Synthesis and characterization of poly(L‐lactide)s and poly(D‐lactide)s of controlled molecular weight

High molecular weight poly(L ‐lactide)s (PLLAs) and poly(D ‐lactide)s (PDLAs) were synthesized in toluene at 70 °C by ring‐opening polymerization of optically pure L ‐lactide and D ‐lactide, using tin(II) 2‐ethylhexanoate (SnOct₂) and 2‐(2‐methoxyethoxy)ethanol as initiator and coinitiator, respectively. Under these conditions, polarimetry as well as ¹³C NMR spectroscopy indicated that the synthesized poly(lactide)s (PLAs) are more than 99% isotactic. The molecular weight was successfully controlled by adjusting the monomer‐to‐initiator molar ratio. Gel permeation chromatography and MALDI‐TOF mass spectrometry analyses showed that the polydispersity index of the PLAs is below 1.1. Moreover, MALDI‐TOF spectra showed two different chain distributions, one characterized by an even number of lactic acid repeat units and the other by an odd number of lactic acid repeat units. The second distribution, indicative of the presence of intermolecular transesterification reactions, appears at the very beginning of the polymerization and its intensity increases with the polymerization time. Finally, a reversible reaction kinetic model was used to determine the monomer equilibrium concentration ([M]_eq = 1.4 ± 0.5%) and the propagation rate constant (k_p = 14.4 ± 0.5 L mol^?1 h^?1) of the polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1944–1955, 2007     

Synthesis of star‐shaped poly(D,L‐lactic acid‐alt‐glycolic acid)‐b‐poly(L‐lactic acid) with the poly(D,L‐lactic acid‐alt‐glycolic acid) macroinitiator and stannous octoate catalyst

Two types of three‐arm and four‐arm, star‐shaped poly(D,L ‐lactic acid‐alt‐glycolic acid)‐b‐poly(L ‐lactic acid) (D,L ‐PLGA50‐b‐PLLA) were successfully synthesized via the sequential ring‐opening polymerization of D,L ‐3‐methylglycolide (MG) and L ‐lactide (L ‐LA) with a multifunctional initiator, such as trimethylolpropane and pentaerythritol, and stannous octoate (SnOct₂) as a catalyst. Star‐shaped, hydroxy‐terminated poly(D,L ‐lactic acid‐alt‐glycolic acid) (D,L ‐PLGA50) obtained from the polymerization of MG was used as a macroinitiator to initiate the block polymerization of L ‐LA with the SnOct₂ catalyst in bulk at 130 °C. For the polymerization of L ‐LA with the three‐arm, star‐shaped D,L ‐PLGA50 macroinitiator (number‐average molecular weight = 6800) and the SnOct₂ catalyst, the molecular weight of the resulting D,L ‐PLGA50‐b‐PLLA polymer linearly increased from 12,600 to 27,400 with the increasing molar ratio (1:1 to 3:1) of L ‐LA to MG, and the molecular weight distribution was rather narrow (weight‐average molecular weight/number‐average molecular weight = 1.09–1.15). The ¹H NMR spectrum of the D,L ‐PLGA50‐b‐PLLA block copolymer showed that the molecular weight and unit composition of the block copolymer were controlled by the molar ratio of L ‐LA to the macroinitiator. The ¹³C NMR spectrum of the block copolymer clearly showed its diblock structures, that is, D,L ‐PLGA50 as the first block and poly(L ‐lactic acid) as the second block. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 409–415, 2002     

In situ formation of Sn(IV) catalyst with increased activity in ε‐caprolactone and L‐lactide polymerization using stannous(II) 2‐ethylhexanoate

Ring‐opening polymerization (ROP) of ε‐caprolactone and L‐lactide (LA) was studied using stannous(II) 2‐ethylhexanoate (Sn(Oct)₂) with N,N‐dimethylformamide‐dimethyl acetal (DMF‐DMA). DMF‐DMA showed a tenfold improvement in catalytic activity over that of Sn(Oct)₂ under the same conditions. It also enhanced the capability to control molecular weight in the synthesis of small molecular weight polymers of polycaprolactone and polylactide (PLA). The high molecular weight polymerization demonstrated a strong capability to control molecular weight for the polymerization of LA: a molecular weight of PLA exceeding 400,000 was obtained at very low catalytic loadings. The individual polymerization rates of other tin reagents with DMF‐DMA also clearly increased. Applying this methodology could drastically reduce the time and cost required for the fabrication of these products to increase the competitive advantage of manufacturers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

