Functional Polyesters with Pendant Double Bonds Prepared by

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Functional Polyesters with Pendant Double Bonds Prepared by Coordination−Insertion and Cationic Ring-Opening Copolymerizations of ε‑Caprolactone with Renewable Tulipalin A

Martin Danko,† Malgorzata Basko,*,‡ Slávka Ď urkácǒ vá,† Andrzej Duda,‡ and Jaroslav Mosnácě k*,† †

Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 845 41 Bratislava, Slovakia Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland

‡

S Supporting Information *

ABSTRACT: The synthesis of functional copolyesters that contain pendant double bonds in their structures is reported by ringopening copolymerization of renewable monomer Tulipalin A (α-methylene-γ-butyrolactone, MBL) and ε-caprolactone (CL) using either coordination−insertion or monomer activated mechanisms. Aluminum tris(isopropoxide) (Al(OiPr)3) was successfully used as a cheap and commonly available catalyst for the coordination−insertion ring-opening copolymerization, and functional copolyesters with dispersities below 1.2 and with contents of MBL units in the copolymer up to 25 mol % were prepared. In addition, linear or multiarm copolymers of MBL with CL were synthesized by cationic ring-opening copolymerization that was conducted using protic acid as a “metal-free” catalyst in combination with mono- or multihydroxyl alcohols as initiators. The molecular characteristics and composition of the functional copolyesters were determined by GPC, NMR, two-dimensional NMR, and MALDI-TOF spectroscopy. The effect of MBL content on thermal properties of the copolyesters was investigated using DCS and TGA analyses. The availability of resulting functional copolyesters containing pendant double bonds toward postfunctionalization was demonstrated by either thermal or photochemical thiol−ene reactions with benzothioxanthene fluorophore or N-acetylcysteine, respectively.

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INTRODUCTION

polymer chain for the covalent immobilization of bioactive molecules such as drugs, peptides, and proteins.4 Therefore, current research efforts are frequently focused on synthetic strategies for the preparation of PCL with functional groups that allow its fabrication into advanced functional materials or chemical, thermal, or photochemical postfunctionalization. There are several studies in which one reactive group per polymer chain was introduced at the one end of PCL chains providing end-functionalized polymers, which were subsequently used either as macromonomers in radical (co)polymerization for the synthesis of comb-like and grafted polymer structures5,6 or for postfunctionalization with biomolecules.7,8 The α,ω-functionalized PCL macromonomers

Aliphatic polyesters occupy a forefront position among polymeric materials, mostly due to their unique combination of useful physicochemical properties with biocompatibility and biodegradability.1 Poly(ε-caprolactone) (PCL) is one of the most extensively investigated degradable polymers that have properties superior to many other polyesters and can be readily synthesized at low costs. Most noteworthy to mention are its low melting temperature (Tm = 60 °C), high thermal stability, high elongation at break, and low modulus. These properties make this polymer attractive for various applications ranging from scaffolds for tissue engineering to food packaging materials.2,3 However, PCL has limitations in terms of its functionality and specific physical properties. In practice, PCL very often cannot meet the requirements of particular applications due to its high hydrophobicity, improper degradation profile, and/or lack of reactive centers in the © XXXX American Chemical Society

Received: March 2, 2018 Revised: April 24, 2018

A

DOI: 10.1021/acs.macromol.8b00456 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Structure of PMBL Repeating Units in the Polymer Chain Formed as a Result of Vinyl-Addition Polymerization (PMBLVA, Route A) or Ring-Opening Polymerization (PBMLROP, Route B)

