Coordination polymerization of lactides, 5. Influence of lactide structure on the transesterification processes in the copolymerization with ε-caprolactone

The influence of lactide configuration on the chain microstructure of lactide/ε-caprolactone copolymer was investigated. The copolymerization was carried out in the presence of Al(acac)₃, AlEt₃ and ZnEt₂ as initiators. The chain microstructure analysis of the copolymers was performed by means of ¹³C NMR spectroscopy. The effect of lactide configuration on the transesterification and randomization of the copolymer chain was determined. The contribution of transesterification processes in the case of D ,L -lactide was found to be considerably higher than that observed when L ,L -lactide was used.     

Coordination polymerization of lactides, 2. Microstructure determination of poly[(L,L-lactide)-co-(ε-caprolactone)] with 13C nuclear magnetic resonance spectroscopy

A detailed analysis of the structure of poly[(L ,L -lactide)-co-(ε-caprolactone)]

1 IUPAC-preferred name is dilactide:

was performed by means of high resolution ¹³C NMR (75 MHz) spectroscopy. The polyesters were obtained using various comonomer ratios and initiators aluminium tris(acetylacetonate) or the system Al(C₂H₅)₃ + Zn(C₂H₅)₂ + H₂O. Signals in spectra were assigned to appropriate structural sequences. Two types of chain structure containing different sequences and depending on the initiator used in copolymerization were found.     

Polymerization of L-lactide and ϵ-caprolactone in the presence of methyl trifluoromethanesulfonate

The cationic polymerization of L -lactide and ?-caprolactone initiated by methyl trifluoromethanesulfonate in nitrobenzene was studied. The monomer conversion and the average molecular weight decrease when the temperature increases. The kinetics constants for L -lactide and ?-caprolactone polymerizations are, respectively, equal to 1,7·10^?3 min^?1 and 1,60·10^?3 min^?1 at 50°C. The thermodynamic parameters were determined from the temperature dependence of the equilibrium monomer concentration. Thus for L -lactide and ?-caprolactone polymerization the enthalpy and entropy values are, respectively, equal to ΔH(L -lactide) = ?24,9 kJ/mol; ΔS(L -lactide) = ?25 J/(mol·K) and ΔH(?-caprolactone) = ?15,35 kJ/mol; ΔS(?-caprolactone) = ?35 J/(mol·K).     

Oligomerization and copolymerization of γ-butyrolactone — a monomer known as unable to homopolymerize, 1. Copolymerization with ε-caprolactone

Copolymerization of γ-butyrolactone (γBL) with ε-caprolactone (εCL) initiated with aluminium isopropoxide trimer ([Al(OⁱPr)₃]₃, (A₃)) is described. Copolymers with molecular weights (M _n) up to 3 · 10⁴ and containing up to 43 mol-% repeating units derived from γBL are prepared. Their molecular weight is controlled by the concentrations of the consumed comonomers and the starting concentration of initiator {M _n = (86.09 · [γBL]_c + 114.14 · [εCL]_c)/3[Al(OⁱPr)₃] + 60.10}. ¹³C NMR and DSC data are indicative of a pseudoperiodic or random copolymer structure.     

Polymerization of methyl methacrylate with V(acac)3 and VOCl3 in combination with methylaluminoxane

The polymerization of methyl methacrylate (MMA) with V(acac)₃ and VOCl₃ in combination with methylaluminoxane (MAO) was investigated. The obtained polymers have a relatively narrow molecular weight distribution (M?_w/M?_n < 1,5). It was found that the isotactic triad content of the polymers increases with increasing temperature and MAO/V and MAO/MMA mole ratios. The interaction between MMA and MAO has an influence on the tacticity of radically (initiator AIBN) obtained polymers: in presence of MAO the fraction of isotactic triads (mm) is higher. The catalytic activity of MAO-containing catalysts (V(acac)₃/MAO and VOCl₃/MAO) was compared with that of AlEt₃-containing catalysts (V(acac)₃/AlEt₃ and VOCl₃/AlEt₃). It was not much different, but for MAO as cocatalyst the isotacticity of the resulting PMMA's was higher, and the polydispersity lower, than with AlEt₃ as cocatalyst.     