About formation of cycles in Sn(II) octanoate‐catalyzed polymerizations of lactides

At first, formation of cycles in commercial poly(l ‐lactide)s is discussed and compared with benzyl alcohol‐initiated polymerizations performed in this work. This comparison was extended to polymerizations initiated with 4‐cyanophenol and pentafluorothiophenol which yielded cyclic polylactides via end‐biting. The initiator/catalyst ratio and the acidity of the initiator were found to be decisive for the extent of cyclization. Further polymerizations of l ‐lactide were performed with various diphenols as initiators/co‐catalysts. With most diphenols, cyclic polylactides were the main reaction products. Yet, only catechols yielded even‐numbered cycles as main reaction products, a result which proves that their combination with SnOct₂ catalyzed a ring‐expansion polymerization (REP). The influence of temperature, time, co‐catalyst, and catalyst concentrations was studied. Four different transesterification reactions yielding cycles were identified. For the cyclic poly(l ‐lactide)s weight average molecular weights (M_w's) up to 120,000 were obtained, but ¹H NMR end group analyses indicated that the extent of cyclization was slightly below 100%. The influence of various parameters like structure of initiator and catalyst and temperature on the formation of cyclic poly(l ‐lactide)s has been investigated. Depending on the chosen conditions, the course of the polymerization can be varied from a process yielding exclusively linear polylactides to mainly cyclic polylactides. Three different reaction pathways for cyclization reactions have been identified. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1915–1925     

Emulsion copolymerization of D,L‐lactide and glycolide in supercritical carbon dioxide

Biodegradable polyesters were synthesized via an emulsion polymerization in supercritical carbon dioxide (SC‐CO₂). Copolymers of lactide and glycolide were synthesized in SC‐CO₂ with stannous octoate as the ring‐opening catalyst and a fluorocarbon polymer surfactant as an emulsifying agent. The conversion of lactide and glycolide was monitored with respect to the reaction time and temperature with ¹H NMR spectroscopy. The conversion of glycolide surpassed 99% within 72 h for an SC‐CO₂ phase maintained at 200 bar and 70 °C. Under the same conditions, lactide conversion reached 65% after 72 h of polymerization. Unpolymerized monomer was removed after the reaction by extraction with an SC‐CO₂ mobile phase. The molecular weights of all the copolymers were measured by gel permeation chromatography. Weight‐average molecular weights (M_w) ranged between 2500 and 30,200 g/mol and polydispersity indices ranged from 1.4 to 2.3 for polymerization times of 6 and 48 h, respectively. Although the molecular weight increased significantly during the first 48 h of reaction, there was no significant difference in the M_w for polymerization times of 48 and 72 h. Emulsion polymerization within the benign solvent SC‐CO₂ demonstrated improved conversion and molecular weight versus polymers synthesized without surfactant. The emulsion polymerization of lactide and glycolide copolymers in SC‐CO₂ is proposed as a novel production technique for high‐purity, biodegradable polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 562–570, 2001     

Synthesis and ring‐opening polymerization of new monoalkyl‐substituted lactides

New monoalkyl‐substituted lactides were synthesized by reaction of α‐hydroxy acids with 2‐bromopropionyl bromide, and polymerized with various catalysts in the presence of benzyl alcohol by ring‐opening polymerization (ROP). The classic tin(II) 2‐ethylhexanoate (Sn(Oct)₂) catalyst was leading to polymers with narrow distribution and predictable molecular weights, in polymerizations in bulk or toluene at 100 °C. The polymerization rate was corresponding to the steric hindrance of the alkyl substituents, such as butyl, hexyl, benzyl, isopropyl, and dimethyl groups. A yield of 83% was obtained with the hexyl‐substituted lactide after 1 h of polymerization. Excellent conversions (97%) could be achieved by using the alternative catalyst 4‐(dimethylamino)pyridine (DMAP). This latter organic catalyst was most efficient in polymerizing the more steric‐hindered lactides with good molecular weight and polydispersity control, in comparison to the tin(II) 2‐ethylhexanoate and tin(II) trifluoromethane sulfonate [Sn(OTf)₂] catalysts. The efficiency of the DMAP catalyst and the variability of the monomer synthesis route for new alkyl‐substituted lactides allow to prepare and to envision a wide range of new functionalized polylactides for the elaboration of tailored materials. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4379–4391, 2004     

