes could catalyze the heteroselective ring-opening polymerization (ROP) of rac-lactide (P_r up to 0.77) with high catalytic activities and a controlled polydispersity. Remarkably, the yttrium and lutetium phosphasalalen complexes could also efficiently catalyze the ROP of rac-β-butyrolactone to produce syndiotactic polymers (P_r up to 0.73) while their salalen analogues were inert, revealing the special effects of the iminophosphorane moiety. Detailed end-group analyses and kinetic investigations suggested that the alkoxo-hydroxo-bridged complexes maintained their binuclear structures in the polymerization.

Biased Lewis Pairs: A General Catalytic Approach to Ether-Ester Block Copolymers with Unlimited Ordering of Sequences

Polymerizing epoxides after cyclic esters remains a major challenge, though their block copolymers have been extensively studied and used for decades. Reported here is a simple catalytic approach based on a metal-free Lewis pair that addresses the challenge. When the Lewis acid is used in excess of a base, selective (transesterification-free) polymerization of epoxides occurs in the presence of esters, while selectivity toward cyclic esters is achieved by an oppositely biased catalyst. Hence, one-pot block copolymerization can be performed in both ester-first and ether-first orders with selectivity being switchable at any stage, yielding ether-ester-type block copolymers with unlimited ordering of sequences as well as widely variable compositions and architectures. The selectivity can also be switched back and forth several times to generate a multiblock copolymer. Experimental and calculational results indicate that the selectivity originates mainly from the state of catalyst-activated hydroxy species.

Alkaline earth metal complexes stabilized by amidine and guanidine ligands: synthesis, structure and their catalytic activity towards polymerization of rac-lactide

A series of amidine ligands RAmDIPP (R is backbone substituent, R = 2-methylpyridine (HL1), N,N,2-trimethylaniline (HL2), N,N-dimethylpropan-1-amine (HL3); DIPP (2,6-diisopropylphenyl) is N substituent) and a (Z)-1,1-diethyl-2,3-bis(2-methoxyphenyl)guanidine ligand (HL4) have been synthesized. Reaction of HL1-HL4 with Ca[N(SiHMe2)2](THF) or Ca[N(SiMe3)2](THF)2, respectively, afforded complexes 1-8 (1: [L1AmDIPPCa(SiHMe2)2]2, 2: L1AmDIPPCaN(SiMe3)2·(THF)2, 3: L2AmDIPPCaN(SiHMe2)2·(THF)2, 4: L2AmDIPPCaN(SiMe3)2·(THF), 5: L3AmDIPPCaN(SiHMe2)2·(THF)2, 6: L3AmDIPPCaN(SiMe3)2·(THF)2, 7: [L4CaN(SiHMe2)2]2, 8: [L4CaN(SiMe3)2]2). All the complexes were well-defined by NMR spectrum analyses and the molecular structures of 3 and 5-8 were further determined by single crystal X-ray diffraction analyses. In combination with an excess of PhOH as the chain transfer agent, complexes 1-8 catalyzed immortal ring-opening polymerization of rac-lactide in a controlled manner and exhibited moderate catalytic activity at room temperature. The end-group fidelity of the resultant polymer was certified by NMR and MALDI-TOF mass spectra.

Synthesis of Aluminum Complexes Bearing 8-Anilide-5,6,7-trihydroquinoline Ligands: Highly Active Catalyst Precursors for Ring-Opening Polymerization of Cyclic Esters

The stoichiometric reactions of 8-(2,6-R¹-4-R²-anilide)-5,6,7-trihydroquinoline (LH) with AlR₃ (R = Me or Et) afforded the aluminum complexes LAlR₂ (Al1⁻Al5,Al1: R¹ = ⁱPr, R² = H, R = Me; Al2: R¹ = Me, R² = H, R = Me; Al3: R¹ = H, R² = H, R = Me; Al4: R¹ = Me, R² = Me, R = Me; Al5: R¹ = Me, R² = Me, R = Et) in high yields. All aluminum complexes were characterized by NMR spectroscopy and elemental analysis. The molecular structures of complexes Al4 and Al5 were determined by single-crystal X-ray diffractions and revealed a distorted tetrahedral geometry at aluminum. In the presence of BnOH, complexes Al1⁻Al5 efficiently initiated the ring-opening homopolymerization of ε-caprolactone (ε-CL) and rac-lactide (rac-LA), respectively, in a living/controlled manner.