Polylactones, 24. Polymerizations of racemic and meso-D,L-lactide with Al?O initiators. Analyses of stereosequences

Racemic D ,L -lactide and meso-D ,L -lactide were polymerized with four different initiators: Al(isopropoxide)₃, AIEt₃/neopentyl alcohol (1 : 1, mole ratio), AIEt₃/(+)-menthol (1 : 1, mole ratio) and methylaumoxane. Most polymerizations were conducted in xylene at 60, 90 and 120°C, but at 60°C the yields were below 10%. ¹H NMR end-group analyses revealed the formation of alkyl ester end-groups from all Al-alkoxide initiators, in agreement with an insertion mechanism. The highest molecular weights were obtained with the methylalumoxane initiator. The stereosequences of the isolated poly(D ,L -lactide)s were analyzed by ¹H and ¹³C NMR spectroscopy on the basis of tetrad effects. poly(D ,L -Iactide)s prepared from racemic D ,L -lactide suggest a stereospecific polymerization favoring syndiotactic growing steps. In the case of meso-D ,L -lactide all polymerizations follow Bernoullian statistics. Transesterification is poor or absent at temperatures ≤ 120°C. However, random stereosequences may be obtained by bulk polymerizations at 180°C. DSC measurements revealed that the glass transition temperature mainly depends on the molecular weight and not on slight differences in the stereosequences.     

(Co)polymers of L-lactide, 1. Synthesis,thermal properties and hydrolytic degradation

Copolymers of L -lactide with D -lactide

1 According to IUPAC the name dilactide is preferred to lactide.

, glycolide

2 IUPAC name: diglycolide.

, ε-caprolactone and trimethylene carbonate, and networks with spiro-bis-dimethylene-carbonate (2,4,7,9-tetraoxaspiro[5.5]undecane-3,8-dione) were prepared in bulk at standardized polymerization conditions. The properties of the nascent copolymers were evaluated with respect to the nature of the comonomer. Copolymerization with comonomers entailing low glass transition temperature simultaneously reduces the crystallinity. 300 MHz ¹H nuclear magnetic resonance is shown to be a useful technique for the determination of the average monomer sequence lengths in ε-caprolactone and trimethylene carbonate copolymers. The presence of crystallizable L -lactide sequences, due to differences in monomer reactivity, has a large effect on the thermal properties of the copolymer as well as on the long-term degradation characteristics.     

Copolymers of L- and D,L-lactide with 6-caprolactone: synthesis and characterization

L - and D ,L -lactide copolymers with 6-caprolactone were synthesized and characterized in a wide range of compositions. The polymerizations were carried out in batch at 130°C, with Sn(II) octoate as a catalyst, a monomers/initiator mole ratio of 5000/1 and a reaction time of 48 h. The copolymers having from 5 to 40 wt.-% of 6-caprolactone are characterized by higher values of non-reacted monomers and deviations of the polymer composition from the feed ratio. In the same interval of compositions, L -lactide copolymers showed progressive hardening with time at room temperature, attributable to the tendency to crystallization of homopolymeric sequences. This behaviour was more deeply studied in the case of poly(L -lactide-co-6-caprolactone) with 30 wt.-% of 6-caprolactone. ¹H NMR at 300 MHz appeared to be a suitable method for the determination of the monomer sequence.     