the polymerization, ΔHp, together with negative entropic factor, coming from the fact that polymerization of most of the monomers is accompanied by an entropy decrease, provides the positive Gibbs free energy in ΔGp = ΔHp − TΔSp for ROP of γ-BL. Thus, its homopolymerization should be thermodynamically forbidden. Indeed, for γ-BL the equilibrium concentration [BL]eq is equal to approximately 3.3 × 103 mol L−1, whereas the monomer concentration in bulk is 13 mol L−1. This explanation has been repeated in all studies related to the copolymerization of the γ-BL monomer and its derivatives and was summarized by Duda.37,38 Very recently, Olsen et al. 39 applied a thermodynamically defined “ceiling” temperature Tc for the evaluation of different monomers and their propensity to undergo ROP. Tc is independent of the catalytic system used but is dependent on concentration. Hence, a preselected Tc value acts as an independent way to describe each monomer’s thermodynamic polymerizability. Duda et al.40,41 showed that the concentration term contribution may outweigh the sum of enthalpic and entropic contributions and may give a negative ΔGp. On the basis of this, they published the formation of PBL oligomers, up to 10 units, using aluminum tris(isopropoxide) trimer as a catalyst (Al(OiPr)3 = A3). Subsequent research has led to successful polymerization of γ-BL using a very active catalyst and performing the polymerization at lower temperature (−40 °C).42,43 By employing lanthanide or yttrium complexes or metal-free phosphazene base organo-catalyst, the ROP of γ-BL proceeded smoothly to high conversions (90%) under ambient pressure producing PBL with Mn up to 30 kg mol−1 and with controlled linear and/or cyclic topologies. In light of these results, the ROP of MBL that led to unsaturated polyester synthesis has recently been performed successfully (Scheme 1, route B), and its low ceiling temperature (Tc = −52 °C for [MBL]0 = 5.0 M) was determined.44 Additionally, polyesters from both γ-BL and MBL can be fully recycled back to the monomers in the presence of a simple catalyst, thus establishing its complete chemical recyclability.44 The strategy to force γ-BL toward ROP using copolymerization with other lactones was also employed.45 For example, γBL was copolymerized with ε-CL while various catalysts, such as Al(OiPr)3,41 tin(II) octoate/ethanolamine, aluminum Schiff’s base complex HAPENAlOiPr, and lithium diisopropyl amide (LDA), have been used.46 Most successful BL incorporation in P(BL-co-CL) was recently achieved by lanthanum-based catalyst, La[N(SiMe3)2]3, for coordination−insertion ROP, providing copolymers with 42 mol % of γ-BL units and with Mn of 20.6 kg mol−1.47 The ring-opening copolymerization of MBL with CL to prepare functional PCL with pendant vinyl groups has also been studied.47,48 A successful ring-opening copolymerization of the lactones has been claimed in the presence of bismuth(III) trifluoromethanesulfonate at 130 °C in bulk.48 Conducting copolymerization at different molar ratios of CL to MBL, products with molar masses in the range of 4.1−13.7 kg mol−1 and different compositions were obtained. Subsequently, bicomponent networks with shape

were also prepared and used for preparation of cross-linked polymer networks with enhanced mechanical properties.9 The introduction of a higher number of pendant reactive groups distributed along the PCL chain was achieved using three different strategies: (i) functionalization of a preformed polymer, (ii) by copolymerization with functional comonomer, or (iii) by polymerization of synthesized functional caprolactone. In the first approach, the functional groups were introduced along the PCL backbone by treating the polymer with strong bases in which it benefits from the ability of the acidic proton in the α-position of the carbonyl group to generate an enolate anion that is further reacted with electrophiles to obtain the pendant functional group.10,11 In the second approach, functional groups were introduced by copolymerization of ε-caprolactone (CL) with comonomers that contained desired reactive moieties.12−15 For example, functional PCL copolymers with (meth)acryloyl or pyridyl disulfide groups were obtained by copolymerization of CL with acryloyl, methacryloyl, or pyridyl disulfide-functionalized cyclic carbonates.16,17 The acryloyl groups were subsequently used for Michael type conjugate addition with varying thiol-containing molecules such as 2-mercaptoethanol, 3-mercaptopropanoic acid, cysteamine, cysteine, and arginine-glycine-aspartic acidcysteine peptide under mild conditions, to provide biodegradable materials with vastly different functionalities (e.g., hydroxyl, carboxyl, amine, amino acid, and peptides) and properties. Introduction of functional pendant groups along the PCL chain by (co)polymerization of allyl18 or α-benzyl carboxylate19 derivatives of CL is an example of the third approach. Recently, there has been an increased interest in the preparation of polymers using compounds that originate from renewable resources. Tulipalin A (α-methylene-γ-butyrolactone, MBL), found naturally in tulips, seems to be a promising monomer thanks to two of its functional groups, including a lactone ring and a highly reactive exocyclic double bond. Chaingrowth polymerization (addition polymerization) of MBL, which involves linking monomer molecules through vinyl double bonds, has been studied in the presence of various types of initiators that are typical for radical,20−28 anionic,29 zwitterionic,30,31 group-transfer,32,33 organocatalytic,34,35 and coordination36 polymerization methods, which exclusively provide polymers that retain a pendant five-membered lactone ring along the polymer chains (Scheme 1, route A). The prepared PBML is an atactic, amorphous material that is insoluble in most common organic solvents and has a high glass transition temperature (Tg = 195 °C) that is characteristic for structural rigidity of the chain segment. The five-membered lactone ring is known by its high stability (low ring strain energy). The γ-butyrolactone (γ-BL) monomer, as the simplest representative of this family, is known to be a nonpolymerizable (or better, hardly polymerizable) monomer by ring-opening polymerization (ROP), which leads to polyester formation. The positive enthalpy of B