Synthesis of dinuclear aluminum complexes bearing bis-phenolate ligand and application in ring-opening polymerization of ε-caprolactone

Dinuclear zwitterionic aluminum complexes were prepared and applied as efficient catalysts toward ROP of ε-CL in a controlled manner.

Synthesis of linear and star poly(ε-caprolactone) with controlled and high molecular weights via cyclic trimeric phosphazene base catalyzed ring-opening polymerization

Linear and star PCLs with controlled and high molecular weights are efficiently prepared by a catalytic system of CTPB/alcohol.

Impact of aryloxy initiators on the living and immortal polymerization of lactide

This report describes two different methodologies for the synthesis of aryl end-functionalized poly(lactide)s (PLAs) catalyzed by indium complexes.

Immortal Ring-Opening Polymerization of rac-Lactide Using Polymeric Alcohol as Initiator to Prepare Graft Copolymer

In the presence of a small molecular protic initiator, immortal ring-opening polymerization (ROP) of lactide (LA) is a highly efficient strategy to synthesize polylactide in a controllable manner, while using polymeric alcohol as an initiator has been less investigated. A series of polymeric alcohols (PS⁻OH) composed of styrene and 4.3%⁻18% hydroxyl functional styrene (diethyl(hydroxy(4-vinylphenyl)methyl)phosphonate, St⁻OH) were synthesized through reversible addition-fragmentation transfer (RAFT) polymerization. Using PS⁻OH as an initiator, the immortal ROP of rac-LA was catalyzed by dibutylmagnesium (MgⁿBu₂) under various ratios of monomer to hydroxyl group within PS⁻OH to generate polystyrene-g-polylactide (PS⁻g⁻PLA) copolymers with different graft lengths. After thermal annealing at 115 °C, the PLA domain aggregated to nanospheres among the PS continuum. The size of the nanospheres, varying from 130.1 to 224.2 nm, was related to the graft density and length of PS⁻g⁻PLA. Nanoporous films were afforded through chemical etching of the PLA component.

Synthesis and AIE properties of PEG–PLA–PMPC based triblock amphiphilic biodegradable polymers

The synthesis of a novel AIE-active micelle based on living immortal polymerization of cyclic esters and a “click” reaction of azide functionalized TPE is described.

Synthesis and characterization of rare-earth metal guanidinates stabilized by amine-bridged bis(phenolate) ligands and their application in the controlled polymerization of rac-lactide and rac-β-butyrolactone

Eight rare-earth metal guanidinates supported by a versatile family of bridged bis(phenolate) ligands were synthesized, and their application in initiating stereoselective ROP of rac-lactide and rac-β-butyrolactone were investigated and compared.

Synthesis and characterization of rare-earth metal complexes supported by a new pentadentate Schiff base and their application in heteroselective polymerization of rac-lactide

Both rare-earth metal salen aryloxides and amides can efficiently initiate the ring-opening polymerization of rac-lactide to give heterotactic-rich polylactides.

Isoselective 3,4-(co)polymerization of bio-renewable myrcene using NSN-ligated rare-earth metal precursor: an approach to a new elastomer

Perfect 3,4-regio- (>99%) and isospecific stereo-selective (mmmm > 99%) PMY were achieved by a lutetium dialkyl complex bearing a NSN-bidentate β-diimidosulfonate ligand for the first time.

Neutral lutetium complex/polyamine mediated immortal ring-opening polymerization of rac-lactide: facile synthesis of well-defined hydroxyl-end and amide-core stereoregular star polylactide

A facile way to obtain well-defined hydroxyl-end and amide-core stereoregular star polylactide.

Facile synthesis and comparative study of poly(l-lactide) with linear-comb and star-comb architecture

The crystallization ability of PLLA with different chain structures.

PLA architectures: the role of branching

Biobased and biodegradable polymers have become more and more interesting in view of waste management and crude oil depletion.