Copolymerization of 2,2-dimethyltrimethylene carbonate with 2-allyloxymethyl-2-ethyltrimethylene carbonate and with ε-caprolactone using initiators on the basis of Li,Al, Zn and Sn

The homopolymerization of 2,2-dimethyltrimethylene carbonate ( 1 ), 2-allyloxymethyl-2-ethyl-trimethylene carbonate ( 2 ) and ε-caprolactone ( 3 ) with sec-butyllithium (sec-BuLi), tri-sec-butoxyaluminium (Al(O-secBu)₃), diethylzinc (ZnEt₂) and dibutyldimethoxytin ((Bu)₂Sn(OMe)₂) in toluene at 80°C results in a reaction product consisting of a high-mole-cular-weight polymer and a homologous series of oligomers. The weight ratio polymer/oligomers, under standardized conditions, depends on the type of initiator. The sequence of monomeric units in the copolymers is statistical, for all initiators used. For sec-BuLi and Bu₂Sn(OMe)₂ the distribution arises both from random addition of the monomers to the active chain end and from transesterification, while with Al(O-secBu)₃ and ZnEt₂ the addition of the monomers is non-selective. For poly[(2,2-dimethyltrimethylene carbonate)-stat-(2-allyloxymethyl-2-ethyltrimethylene carbonate)] the carbonyl region of the ¹³C NMR spectrum is suitable for diad analysis, for poly[(2,2-dimethyltrimethylene carbonate)-stat-(ε-caprolactone)] the methyleneoxy region is most indicative. The assigment of the diads was made on the basis of model compounds.     

Polylactones, 39. Zn lactate-catalyzed copolymerization of L-lactide with glycolide or ε-caprolactone

Copolymerization of glycolide (Glyc) and L -lactide (L -Lac) were conducted at 150°C in bulk to prepare random copolyesters. Four resorbable catalysts were used and compared: ZnCl₂, ZnI₂, Zn stearate (ZnSte₂) and Zn lactate (ZnLac₂). Separate time-conversion curves were recorded for glycolide and lactide by means of ¹H NMR spectroscopy. Glycolide polymerized faster than L -lactide under all circumstances, but the difference was smallest when ZnLac₂ was used as catalyst. Sequence analyses were performed using ¹³C NMR spectroscopy. Completely random sequences were never obtained, but the sequences resulting from a catalysis with ZnLac₂ came closest to the desired randomness. Preparative 1 : 1 (mole ratio) polymerizations were conducted with ZnSte₂ and ZnLac₂. High yields were obtained with both catalysts, but ZnLac₂ yielded far higher molecular weights. Furthermore, ZnLac₂-catalyzed copolymerizations were conducted with variation of the Glyc/Lac ratio. It was found that small scale and large scale copolymerizations yield copolyesters having different properties, because the superheating in the early stage of the polymerizations (resulting from glycolide) favors the transesterification. Finally, 1 : 1 copolymerization of ε-caprolactone and L -lactide were studied in bulk at 150°C. Again high yields, high molecular weights and nearly random sequences were obtained.     

Amino-termined poly(L-lactide)s as initiators for the polymerization of N-carboxyanhydrides: synthesis of poly(L-lactide)-block-poly(α-amino acid)s

Poly(L -lactide)-block-poly(L -amino acids) block copolymers were prepared via polymerization of α-amino acid N-carboxyanhydrides with amino-terminated poly(L -lactide)s as macroinitiators. Two types of macroinitiators were used, one with an aminopropoxy head group (number-average molecular weight M _n = 22000) and the other one with a phenylalanine end group (M _n = 18000). The first macroinitiator was obtained by polymerization of (L ,L )-lactide with an initiator prepared in situ from diethylzinc Et₂Zn and N-tert-butoxycarbonyl-1-amino-3-propanol, followed by deprotection of the amino group. The second macroinitiator was obtained by endcapping of poly(L -lactide) with N-tert-butoxycarbonylphenylalanine and deprotection of the amino group. ¹H- and ¹³C NMR spectroscopies confirm the block structure of the copolymers obtained. In differential scanning calorimetry curves only one melting transition characteristic of the poly(L -lactide) block is observed, on further heating decomposition occurs. By thermogravimetry two steps of decomposition are observed, the first one being assigned to the decomposition of the poly(L -lactide) block, and the second one to that of the poly(amino acid) block, by comparison with the thermal behaviour of the corresponding homopolymers.     