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Table 1. Reaction Conditions and Results of Coordination−Insertion Ring-Opening Copolymerizations of MBL with CL Performed Using Al(OiPr)3 as Both the Catalyst and Initiator in Toluene as a Solvent entry

[M]0a (mol L−1)

[MBL]0/[Cl]0a

[Al] × 10−3 (mol L−1)

T (°C)

tb (h)

f MBLc (mol %)

FMBLc (mol %)

Mn(th)d × 103 (g mol−1)

Mn(GPC)e × 103 (g mol−1)

Đe

1A 2A 3A 4A 5A 6Af 7A 8A 9A 10A 11A 12A 13A

2.0 2.1 2.0 3.0 6.6 9.3 1.3 3.7 2.0 2.0 6.0 9.9 6.0

0.9 0.9 0.25 2 5 10.6 6.5 0.37 1 1 5 10 5h

2.7 0.6 2.9 2.7 2.7 2.2 2.7 2.5 3.3 1.7 3.3 2 6.7

rt rt rt rt rt rt rt 0 0 0 0 0 rt

4 120 1 2 5 4 24 6 7 170 48 48 4

47.4 47.6 20.0 66.6 83.3 91.4 86.6 30.3 50.0 50.0 83.3 90.9 83.3

5 5 1 8 13 17 14 3 7 7 16 25 22

14.7 72.8 21.2 15.2 17.6 16.3 2.8 42.2 12.2 24.2 13.3 22.0 7.1

25.0 60.8 43.2 17.6 20.5 10.2 4.4 (3.7)g 87.7 18.7 45.3 13.9 15.5 7.04

1.11 1.51 1.25 1.27 1.32 1.26 1.15 1.16 1.17 1.21 1.19 1.12 1.14

a

[M]0 and [MBL]0/[CL]0 stay for total feed monomer concentration and relative ratio of feed MBL and CL monomers concentrations, respectively. All polymerizations were run until the CL conversion reached almost 100%. cf MBL and FMBL stay for feed content of MBL and content of MBL in copolyester based on 1H NMR, respectively. dCalculated based on the equation Mn(th) = 60 + ([CL]0/3 × [Al]) × 114 × (conv CL/100) + ([MBL]0/3 × [Al]) × 98 × (FMBL/100). eBased on GPC performed in THF as eluent and determined according to PS standards. fPerformed in bulk. gCalculated from NMR based on the equation Mn(NMR) = 60 + [(I(4.05) + I(4.15))/I(5.00)] × 114 + [(I(4.20) + I(4.29))/I(5.00)] × 98. h CL monomer was added dropwise to MBL solution with final concentrations of 5.0 mol L−1 of MBL and 1.0 mol L−1 of CL. b

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memory effects were prepared through the chemical crosslinking of unsaturated polyesters and methacrylates. However, it was later indicated that the polymerization product obtained from the described conditions contained a mixture of P(MBLco-CL) copolyester formed by the ring-opening polymerization and the PMBL homopolymer formed by the vinyl-addition process (PMBLVA).47 A better-defined initiating system based on f-block lanthanide (Ln) catalysts, Ln[N(SiMe3)2]3, in which Ln = La, Sm, and Nd, was proposed by Hong.47 According to this paper, under the reaction conditions (relatively nonpolar solvent and low temperature, 0 or −20 °C), the effective copolymerization of MBL and CL was achieved without any detectable PMBLVA homopolymer. Thus, functional polyesters with up to 40 mol % of incorporated double bond functionalities along the polymer chain were prepared. However, due to the high activity of the catalysts, the copolyesters with broader dispersities (in the range of 1.47− 1.9) were obtained. In comparison to previously used Ln-based catalysts, in this article we report a complex study of ROP of CL with MBL using commonly available and cheaper catalysts such as aluminum tris(isopropoxide) ((Al(OiPr)3) for coordination− insertion ROP and triflic acid or diphenyl phosphate for cationic ROP. Unlike previously used catalysts, using (Al(OiPr)3 allowed for the preparation of well-defined functional polyesters with dispersities below 1.2, while MBL content in the copolymer as high as 25 mol % was still obtained. For the first time we also report cationic copolymerization according to an “activated monomer” mechanism in the presence of alcohol as an initiator and protic acid as a catalyst for the studied system. Application of a “metal-free” catalyst, such as diphenyl phosphate, provided a slightly broader dispersity (Đ = 1.4− 1.7); however, the versatility of this system is illustrated by combination with multifunctional alcohols, allowing for the preparation of telechelic or star functional oligoesters, not published so far. Finally, the possibility of postfunctionalization of the vinyl groups that are present in the copolymers was proven by reactions with thiols.