Stannous(II) trifluoromethane sulfonate: a versatile catalyst for the controlled ring‐opening polymerization of lactides: Formation of stereoregular surfaces from polylactide “brushes”

A general method for the controlled synthesis of polylactide in solution and from solid supports is presented. The evaluation of stannous(II) trifluoromethane sulfonate [Sn(OTf)₂] and scandium(III) trifluoromethane sulfonate [Sc(OTf)₃] as catalysts for the ring‐opening polymerization (ROP) of L ‐, D ‐, and L ,D ‐lactide is described as a route to polylactide using mild and highly selective conditions. These triflate catalysts must be used in conjunction with a nucleophilic compound such as an alcohol that is the actual initiating species via the active metal alkoxide species. Consistent with this process, ¹H NMR analysis revealed that the α‐chain‐end bears the ester from the initiating alcohol, and upon hydrolysis of the active metal alkoxide chain end, a ω‐hydroxyl chain end was clearly detected. Polymers of predictable molecular weights and narrow polydispersities were obtained in high yields in accordance with a controlled polymerization process. The addition of base either as a solvent or additive significantly enhanced the polymerization rate with minimal loss to the polymerization control. The ROP of lactide isomers from an initiator, HO(CH₂CH₂O)₃(CH₂)₁₁SH, self‐assembled onto a gold surface using Sn(OTf)₂ produced polylactide brushes under living conditions and provides the opportunity to prepare stereoregular or chiral surfaces by polymerization of enantiomerically pure monomers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3529–3538, 2001     

Synthesis and characterization of multiblock copolymers composed of poly(5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one) outer blocks and poly(L‐lactide) inner blocks

Ethylene glycol (EG) initiated, hydroxyl‐telechelic poly(L ‐lactide) (PLLA) was employed as a macroinitiator in the presence of a stannous octoate catalyst in the ring‐opening polymerization of 5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one (MBC) with the goal of creating A–B–A‐type block copolymers having polycarbonate outer blocks and a polyester center block. Because of transesterification reactions involving the PLLA block, multiblock copolymers of the A–(B–A)_n–B–A type were actually obtained, where A is poly(5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one), B is PLLA, and n is greater than 0. ¹H and ¹³C NMR spectroscopy of the product copolymers yielded evidence of the multiblock structure and provided the lactide sequence length. For a PLLA macroinitiator with a number‐average molecular weight of 2500 g/mol, the product block copolymer had an n value of 0.8 and an average lactide sequence length (consecutive C₆H₈O₄ units uninterrupted by either an EG or MBC unit) of 6.1. For a PLLA macroinitiator with a number‐average molecular weight of 14,400 g/mol, n was 18, and the average lactide sequence length was 5.0. Additional evidence of the block copolymer architecture was revealed through the retention of PLLA crystallinity as measured by differential scanning calorimetry and wide‐angle X‐ray diffraction. Multiblock copolymers with PLLA crystallinity could be achieved only with isolated PLLA macroinitiators; sequential addition of MBC to high‐conversion L ‐lactide polymerizations resulted in excessive randomization, presumably because of residual L ‐lactide monomer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6817–6835, 2006     

Synthesis of four‐arm star poly(L‐lactide) oligomers using an in situ‐generated calcium‐based initiator

Using an in situ‐generated calcium‐based initiating species derived from pentaerythritol, the bulk synthesis of well‐defined four‐arm star poly(L ‐lactide) oligomers has been studied in detail. The substitution of the traditional initiator, stannous octoate with calcium hydride allowed the synthesis of oligomers that had both low PDIs and a comparable number of polymeric arms (3.7–3.9) to oligomers of similar molecular weight. Investigations into the degree of control observed during the course of the polymerization found that the insolubility of pentaerythritol in molten L ‐lactide resulted in an uncontrolled polymerization only when the feed mole ratio of L ‐lactide to pentaerythritol was 13. At feed ratios of 40 and greater, a pseudoliving polymerization was observed. As part of this study, in situ FT‐Raman spectroscopy was demonstrated to be a suitable method to monitor the kinetics of the ring‐opening polymerization of lactide. The advantages of using this technique rather than FTIR‐ATR and ¹H NMR for monitoring L ‐lactide consumption during polymerization are discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4736–4748, 2009     