Influence of ethyl benzoate on copolymerization of styrene and 1-hexene with the titanium-based systems (α)-TiCl3(H) and TiCl3 · 1/3 AlCl3

The influence of ethyl benzoate (E.B.) on the copolymerization of styrene with 1-hexene, initiated by isospecific Ziegler-Natta catalysts α-TiCl₃(H)-AlEt₃ and TiCl₃ · 1/3 AlCl₃-AlEt₃ (mole ratio Al/Ti = 3), was examined. Ethyl benzoate was found to reduce the activity of the catalysts. In addition it leads, depending on the Ti catalyst, to opposite effects on the apparent reactivity order of the two monomers. Incorporation of styrene into the copolymers is reduced when E. B. is added to TiCl₃ · 1/3 AlCl₃-AlEt₃. On the contrary, a much higher incorporation of styrene is observed with α-TiCl₃(H)-AlEt₃ in the presence of E. B. For this system the calculated reactivity ratio varies strongly with increasing proportion of E. B.: for a mole ratio AlEt₃/E. B. = 3, r_S = 0,94 and r_H = 1,46 and for AlEt₃/E. B. = 2, r_S = 8 and r_H = 0,1. Changes in the stereoregularity of copolymers suggest that E. B. leads to an inhibition of the less stereospecific sites for TiCl₃ · 1/3 AlCl₃-AlEt₃, whereas its addition suppresses the stereospecificity of the α-TiCl₃(H)-AlEt₃ catalyst. Contributions of conventional cationic and/or radical processes to the copolymerization reaction were examined and may be ruled out.     

Functionalized monomers, 3. An investigation of initiation systems used in the anionic and complex-coordination polymerization of oxirane groups in 4-vinylphenyloxirane

The effect of various types of alkoxides and of a complex coordination initiator based on Et₂Al? O? AlEt₂, of the polarity of the medium and of the temperature on the rate of polymerization of 4-vinylphenyloxirane ( 1 ) and on the structure of the resulting polymers was studied as part of the investigation of the activity of the oxirane ring in the structure of ( 1 ) under the conditions of anionic polymerization. In a nonpolar medium the activity of the initiators increases in the series sodium methoxide < potassium tert-butoxide < Et₂Al? O? AlEt₂. The complex Et₂Al? O? AlEt₂ preferentially initiates the polymerization of 1 via the oxirane cycles, giving rise to soluble polymers, the structure of which, determined on the basis of their ¹³C NMR spectra, corresponds to that of polyethers substituted with 4-vinylphenyl groups. In the presence of aluminium isopropoxide the polymerization of 1 via the vinyl groups proceeded with the formation of a solution of a polymer which becomes insoluble after the termination of polymerization. Copolymerization of 1 with phenyloxirane, initiated with the complex Et₂Al? O? AlEt₂, yields soluble copolymers containing a fraction of monomeric units of 1 , higher than that of 1 in the feed.     

Coordination polymerization of lactides, 4. The role of transesterification in the copolymerization of L,L-lactide and ε-caprolactone

Two modes of transesterification in copolymerization of L ,L -lactide with ε-caprolactone in the presence of initiators containing Al and Zn were found. The quantitative analysis of the contributions of both types of transesterification reactions is revealed. The transesterification ability of initiators containing zinc is greater than that of initiators containing aluminium. Decrease of the reaction temperature tends to reduce both the first and the second mode of transesterification, leading to an elongation of blocks in the copolymer chains.     