EXPERIMENTAL SECTION

Coordination−Insertion Ring-Opening Polymerization. Polymerizations were performed using Al(OiPr)3 as a catalyst. Either toluene or THF was used as solvent, or bulk polymerization was employed. Polymerization mixtures were prepared in sealed glass ampules using a standard high-vacuum technique. A break-seal equipped with a glass hammer and containing Al(OiPr)3/solvent solution and a break-seal with a monomer were sealed to the reacting glass vessel. The concentrations of reactants and reaction conditions are given in Table 1. The solvent was distilled to this vessel under a vacuum (remained amount for total volume), and the vessel was closed after freezing in liquid nitrogen. Break-seals were broken, and all components were mixed at the temperature required for a particular polymerization (see Table 1; rt was in the range of 19−21 °C). In the case of bulk polymerization, the solvent was evaporated on the vacuum line before sealing the ampule. The resulting polymers were precipitated into cold methanol, filtered, washed several times with methanol, and dried under a vacuum for several hours. In the case of copolyester with highest content of MBL (25 mol %), the precipitated fine dispersion of polymer was separated by centrifugation. Cationic Ring-Opening Polymerization. All experiments were performed under an argon atmosphere. After purging a roundbottomed flask with argon and closing it with a rubber septum, an MBL and dried DCM were added with a syringe. Next, an alcohol and protic acid were introduced into the stirred reactive mixture with a syringe. CL was slowly introduced over the course of 2 h to maintain a low instantaneous concentration. For copolymerizations conducted at low temperature, the reactive mixture was prepared in an ampule closed with a Rotaflo stopcock, and CL was added in one portion. The reaction was performed at the desired temperature, and then a sample of reaction mixture was withdrawn for analysis. In the case of chain extension polymerizations, to the remaining copolymerization mixture a second portion of CL was added at the same supply rate. The reaction was stopped by the addition of solid CaO to neutralize the acid catalyst. After filtration, the polymer was isolated by precipitation into methanol (for high molar masses) or into a hexane/diethyl ether mixture. The resulting copolymers were analyzed after drying under a vacuum. Typical 1H and 13C NMR spectra of P(MBL-co-CL) copolyesters with an isopropanol initiating moiety are as follows: 1 H NMR (400 MHz, CDCl3): δ (ppm) = 1.23 (d, 2 × 3H/DPn, (CH3)2CHO−), 1.31−1.40 (m, 2H, −C(O)(CH2)2CH2(CH2)2O−), 1.59−1.71 (m, 4H, −C(O)CH2CH2CH2CH2CH2O−), 2.32 (t, 2H, C


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Macromolecules −C(O)CH2(CH2)4O−), 2.64 (t, 2H × FMBL, −C(O)C(CH2)CH2CH2O−), 4.05 (t, 2H, −C(O)(CH2)4CH2O−), 4.15 (t, 2H × FMBL, −C(O)(CH2)4CH2O-, CL-MBL), 4.20 (t, 2H × FMBL, −C(O)C(CH2)CH2CH2O-, MBL-CL), 4.29 (t, 2H × FMBL, −C(O)C(CH2 )CH2 CH 2 O-, MBL-MBL) 5.00 (q, 1H/DP n , (CH3)2CHO−), 5.61 and 6.23 ((2 × 1H) × FMBL, −C(O)C( CH2)CH2CH2O−). The MBL content in the P(MBL-co-CL) was determined by 1H NMR analysis (CDCl3) and was calculated according to the formula MBL mol % = [2 × IMBL/(IMBL + ICL)] × 100%, where IMBL is the peak area at 5.61 ppm and ICL is the peak area at 4.10 ppm. 13 C NMR (100 MHz, CDCl3): δ (ppm) = 21.9 (2 × C/DPn, (CH3)2CH−O−), 24.5 (1C, −C(O)(CH2)2CH2(CH2)2O−), 25.5 (1C, −C(O)CH2CH2CH2CH2CH2O−), 28.4 (1C, −C(O)CH2CH2CH2CH2CH2O−), 31.4 (1C × FMBL, −C(O)C(CH2)CH2CH2O−), 34.1 (1C, −C(O)CH2(CH2)4O−), 62.4 (1C × FMBL, −C(O)C( CH2)CH2CH2O-, MBL-CL), 63.1 (1C × FMBL, −C(O)C(CH2)CH2CH2O-, MBL-MBL), 64.2 (1C, −C(O)(CH2)4CH2O-, CL-CL), 64.7 (1C × FMBL, −C(O)(CH2)4CH2O-, CL-MBL), 126.9 and 136.7 (2C × FMBL, −C(O)C(CH2)CH2CH2O−), 166.3 (1C, CO, MBL-MBL),166.5 (1C, CO, CL-MBL), 173.3 (1C, CO, MBLCL),173.5 (1C, CO, CL-CL). Thiolene Postmodification. Thiolene modifications were carried out in a Schlenk tube under an Ar atmosphere using either fluorescent BTXI-SH under thermal initiation or N-acetylcysteine under photoinitiation. In the first approach, BTXI-SH dissolved in dichloromethane was added to the dichloromethane solution of 0.2 g of P(MBL-co-CL) with 3 mol % of MBL units (Table 1, entry 8A). A 5 molar excess of BTXI-SH with respect to the MBL units was used for this reaction. After the addition of AIBN (0.5 equiv), the reaction was stirred for 8 h at 50 °C. In the second approach, the same ratio of reagents was used, but the reaction was carried out in ethyl acetate using DMPA (0.1 equiv) as photoinitiator and under light irradiation for 6 h using a medium-pressure mercury lamp as a source of light with a wavelength >360 nm. Polymers were isolated twice by reprecipitation from dichloromethane to cold methanol and were filtered, washed three times with methanol, and dried under vacuum for 4 h.