Controlled preparation of poly(ethylene glycol) and poly(L‐lactide) block copolymers in the presence of a monomer activator

Polymerization of L ‐lactide (LA) was performed in the presence of trifluoromethanesulfonic acid (CF₃SO₃H) via an activated monomer mechanism to synthesize various block copolymers composed of polyethyleneglycol (PEG) and poly(L ‐lactide) (PLLA). The PLLAs obtained had molecular weights close to theoretical values calculated from LA/PEG molar ratios and exhibited monomodal GPC curves. A ¹H NMR spectroscopic study showed that the LA carbonyl carbon signal exhibited a change in chemical shift to lower field, caused by electron delocalization of the carbonyl carbon by CF₃SO₃H. We successfully prepared PEG and PLLA block copolymers using this activated monomer mechanism. We concluded that synthesis proceeded by LA ring‐opening polymerization caused by PEG in the presence of CF₃SO₃H to yield PEG and PLLA block copolymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5917–5922, 2009     

Synthesis and characterization of allyl functionalized poly(α-hydroxy)acids and their further dihydroxylation and epoxidation

In this study, the synthesis of an allyl functionalized aliphatic polyester and the subsequent oxidation of the double bonds was investigated. Allylglycolide (3-allyl-1,4-dioxane-2,5-dione) was synthesized and its homopolymer and copolymers with l-lactide were prepared by ring opening polymerization in the melt using benzyl alcohol and SnOct₂ as initiator and catalyst, respectively. The polymerizations proceeded with high yields and conversions and good control over molecular weights and copolymer composition. The obtained polymers were amorphous materials and their T_g increased with increasing lactide content. Dihydroxylation of the double bonds in poly(allylglycolide) and copolymers with lactide was attempted with osmiumtetroxide/4-methylmorpholine-4-oxide (OsO₄/NMO). However, particularly the polymers rich in allylglycolide could not be isolated after dihydroxylation because they likely underwent degradation during workup. Optimizing the reaction conditions gave partially dihydroxylated copolymers only for copolymers with high lactide content (50 and 75 mol%) with a conversion of the double bonds of only ∼60%. GPC analysis showed that chain scission had occurred during the dihydroxylation reaction and/or workup.The allyl groups of poly(allylglycolide) homopolymers and copolymers with lactide were oxidized using m-chloroperoxy benzoic acid (mCPBA) to yield the corresponding epoxidated polymers in high yield. NMR analysis showed that conversion of the double bonds to epoxides was quantitative, whereas GPC analysis showed that the epoxidation was not associated with chain scission. All epoxidated polymers were amorphous materials with a T_g depending on the composition.     

Chemical Recycling of End-of-Life Poly(lactide) via Zinc-Catalyzed Depolymerization and Polymerization

The chemical recycling of poly(lactide) was investigated based on depolymerization and polymerization processes. Using methanol as depolymerization reagent and zinc salts as catalyst, poly(lactide) was depolymerized to methyl lactate applying microwave heating. An excellent performance was observed for zinc(II) acetate with turnover frequencies of up to 45000 h⁻¹. In a second step the monomer methyl lactate was converted to (pre)poly(lactide) in the presence of catalytic amounts of zinc salts. Here zinc(II) triflate revealed excellent performance for the polymerization process (yield: 91 %, M_n ∼8970 g/mol). Moreover, the (pre)poly(lactide) was depolymerized to lactide, the industrial relevant molecule for accessing high molecular weight poly(lactide), using zinc(II) acetate as catalyst.     

Anionic iron(II) alkoxides as initiators for the controlled ring‐opening polymerization of lactide

Hetero‐bimetallic Fe(II) alkoxide/aryloxides were evaluated as initiators for the ring‐opening polymerization of rac‐lactide. [(THF)NaFe(O^tBu)₃]₂ ( 1 ) and [(THF)₄Na₂Fe(2,6‐diisopropylphenolate)₄] ( 2 ) (THF = tetrahydrofuran) both polymerized lactide efficiently at room temperature, with complex 1 affording better control over the molecular weight parameters of the resultant polymer. At conversions below 70%, a linear increase in molecular weight with conversion was observed, indicative of a well‐controlled polymerization process. Complex 2 is the first example of a dianionic Fe(II) alkoxide and has been structurally characterized to reveal a distorted square planar FeO₄ array in which both Na counterions bridge two aryloxide ligands and are further complexed by two THF ligands. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3798–3803, 2003     