Polylactones, 47. A-B-A triblock copolyesters and random copolyesters of trimethylene carbonate and various lactones via macrocyclic polymerization

2,2-Dibutyl-2-stanna-1,3-dioxepane (DSDOP)-initiated copolymerizations of trimethylene carbonate (TMC) with β-D ,L -butyrolactone (β-D ,L -BL), δ-valerolactone (δ-VL), ε-caprolactone (ε-CL) or L -lactide (Lac) were performed in chlorobenzene at 80°C. When equimolar mixtures of TMC and a lactone were copolymerized, copolyesters having nearly random sequences were obtained in the case of β-D ,L -BL, δ-VL and ε-CL, whereas an almost perfect homopolymerization was observed for L -lactide. Furthermore, a series of sequential copolymerizations was conducted so that the TMC was polymerized first. After removal of the Bu₂Sn group with 1,2-dimercaptoethane, telechelic A-B-A triblock copolymers having free OH endgroups were obtained from all four lactones. In another series of sequential copolymerizations, the lactones were used as the first monomer. Again a telechelic triblock copolyester was obtained with β-D ,L -BL, whereas copolyesters with largely randomized sequences were isolated, when δ-VL or ε-CL were used as first monomer. With L -lactide as the first monomer an almost perfect homopoly(L -lactide) was obtained and most of the TMC remained unreacted, in close analogy to the attempted random copolymerizations.     

Polylactones, 37. Polymerizations of L-lactide initiated with Zn(II) L-lactate and other resorbable Zn salts

Zn(II) L -lactate (ZnLac₂) was prepared either from ZnO and ethyl L -lactate or with slightly higher optical purity from ZnO and L -lactide. Using water-free ZnLac₂ L -lactide was polymerized in bulk at 120°C or 150°C. Higher yields and higher molecular weights were found at 150°C. The highest number average molecular weights (M?_n around 70 000) were obtained at monomer/initiator (M/I) ratios of 4000. Despite the high reaction temperature the isolated poly(L -lactide)s were 100% optically pure. Analogous polymerizations were also conducted with Zn(II) L -mandelate or Zn(II) stearate with inferior results. Poor yields and molecular weights were found, when zinc glycolate salt was used as catalyst. Furthermore, numerous polymerizations were conducted with ZnCl₂, ZnBr₂ or ZnI₂ as initiators. Again poly(L -lactide)s with 100% optical purity were isolated, but most molecular weights were lower and never higher than those obtained with ZnLac₂. Therefore, ZnLac₂ proved to be the most favorable and fully resorbable (biocompatible) initiator of this study. Finally, the combination of ZnLac₂ with a primary alcohol, which plays the role of a coinitiator, allows a broad variation of the molecular weight and the introduction and modification of an ester endgroup. This approach also allows the incorporation of bioactive alcohols such as α-tocopherol, stigmasterol or testosteron in the form of covalently bound ester endgroups.     

Macrocycles, 6. MALDI-TOF mass spectrometry of tin-initiated macrocyclic polylactones in comparison to classical mass-spectroscopic methods

2,2-Dibutyl-2-stanna-1,3-dioxepane ( 1 ) was used as cyclic initiator for the (ring expansion) polymerization of L -lactide, ε-caprolactone, β-D,L -butyrolactone and 1,4-dioxane-2-one. The polymerizations were conducted in bulk at 80 and 120°C with monomer/initiator mole ratios of 6, 10, and 20. When the resulting macrocyclic oligolactones (DP-6) were subjected to mass spectroscopy total thermal degradation via transesterification took place and the original molecule peak was not detectable. In the case of fast atom bombardment (FAB) and matrix-assisted laser desorption ionization/time of flight (MALDI-TOF) measurements the alcohols or phenols used as matrix materials cleave the Sn—O bonds of the macrocycles and the MALDI-TOF mass spectra showed the molecule peaks of OH-terminated telechelic polylactones. Stoichiometric insertion of γ-thiobutyrolactone into the Sn—O bonds yielded stabilized macrocyclic polylactones containing the less reactive Sn—S bonds, which allowed the recording of FAB and MALDI-TOF mass spectra showing the molecule peaks of intact tin-containing cyclic oligo- and polylactones.     