the study of CL and MBL copolymerization in this work. Using Al(OiPr)3 as a catalyst for polymerization of MBL at temperatures as high as 80 °C also led to the formation of a PMBLVA homopolymer through vinyl addition. The amount of PMBLVA was higher when the reaction was carried out in THF instead of toluene because 40% and 8% of PMBLVA were isolated, respectively. The molar mass of the formed PMBLVA was 73 and 56 g mol−1 for polymerizations in THF and toluene, respectively, as determined by GPC in DMAc as an eluent. The extent of possible thermal initiation of MBL with highly reactive vinyl bond toward radical polymerization was higher at elevated temperatures, while a higher extent of peroxides can be formed in THF as compared with toluene solvent. Since Al(OiPr)3 is active enough for the ROP of lactones, such as LA and CL, also at lower temperatures,53 we performed all further experiments either at rt or at 0 °C to prevent the PMBLVA formation. To check the ability of Al(OiPr)3 to open MBL ring at rt, we first performed the ROP of pure MBL at rt in bulk. No polymer was formed, which was expected based on the known positive Gibbs free energy for the ROP of γ-butyrolactones as well as the ceiling temperature of −56 °C estimated recently for ROP for a 5 mol L−1 MBL solution.44 However, in the 1H NMR product of ring-opening of MBL, a ring can be recognized confirming the ability of Al(OiPr)3 to react with the MBL ring. New well-resolved signals in the region for a methylene double bond at 5.6 and 6.2 ppm appeared (Figure S1). These signals are related to ring-opening MBL derivatives with the retention of a vinyl double bond and are related to isopropyl 4-hydroxy-2-methylene butanoate54 in 25% conversion and dimers and/or trimers in approximately 8% conversion. Further, we investigated the copolymerization of MBL and CL lactones with various feed monomer compositions using Al(OiPr)3 as a catalyst (Table 1). Keeping the reaction temperature at rt or at 0 °C, exclusively linear copolyesters P(MBL-co-CL) were formed, exhibiting NMR proton and carbon signals at characteristic positions. In 1H NMR, the proton signals 1, 2, and 3 belong to MBL units and protons a, b, c, d, and e belong to CL units (Figures 1 and 2). The functionality of prepared polyesters was confirmed by vinyl double bond retention with characteristic proton signals 1 at 6.23 and 5.61 ppm in the 1H NMR (Figure 1) as well as carbon signals 1 and 4 at 126.9 and 136.7 ppm (Figure 2) in 13C NMR. The precise assignment of the signals in the 1H and 13C NMR spectra with chemical shifts for the P(MBL-co-CL) hetero- and homosequences in the NMR spectra have not been described in the literature so far. Therefore, HMBC and HSQC 2D NMR techniques were used to verify the structure of the copolyesters (Figure 3). Two-dimensional NMR that was applied in this study combined the results of proton−carbon data to help understand the near-neighbor relationships in the structure and allowed us to prove the signal assignments in 1H NMR in the 4.00−4.35 ppm region and in 13C NMR spectra in the 61−65 ppm region. First, the analysis of the HMBC spectrum (which detects heteronuclear correlations over a range of approximately 2−3 bonds in aliphatic chains), presented in Figure 3A, allowed us to assign the signals in the 4.00−4.35 ppm region. Thus, in addition to a strong signal at 4.05 ppm from the oxo-methylene protons of CL in the CL− CL homosequence, the triplet signal a′ observed at 4.15 ppm was attributed to oxo-methylene protons of CL in the CL-MBL heterosequence based on their correlation with the carbon three bonds away from the adjacent MBL carbonyl group that

