Horseradish Peroxidase Mediated Free Radical Polymerization of Methyl Methacrylate

This paper reports the free radical polymerization of methyl methacrylate (MMA) catalyzed by horseradish peroxidase (HRP). A novel method was developed whereby MMA polymerization can be carried out at ambient temperatures in the presence of low concentrations of hydrogen peroxide and 2,4-pentanedione in a mixture of water and a water-miscible solvent. Polymers of MMA formed were highly stereoregular with predominantly syndiotactic sequences (syn-dyad fractions from 0.82 to 0.87). Analyses of the chloroform-soluble fraction of syndio-PMMA products by GPC showed that they have number-average molecular weights, Mn, that range from 7500 to 75 000. By using 25% v/v of the cosolvents dioxane, tetrahydrofuran, acetone, and dimethylformamide, 85, 45, 7 and 2% product yields, respectively, resulted after 24 h. Increasing the proportion of dioxane to water from 1:3 to 1:1 and 3:1 resulted in a decrease in polymer yield from 45 to 38 and 7%, respectively. Increase in the enzyme concentration from 70 to 80 and 90 mg/mL resulted in increased reaction kinetics. By adjustment of the molar ratio of 2,4-pentanedione to hydrogen peroxide between 1.30:1.0 and 1.45:1.0, the product yields and Mn values were increased. On the basis of the catalytic properties of HRP and studies herein, we believe that the keto−enoxy radicals from 2,4-pentanedione are the first radical species generated. Then, initiation may take place through this radical or by the radical transfer to another molecule

Biocatalytic Route to Well-Defined Macromers Built around a Sugar Core

By using 4-C-hydroxymethyl-α-d-pentofuranose as the sugar core and lipase-catalyzed transformations, a macromer was constructed with exceptional control of substituent placement around the carbohydrate core. The key synthetic transformations performed were as follows:  (1) selective lipase-catalyzed acrylation along with prochiral selection of 4-C-hydroxymethyl-1,2-O-isopropylidene-α-d-pentofuranose (diastereomeric excess up to 93%); (2) the ring-opening of ε-caprolactone, ε-CL, from the remaining primary hydroxyl group to give an acryl-sugar capped macromer (Mn = 11 300, Mw/Mn = 1.36, initiator efficiency 50−55%, O-isopropylidene group at the sugar core without any substantial loss in macromer molecular weight; and (5) homopolymerization of the corresponding macromer. In principle, the method developed is flexible so that it can be used to generate a wide array of unusual macromers and heteroarm stars. In the absence of biocatalytic transformation, such structural control would be extremely difficult or currently impossible to obtain

Versatile Copolymers from [l]-Lactide and [d]-Xylofuranose

The new monomer 1,2-o-isopropylidene-[d]-xylofuranose-3,5-cyclic carbonate (IPXTC) was prepared. The organometallic catalysts AlR3−H2O (R = ethyl, isobutyl), ZnEt2−H2O, and Sn(Oct)2 were evaluated for the copolymerization of [l]-lactide ([l]-LA) with IPXTC. This work showed that Sn(Oct)2 was preferred for the formation of high molecular weight copolymers. For example, a copolymerization ([l]-LA/IPXTC = 83:17 mol/mol) at 120 °C for 6 h gave poly([l]-LA-co-7 mol % IPXTC) with an Mn and polydispersity (Mw/Mn) of 78 400 and 1.9, respectively. The comonomer reactivity ratios were 4.15 and 0.255, respectively, for [l]-LA and IPXTC copolymerizations conducted at 120 °C, M/C = 200, and Sn(Oct)2 as catalyst. Structural investigations by NMR revealed that [l]-LA/IPXTC copolymers had short average IPXTC repeat unit segment lengths. Increased copolymer IPXTC content resulted in products with lower melting transition temperatures but higher glass transition temperatures. To obtain hydroxyl functionalized P([l]-LA) copolymers, the pendant IPXTC ketal protecting group was removed. The deprotection was performed in CH2Cl2 using CF3COOH/H2O without substantial molecular weight decrease. Hence, an efficient route has been developed to synthesize high molecular weight PLA-based copolymers that consist of [l]-lactic acid and [d]-xylofuranose repeat units. The [d]-xylofuranose repeat units have vicinal diol groups that will facilitate further functionalization and modification of these copolymers. The “tailorability” of the new copolymers is expected to be of great value for the development of important new bioresorbable medical materials