Bio‐safe synthesis of linear and branched PLLA

The catalytic activities of Bi(III) acetate (Bi(OAc)₃) and of creatinine towards the ring‐opening polymerization of L ‐lactide have been compared with those of a stannous (II) ethylhexanoate ((SnOct)₂)‐based system and with those of a system catalyzed by enzymes. All four were suitable catalysts for the synthesis of high and moderate molecular weight poly(L ‐lactide)s and the differences in reactivity and efficiency have been studied. Linear and branched poly(L ‐lactide)s were synthesized using these bio‐safe initiators together with ethylene glycol, pentaerythritol, and myoinositol as coinitiators. The polymerizations were performed in bulk at 120 and 140 °C and different reactivities and molecular weights were achieved by adding different amounts of coinitiators. A molecular weight of 105,900 g/mol was achieved with 99% conversion in 5 h at 120 °C with a Bi(OAc)₃‐based system. This system was comparable to Sn(Oct)₂ at 140 °C. The reactivity of creatinine is lower than that of Bi(OAc)₃ but higher compared with enzymes lipase PS (Pseudomonas fluorescens). A ratio of Sn(Oct)₂M_o/I_o 10,000:1 was needed to achieve a polymer with a reasonable low amount of tin residue in the precipitated polymer, and a system catalyzed by creatinine at 140 °C has a higher conversion rate than such a system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1214–1219, 2010     

Ring‐opening polymerization of lactides catalyzed by magnesium complexes coordinated with NNO‐tridentate pyrazolonate ligands

A series of magnesium benzylalkoxide complexes, [LⁿMg(μ‐OBn)]₂ ( 1 – 14 ) supported by NNO‐tridentate pyrazolonate ligands with various electron withdrawing‐donating subsituents have been synthesized and characterized. X‐ray crystal structural studies revealed that Complexes 1 – 3 , 5 , 7 , 9 , and 10 are dinuclear bridging through benzylalkoxy oxygen atoms with penta‐coordinated metal centers. All of these complexes acted as efficient initiators for the ring‐opening polymerization of L‐lactide and rac‐lactide. Based on kinetic studies, the activity of these metal complexes is significantly influenced by the electronic effect of the ancillary ligands with the electron‐donating substituents at the phenyl rings enhancing the polymerization rate. In addition, the “living” and “immortal” character of 6 has paved a way to synthesize as much as 40‐fold polymer chains of polylactides with a very narrow polydispersity index in the presence of a small amount of initiator. Among all of magnesium complexes, Complex 6 exhibits the highest stereoselectivity toward ring‐opening polymerization of rac‐lactide with Pr up to 88% in THF at 0 °C. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Sn(OTf)2 and Sc(OTf)3: Efficient and versatile catalysts for the controlled polymerization of lactones

A general and novel method for the controlled synthesis of aliphatic polyesters is presented. The evaluation of stannous (II) trifluoromethane sulfonate [Sn(OTf)₂] and scandium (III) trifluoromethane sulfonate [Sc(OTf)₃] as catalysts for the ring‐opening polymerization (ROP) of various lactones is described as a route to polyesters under mild and highly selective polymerization conditions. Size exclusion chromatograms of poly(ϵ‐caprolactone) initiated from ethanol in the presence of either Sn(OTf)₂ or Sc(OTf)₃ demonstrate the facile synthesis of narrowly dispersed products. Predictable molecular weights, typical of a living or controlled polymerization, were obtained with high yields. These catalysts are versatile and applicable toward the ROP of other cyclic (di)esters, including β‐butyrolactone, which produces the synthetic analogue of the biopolymer poly(β‐hydroxybutyrate). © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2067–2074, 2000     

Dendritic homopolymers and block copolymers: Tuning the morphology and properties