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RESULTS AND DISCUSSION Coordination−Insertion ROP. Tin octoate, in combination with alcohols, and Al(OiPr)3 are among the most utilized catalytic systems for ROP of cyclic lactones, working under a coordination−insertion mechanism. The first is suitable for LA and CL polymerizations at higher temperatures. Because of the relatively high double bond reactivity of MBL, comparable to acrylates, the formation of PMBLVA homopolymer through vinyl addition was observed when MBL was heated to 80 °C using tin octoate as a catalyst in combination with butyl alcohol, similar to when Bi(TfA)3 was used as a catalyst.47 The PMBLVA formation was simply visible as the formation of a turbid dispersion due to its insolubility in the reaction mixture. Additionally, this was also confirmed in the NMR spectra using DMSO-d6 as a solvent in which PMBLVA is soluble. In the 1H NMR spectra, decreases of signals from vinyl group protons at 5.6 and 6.2 ppm were accompanied by the appearance of wide signals of protons related to main-chain polyacrylates at 1.5− 2.3 ppm. Al(OiPr)3 has been widely used as catalyst/initiator in the ROP of cyclic lactones working by a coordination−insertion mechanism.49−51 Its trimer form was found to be much more active in the ROP polymerization of ε-CL and L,L-lactide as its tetramer form52 and was also successfully employed for preparation of high molar mass (∼30 kg mol−1) copolymer of CL and γ-BL with up to 43 mol % of γ-BL units incorporated in the copolyester.41 Thus, Al(OiPr)3 was logically chosen for D


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Macromolecules

on correlation with corresponding oxo-methylene protons (Figure 3B). The chemical shifts of the carbons a from CL in the CL−CL homosequence is easily visible at 64.1 ppm and carbons 3′ from MBL in the MBL−MBL homosequence were recognized at 63.1 ppm. The carbon signal a′ that corresponds to CL in the CL−MBL heterosequence is present at 64.7 ppm, and the carbon signal 3 that corresponds to MBL in the MBL−CL heterosequence is centered at 62.4 ppm. The precise assignment of the signals, especially in the region of 4.00− 4.35 ppm in the 1H NMR spectra, is important for correct determination of both monomer conversion and the molar content of MBL in the final copolyesters, as discussed later in the paper. At an approximately equimolar MBL/CL ratio and total monomer concentration of approximately 2.0 mol L−1 (Table 1, entry 1), the ROP process was well-controlled as confirmed by the kinetics of polymerization as well as by the molecular characteristics of the final copolyester. The reaction was run until complete CL conversion was reached. Contrary to that, the conversion of MBL was only approximately 6% and did not increase with a prolonged polymerization time. Figure 4A shows an example of the kinetic profiles for MBL and CL monomers, both following the first-order kinetics. The determined apparent rate coefficient of MBL (kMBL = 0.019 mol L−1 s−1) was approximately 50 times lower than that of CL (kCL = 1.034 mol L−1 s−1). The molar mass of copolyester increased linearly with total monomer conversion (Figure 4B) and reached Mn = 25 kg mol−1. A unimodal molar mass distribution of polyesters during the polymerization is shown in GPC traces (Figure 5). Good control over the polymerization process was also confirmed by a narrow dispersity, Đ ∼ 1.11, of the final copolyester. Lower molar masses were determined from GPC compared to those calculated based on conversion from NMR. However, the hydrodynamic radius of P(MBL-coCL) chains can be expected to be different compared to PS standards. It was shown that the correction factor for polylactides or polycaprolactones in dichloromethane is

Figure 1. 1H NMR spectra of P(MBL-co-CL) with 25 mol % MBL prepared by ROP using MBL/CL/Al(OiPr)3 with molar ratio 9/0.9/ 0.002 after 48 h at 0 °C and after precipitation to methanol (Table 1, entry 12A) performed in CDCl3.

has a signal in the 13C NMR at 166.5 ppm. On the other hand, the carbon from the CL carbonyl group in the MBL−CL heterosequence that has a signal at 173.3 ppm in 13C NMR was correlated to oxo-methylene protons three bonds away that belonged to an adjacent MBL unit and had a triplet signal 3 at 4.22 ppm in 1H NMR. Additionally, oxo-methylene protons 3′ of the MBL−MBL homosequences were identified at 4.29 ppm in the 1H NMR spectrum based on correlation with the carbon of an adjacent MBL carbonyl group that had a signal at 166.3 ppm in 13C NMR. Next, on the basis of HSQC spectrum analysis, the assignment of oxo-methylene carbon signals in the 13 C NMR spectra in the range of 62−65 ppm was verified based

Figure 2. 13C NMR spectra of P(MBL-co-CL) with 25 mol % MBL prepared by ROP using MBL/CL/Al(OiPr)3 with molar ratio 9/0.9/0.002 after 48 h at 0 °C and after precipitation to methanol (Table 1, entry 12A) measured in CDCl3. E


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Macromolecules

Figure 3. (A) HMBC and (B) HSQC spectrum of P(MBL-co-CL) with 25 mol % MBL prepared by ROP using MBL/CL/Al(OiPr)3 with a molar ratio 9/0.9/0.002 after 48 h at 0 °C and after precipitation to methanol (Table 1, entry 12A) measured in CDCl3.