Ring-closing depolymerization of poly(ε-caprolactone)

Ring-closing depolymerization of poly(ε-caprolactone) in toluene solution with catalytic amounts of Bu₂Sn(OMe)₂ results in a mixture of cyclic oligomers, the equilibrium concentration of which corresponds to the concentration predicted according to the Jacobson-Stockmayer theory. ε-Caprolactone, however, is absent from this reaction mixture. When poly(ε-caprolactone) is depolymerized in the melt at 260°C in the presence of a catalyst, beside ε-caprolactone its cyclic dimer and trimer are distilled off. The composition is dependent on the catalyst used, e.g., Bu₂Sn(OMe)₂ produces 95,4 wt.-% of ε-caprolactone and 3,7 wt.-% of its dimer while with Ti(OiPr)₄ (iPr: isopropyl) 51,4 wt.-% ε-caprolactone and 49,6 wt.-% of its dimer are obtained. Block copolymers containing poly(ε-caprolactone) result in a similar product distribution, however, the second block may influence the rate of decomposition. The activation energy of the catalysed (0,5 mol-% Bu₂Sn(OMe)₂) and uncatalysed ring-closing depolymerization were determined to be 63 kJ/mol, and 87 kJ/mol, respectively.     

Polyoxirannes et polythiirannes comportant un centre chiral dans la chaîne latérale, 3. Polymérisation de (2RS,3S)- et (2R,3S)-épithio-1,2 méthyl-3 pentanes

Different mixtures of (2R,3S)- and (2S,3S)-1,2-epithio-3-methylpentanes ( 1 ) were polymerized using anionic (sodium), stereoselective (ZnEt₂/H₂O, cadmium tartrate) and stereoelective (ZnEt₂/R(?)-3,3-dimethyl-1,2-butanediol (R(?)DMBD)) initiator systems. In all cases a preferential consumption of the (2R,3S)-diastereoisomer was observed. The reactivity ratios of the two diastereoisomers (2R,3S) and (2S,3S) in the copolymerization, initiated with sodium, were determined: r₁ = 1,40 ± 0,08 and r₂ = 0,54 ± 0,05. A low but significant diastereoisomeric enrichment was observed in stereoselective polymerization. It results from different consumption rates for the two diastereoisomers on enantiomorphic sites of the initiator. The election obtained with a stereoelective system is much higher. It is, however, not very different from that observed with isopropylthiiranne, which does not bear an additional chiral center in the alkyl substituant.     

Structure of active species in the cationic polymerization of β-propiolactone and ε-caprolactone

The study of the structure of the end-groups of poly(β-propiolactone) and poly(ε-caprolactone) by ¹H, ³¹P{¹H} NMR, and IR spectroscopy revealed that the mode of ring scission in the monomer in the first propagation step depends on the initiator used. In the initiation of the β-propiolactone and ε-caprolactone polymerization with halonium salts ((CH₃)₂Br^⊕SbF₆^? or (CH₃)₂I^⊕SbF₆F₆^?) the exocyclic oxygen atoms in the monomer molecules are attacked and oxonium ions are formed. The propagation steps proceed with alkyl-oxygen bond scission in the active center and with attack at the exocyclic oxygen atom in the subsequent monomer molecule. Thus, the tertiary oxonium ions are the exclusive active species from the very beginning of the polymerization. Acylium cations (CH₃CO^⊕ SbF₆^? or CH₃CH₂CO^⊕SbF₆^?) as initiators attack both exo-and endocyclic oxygen atoms in the monomers and the initiation proceeds with almost equal proportions of alkyl-oxygen and acyl-oxygen bond scission. Thus, in the polymerization of β-propiolactone the concentration of the acylium growing species decreases with increasing number of propagation steps and finally the tertiary oxonium ions are becoming the exclusive growing species. In the polymerization of ε-caprolactone the active acylium centers apart from propagation participate also in side reactions leading to the cleavage of a proton.