The synthesis and characterization of dendritic homopolymers and block copolymers of ?‐caprolactone and lactide (L ‐lactide and racemic lactide) were performed with multifunctional initiators in combination with living polymerization and the selective placement of branching junctures in a divergent growth strategy. A hexahydroxy‐functional 2,2‐bis(hydroxymethyl) propionic acid derivative was used as an initiator for the stannous‐2‐ethylhexanoate‐catalyzed living ring‐opening polymerization of ?‐caprolactone, L ‐lactide, and racemic L ,D ‐lactide. Branching junctions at the chain ends were introduced with benzylidene‐protected 2,2‐bis(hydroxymethyl) propionic acid. Subsequent generations were then polymerized, after deprotection, from these star‐shaped macroinitiators. Successive chain end capping and initiation produced three generations of polymers with molecular weights in excess of 130,000 g/mol and narrow polydispersities (<1.20). It was possible to prepare diblock and triblock copolymers with phase‐separated morphologies, and with L ‐lactide or D ,L ‐lactide, semicrystalline and amorphous morphologies were demonstrated. The polymers were characterized by ¹H NMR, ¹³C NMR, size exclusion chromatography, and differential scanning calorimetry. The compositions of the block copolymers and the conformational structures of the optically active polymers were also confirmed by optical rotation measurements. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1174–1188, 2004     

Phosphines: Nucleophilic organic catalysts for the controlled ring‐opening polymerization of lactides

A new metal‐free synthetic approach to the controlled ring‐opening polymerization (ROP) of lactide with nucleophilic phosphines as transesterification catalysts is described. P(Bu)₃, PhPMe₂, Ph₂PMe, PPh₃, and related phosphines are commercially available, inexpensive catalysts that generate narrowly dispersed polylactides with predictable molecular weights. These organic catalysts must be used in combination with an initiator, such as an alcohol, to generate an alcoholate ester α‐end group upon ROP. A likely polymerization pathway is through a monomer‐activated mechanism, with minimal active species, facilitating narrow molecular weight distributions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 844–851, 2002; DOI 10.1002/pola.10168     

A “Mitsubishi emblem” tetrauclear aluminum complex containing two unsymmetric N2O2‐ligands and a symmetric NO3‐ligand as initiator for polymerization of rac‐lactide

A novel hydroxy‐, methoxy‐, and phenoxy‐bridge “Mitsubishi emblem” tetranuclear aluminum complex ( 1 ) is synthesized from an unsymmetric amine‐pyridine‐bis(phenol) N₂O₂‐ligand (H₂L¹) and a symmetric amine‐tris(phenol) NO₃‐ligand (H₂L²). Two same configuration chiral nitrogen atoms are formed in the tetranuclear Al complex upon coordination of the unsymmetric tertiary amine ligand to central Al. Complex 1 initiates controlled ring‐opening polymerization (ROP) of rac‐lactide and afford polylactide (PLA) with narrow molecular weight distributions (M_w/M_n = 1.05–1.19). The analysis of ¹H NMR spectra of the oligomer indicates that the methoxy group is the initiating group and the ring‐opening polymerization of lactide follows a coordination‐insertion mechanism. The Homonuclear decoupled ¹H NMR spectroscopy suggests the isotactic‐rich chains is preferentially formed in PLA. The study on kinetics of the ROP of lactide reveals the homopropagation rate is higher than the cross‐propagation rate, which is in agreement with the observed isotactic selectivity in the ROP of rac‐lactide. The stereochemistry of the polymerization was also supported by activation parameters. The introduction of unsymmetric ligand H₂L¹ has an effect on stereoslectivity of polymerization. This result may be of interest for the design of multinuclear metal complex catalysts containing functionalized ligands. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2084–2091     

Synthesis and characterization of polytulipalin‐g‐polylactide copolymers

Graft copolymers of poly(tulipalin A) (PT) and poly(DL‐lactide) (PDLLA) (PT‐g‐PDLLA) having various graft lengths and ratios were synthesized by free‐radical copolymerization of α‐methylene‐γ‐butyrolactone (MBL) and PDLLA macromonomers (HEMA‐PDLLA) terminated by 2‐hydroxyethyl methacrylate (HEMA)‐terminated. HEMA‐PDLLA were synthesized by ring opening polymerization (ROP) of DL‐lactide in the presence of HEMA. Both HEMA‐PDLLA and the copolymers were characterized by NMR spectroscopy and gel permeation chromatography (GPC). The thermal properties of the graft copolymers were found to depend on the graft length and the ratio. The copolymers consisting of PDLLA side chains of M_n = 500 Da showed a single T_g between T_gs of the two component polymers, suggesting a miscible state of PT and PDLLA. In contrast, the copolymers consisting of PDLLA side chains of M_n = 1100, 2000, and 7000 Da showed two isolated T_g, suggesting two segregated domains. The AFM phase images of the copolymers supported the single and phase‐separated morphologies for the former and latter systems, respectively. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012