Figure 4. (A) Kinetic profiles of MBL (triangles) and CL (circles) monomer consumption and (B) the changes of Mn with monomer conversion determined by GPC with PS standards during ROP using MBL/CL/Al(OiPr)3 with a molar ratio 0.9/1/0.0027 at rt (Table 1, entry 1A).

A decrease of the Al(OiPr)3 concentration by approximately 4 times (from 2.7 to 0.6 mmol L−1) with keeping the similar monomer concentrations was applied to prepare a higher molar mass polymer and led to a significant decrease in the polymerization rate (Table 1, entry 2A). After 120 h, a copolyester with a molar mass of approximately 60.8 kg mol−1 and a dispersity of 1.51, as determined by GPC, was obtained. The broad dispersity could be explained by a higher extent of transesterification reactions. Nevertheless, the molar content of MBL units of 5 mol % was similar to the previous experiment. Subsequently, various ratios and concentrations of MBL and CL monomers were investigated while the concentration of Al(OiPr)3 was kept at a nearly constant value (Table 1, entries 3−7). Using 4 times excess of CL compared to MBL led to complete CL conversion after 1 h; however, the final polyester only contained approximately 1 mol % of MBL units. Increasing the molar content of MBL units in the copolyester was observed with an increase of the feed molar content of MBL. A content of MBL in polyester equal to 17 mol % was obtained when the MBL/CL feed ratio of 8.5/0.8 was used. In this case, the polymerization was performed in bulk, and it should be mentioned that the prolonged polymerization from 4 h (time when complete CL conversion was obtained) up to 7 h also led to formation of a PMBLVA homopolymer through the

Figure 5. GPC traces of P(MBL-co-CL) polyester during the ROP copolymerization process of MBL/CL/Al(OiPr)3 with a molar ratio 0.9/1/0.0027 at rt (Table 1, entry 1A). After 1 h, Mn = 16.0 kg mol−1 and Đ = 1.10; after 4 h, Mn = 25.0 kg mol−1 and Đ = 1.11.

approximately 0.68.55 Thus, even though the THF was used as an eluent, overestimated values from GPC can still be expected for P(MBL-co-CL) with high CL content. F


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Macromolecules vinyl addition. PMBLVA formation was overcome by decreasing the monomer concentration (Table 1, entry 7A) that led, however, also to a slight decrease of the MBL content in polyester down to 14 mol %. Since a low monomer−initiator ratio was used, a low molar mass polymer was obtained. The low molar mass of the copolyester allowed NMR analysis to also be used to determine the degree of polymerization (DPn) by simple comparison of peak integrals related to monomer units (4.06, 4.16, and 4.21 ppm) and an isopropoxy initiating moiety (5.00 or 1.24 ppm) and to compare the Mn with that from GPC. The Mn determined from NMR (3.7 kg mol−1) was slightly lower than Mn determined from GPC using PS standards (4.4 kg mol−1), confirming a different hydrodynamic volume of the polyester (Table 1, entry 7A). The dispersity determined from GPC was as narrow as 1.15, i.e., slightly narrower than in the case of polyesters prepared at higher monomer concentrations, where the dispersity was in the range 1.25−1.32. Comparing our results of the MBL/CL/Al(OiPr)3/toluene system with similar using γ-BL instead MBL performed at 20 °C earlier,41 a copolyester with γ-BL content 11 and 22 mol % was obtained when equimolar γ-BL/CL feed ratio with total monomer concentrations [M]0 = 1 or 5 mol L−1 was used, whereas in our system with [M]0 = 2 mol L−1 only 5 mol % of MBL was incorporated in the copolymer. Similarly, batch polymerization for γ-BL/CL 10/1 provided copolyester with 43 mol % of BL compared to only 17% of MBL incorporated under similar conditions. In the case of γ-BL derivative, such as α-bromo-γ-butyrolactone (αBrγBL), a linear relationship between the feed ratio and its composition in the copolymers with CL and LA was observed even using relatively less active coordination−insertion ROP catalyst such as Sn(Oct)2.5612 mol % of αBrγBL was incorporated into the copolymer when 28 mol % of αBrγBL was used in feed mixture. It is worth to notice here that Zhoe et al.48 reported that MBL was not copolymerized when Sn(Oct)2 was used as a catalyst. Thus, it seems that presence of exocyclic methylene double bond in the α position of butyrolactone ring in MBL structure decreases its reactivity toward coordination−insertion ring-opening copolymerization with CL. Based on the thermodynamic equation ln[M]eq = ΔHp/RT − ΔS0p/R, where [M]eq is an equilibrium monomer concentration and ΔHp is a negative for MBL (ΔHp = −5.9 kJ mol−1),44 more MBL units should be incorporated in the polymer chain at lower temperatures.47 Therefore, in order to investigate the possibility of further increasing of the content of MBL in the copolyesters, polymerizations were performed at 0 °C. Figure 6 summarizes the dependence of MBL content incorporated into the polyester chains (FMBL) on MBL feed content ( f MBL) for two various temperatures: rt and 0 °C. A gradual increase in FMBL with f MBL was observed. As expected, thermodynamically driven copolymerization did not depend on catalyst concentration, but the decrease of reaction temperature from rt to 0 °C led to increased MBL incorporation from 5 to 7 mol % in the case of a 1/1 feed MBL/CL ratio, from 13 to 16 mol % for a 5/1 feed MBL/CL ratio, and from 17 to 25 mol % for a 10/1 feed MBL/CL ratio. In addition, the polyesters prepared at 0 °C had narrower dispersities in the range 1.12−1.21 (Table 1), confirming better control during polymerization at lower temperature. It is worth noting that in the case of copolyesters with higher content of MBL units the molar masses determined from GPC based on PS standards became more similar to the theoretical molar masses, probably due to the significant effect