<i>Candida</i><i>a</i><i>ntarctica</i> Lipase B-Catalyzed Transesterification: New Synthetic Routes to Copolyesters

The catalysis by an immobilized preparation of Candida antartica lipase B (Novozyme-435) of transesterification or transacylation between poly(ε-caprolactone), PCL, and poly(ω-pentadecalactone), PPDL, was studied. These reactions between macromolecules were performed in toluene or without solvent (bulk) at 70−75 °C. In bulk, for PCL (Mn = 9.2 × 103) and PPDL (Mn = 4.3 × 103), PDL*CL/CL*PDL diad sequences were observed by 13C NMR within 30 min. By increasing the reaction time from 30 to 60 min, the average-sequence length of CL (μCL) and PDL (μPDL) repeat units along chains decreased from 18 to 2 and 23 to 2, respectively. Transacylation between PCL (Mn = 44.0 × 103, PDI 1.65) and PPDL (Mn = 40.0 × 103, PDI 1.71) was also studied. To reduce diffusion constraints, the reaction was performed in toluene. Multiblock copolymers (Mn = 18.2 × 103 g/mol, PDI 1.92) were formed after 1 h. By increasing the reaction time to 30 h, random Poly(CL-co-PDL) (Mn = 31.2 × 103 g/mol, PDI 1.87) was formed. Transacylation reactions between polyesters are believed to involve intrachain cleavage by the lipase to form an enzyme-activated-chain segment, followed by reaction of this activated segment with the terminal hydroxyl unit of another chain. This hypothesis is supported by the finding that acetylation of chain end hydroxyl units causes a large decrease in the rate of transacylation between PCL and PPDL chains

Protease Catalyzed In Situ C-Terminal Modification of Oligoglutamate

One-pot biotransformations gave oligo(γ-l-Et-Glu) decorated with selected amine-functionalized end-groups at C-termini. Motivations for this work were to (i) control the end group structure of peptides synthesized by protease-catalyzed peptide synthesis and (ii) incorporate end-groups that can be used directly or after further modification as polymerizable entities. Papain, bromelain, α-chymotrypsin, Multifect P-3000, and Purafect prime 4000 L were used as catalysts for oligomerization of γ-l-(Et)2-Glu in the presence of monofunctional amines. The series of amine nucleophiles (NH2-R, acyl acceptors) studied mimic phenylalanine in that they possess aromatic rings linked to amine groups by one or more methylenes. Generally, addition of increased quantities of NH2-R from 0 to 30, 50, and 70 mol % with respect to γ-l-(Et)2-Glu results in decreased % yield, but increased mol % of NH2-R end-capped oligo(γ-l-Et-Glu)-NH-R (determined by NMR). Irrespective of the protease used, 2-thiophene methyl amine (TPMA) gave the highest fraction of oligo(γ-l-Et-Glu)-NH-R chains. For example, using Multifect P-3000 and a feed ratio of TPMA-to γ-l-(Et)2-Glu of 7:3, >90 mol % of oligopeptides formed had TPMA C-terminal groups. With all five proteases studied herein, l-phenylalanine and l-histidine did not produce end-capped oligo(γ-l-Et-Glu). In contrast, l-phenylalanine analogs benzylamine (BzA) and l-phenylalaninol (F-OH), both of which lack the α-carboxyl group, gave substantial quantities of oligo(γ-l-Et-Glu)-F-OH or -BzA chains. Hence, the results of this study prove that the promiscuity of proteases used herein can be exploited to create a diverse family of desired end-functionalized oligopeptides. MALDI-TOF spectra recorded of oligo(γ-l-Et-Glu) with amine nucleophiles showed molecular ions that affirmed the formation of corresponding NH2-R functionalized oligo(γ-l-Et-Glu)

Functionalized Polylactides: Preparation and Characterization of [l]-Lactide-<i>co</i>-Pentofuranose

The new monomer 1,2-O-isopropylidene-3-benzyloxy-pentofuranose-4,4‘-cyclic carbonate (IPPTC) was prepared. IPPTC has both a ketal-protected diol and a benzyl ether-protected hydroxyl. Thus, these two sets of hydroxyl groups can be independently deprotected to give IPPTC repeat units with one, two, or three free hydroxyl groups. Stannous octanoate at 130 °C was used for the copolymerization of [l]-LA with IPPTC. When fLA/fIPPTC was 91/9, the percent yield, Mn, and percent incorporation of IPPTC units were 78%, 77 800 g/mol, and 4 mol %, respectively. By the method of Fineman and Ross, the [l]-LA and IPPTC comonomer reactivity ratios were 8.6 and 0.51, respectively. Relative to poly([l]-LA), incorporation of IPPTC units into [l]-LA/IPPTC copolymers gave products that are lower melting (112 °C, 14 mol % IPPTC) and have higher thermal stabilities and higher glass transition temperatures (69 °C, 100 mol % IPPTC). The liberation of hydroxyl pendant groups by the selective removal of the benzyl ether, the ketal groups, or both was possible without substantial loss in the product molecular weight. The removal of the protecting groups of poly([l]-LA-co-4.0 mol % IPPTC) did not change the Tm value (160 °C) but did alter the copolymer crystallization kinetics and thermal stability

Copolymerizations of ω-Pentadecalactone and Trimethylene Carbonate by Chemical and Lipase Catalysis

Copolymerizations of ω-pentadecalactone (PDL) with trimethylene carbonate (TMC) were studied using chemical and enzyme catalysts. By using stannous octanoate, methylaluminoxane (MAO), or aluminum isopropoxide, copolymerizations of PDL with TMC with 1:1 feed ratio resulted in either homo-polyTMC or PDL/TMC block copolymers. These catalysts polymerize TMC more rapidly than PDL. A copolymerization catalyzed by MAO gave poly(TMC-co-16 mol % PDL) with Mn 26.4 × 103g/mol and randomness numder (B) about 1.1. The sodium ethoxide-catalyzed copolymerization led to products with low Mn (3) but nearly random sequence distribution. The copolymerization of PDL with TMC was also studied by using lipase catalysts. Of the six lipases evaluated for PDL/TMC copolymerizations in toluene at 70 °C, an immobilized form of lipase B from Candida antarctica (Novozyme-435) was preferred. Changing the PDL/TMC comonomer feed ratio from 1:10 to 10:1 (mol/mol) provided copolymers that ranged in Mn and PDL mol % from 7.3 × 103 to 25.2 × 103 and 28 to 88, respectively. In contrast to the chemical catalyst systems, Novozyme-435 catalysis showed that PDL was consumed more rapidly than TMC. Also, in contrast to most of the chemical catalysts, 1H and 13C NMR analyses showed that the copolymers from Novozyme-435 catalysis were able to give a random distribution of the repeat units at extended reaction times. Furthermore, in contrast to TMC polymerization in the presence of preformed polyPDL with MAO, Novozyme-435 catalyzed polymerization led to random copolymers

Probing Water-Temperature Relationships for Lipase-Catalyzed Lactone Ring-Opening Polymerizations

Polymerizations of ε-CL catalyzed by Novozyme-435 (immobilized Lipase B from Candida antarctica) were studied at temperatures between 20 and 108 °C. The monomer conversion to polymer was remarkably rapid at ambient temperature. At 20 °C by 7 h, ε-CL conversion and product Mn were >97% and 17 800, respectively. Contrary to previous reports, the number of chains formed, as well as the product molecular weight, was almost identical for polymerizations at constant enzyme water content between 60 and 108 °C. Thus, differences in reaction temperature over a 48 °C range did not “free” water from “bound” states so that it could function for chain initiation. At 60 °C, variation in the enzyme water content from 0.6 to 1.9% increased the number of chains formed but did not change the polymerization propagation kinetics. Therefore, the enzyme water content and not the reaction temperature regulated the product molecular weight. In contrast, at 108 °C, an increase in the reaction water content from 0.6 to 1.8% increased both the number of chains and the polymerization propagation kinetics. Explanations for these differences in behavior as a function of temperature and water contents are discussed

Mild, Solvent-Free ω-Hydroxy Acid Polycondensations Catalyzed by <i>Candida </i><i>a</i><i>ntarctica</i> Lipase B

Immobilized Candida antarctica Lipase B (Novozyme-435) was studied for bulk polyesterifications of linear aliphatic hydroxyacids of variable chain length. The products formed were not fractionated by precipitation. The relative reactivity of the hydroxyacids was l6-hydroxyhexadecanoic acid ≈ 12-hydroxydodecanoic acid ≈ 10-hydroxydecanoic acid (DPavg ≅ 120, Mw/Mn ≤ 1.5, 48 h, 90 °C) > 6-hydroxyhexanoic acid (DPavg ≅ 80, Mw/Mn ≤ 1.5, 48 h, 90 °C). Remarkable improvements in molecular-weight buildup resulted from leaving water in the reaction. By 4 h, without application of vacuum, the DPavg for 12- and 16-carbon hydroxyacids was about 90. In contrast, with identical substrates and water removal, the DPavg at 4 h was about 23. Large differences in the molecular-weight build up of 12-hydroxydodecanoic acid were observed for catalyst concentrations (%-by-wt relative to monomer) of 0.1, 0.5, 1, and 10. Nevertheless, by 24 h, with 1% catalyst containing 0.1% lipase, poly(12-hydroxydodecanoic acid) with Mn 17 600 was formed. For 12-hydroxydodecanoic acid polymerization at 90 °C, the catalyst activity decreased by 7, 18, and 25% at reaction times of 4, 24, and 48 h, respectively. Furthermore, the retention of catalyst activity was invariable as a function of the substrates used

Synthesis, Modification, and Characterization of l-Lactide/2,2-[2-Pentene-1,5-diyl]trimethylene Carbonate Copolymers

This paper explores the copolymerization of l-lactide (l-LA) with 2,2-[2-pentene-1,5-diyl]trimethylene carbonate (cHTC). Since cHTC has a cyclohexene group, this provided a route for preparing poly(lactic acid), (PLA), based chains decorated with controlled quantities of CC substituents. Ring-opening copolymerizations of l-LA with cHTC were successfully conducted in bulk by using AlR3−H2O (R = ethyl, isobutyl), Al(OiPr)3, ZnEt2−H2O and Sn(Oct)2 as catalysts. Comparison of these copolymerizations showed that the Sn(Oct)2 catalyst system gave copolymers of relatively higher molecular weight. Increasing the reaction time of Sn(Oct)2 catalyzed copolymerizations from 6 to 24 h resulted in higher copolymer cHTC content and yield but lower copolymer molecular weight. Variation of the comonomer feed ratio was useful in regulating the content of cyclohexene pendant groups in the copolymer. However, regardless of the catalyst used, the mole percent of cHTC incorporated into the copolymer was lower than that used in the monomer feed. Determination of the comonomer reactivity ratios for Sn(Oct)2 catalyzed copolymerizations gave values of 8.8 and 0.52 for l-LA and cHTC, respectively. All gel permeation chromatography (GPC) traces showed unimodal molecular weight distributions. Determination by 13C-NMR of the copolymer sequence fractions HLL, LLL, LLH, HLH, HL, and LH (H = cHTC units, L = l-lactyl units) showed that they were close to those calculated by assuming a Bernoulli statistical propagation. On the basis of these results and the effects of reaction conditions on the copolymer sequence distribution, a mechanism which involves insertion of cHTC into the polymer chain was proposed. Studies by differential scanning calorimetry (DSC) showed that cHTC units in the copolymers disrupted ordering of the l-PLA crystalline phase. Furthermore, the glass transition temperatures (Tg) ranged from 60 (l-PLA) to 33 °C (P(cHTC)). Conversion of CC to epoxy side groups was successfully carried out by using 3-(chloroperoxy)benzoic acid at room temperature with only small decreases in copolymer molecular weight