Figure 6. Content of incorporated MBL units (FMBL) in copolymer P(MBL-co-CL) dependent on the MBL/CL polymerization feed ratio ( f MBL) obtained from ROP using Al(OiPr)3 as catalyst at rt and 0 °C.

of MBL units on the hydrodynamic volume of PCL. In the case of P(MBL-co-CL) with the highest MBL content (25 mol %), the presence of MBL could also be confirmed by FTIR spectra, where a peak at 1632 cm−1 (characteristic for vibration related to carbon−carbon double bond) was observed (Figure S3). Since the decrease in temperature was accompanied by a decrease in the polymerization rate, a further decrease of temperature was not investigated, since too long of a polymerization time would be needed to obtain complete CL conversion. Generally, the conversion of MBL was low in the presented polymerizations, especially at higher MBL/CL feed ratios when MBL conversions were in the range 2−4%, even though a high content of MBL was incorporated into the copolyester. Varying experimental conditions only produced small changes in MBL conversion. To increase both the conversion and the molar content of MBL incorporated into copolyester, we added a CL comonomer dropwise into the MBL containing the polymerization mixture (Table 1, entry 13A), thus maintaining a high excess of MBL compared to CL during the entire polymerization process performed at rt. The final concentrations of MBL and CL were 5 and 1 mol L−1, respectively. The MBL conversion was approximately 7% compared to the 3−4% conversion obtained with the batch polymerization process (Table 1, entries 5A and 11A). At the same time, the MBL content in polyesters increased up to 22 mol % compared to the 13 and 16 mol % obtained from the batch polymerization performed at rt and 0 °C, respectively (Table 1, entries 5A and 11A, respectively). Cationic ROP. The cationic polymerization, according to the activated monomer mechanism, may be applied for the polymer synthesis conducted under metal-free conditions, from a wide range of the oxygen-containing heterocyclic monomers.57 The development of initiating systems based on easily available acids and alcohols is highly demanded as it provides access to polymers that are free of metallic contamination, which may be responsible for the degradation or coloration of such polymers. The mechanism of cyclic ester polymerization (mainly lactide and CL) carried out in the presence and absence of alcohol and catalyzed by protic acids has been demonstrated.58−63 To study the cationic ring-opening copolymerization of CL with MBL in the presence of alcohol as an initiator and protic acid as a catalyst, screening experiments were conducted for G


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Table 2. Reaction Conditions and Results of Cationic MBL/CL Copolymerization According Activated-Monomer Mechanism Using Isopropyl Alcohol as an Initiator entry 1C 2C 3Cc 4Cc 5C 6C 7Cc

8Cc

[M]0a (mol L−1)

[MBL]0/[Cl]0a

4.3 3.5 4.6 10.6 10.6 8.1 4.2 10.7 10.6 10.6 10.6

0.4 1.9 0.64 1 2.2 3.5 1 5.3 4.6 3.4 2.2

[i-PrOH] (mol L−1)

[H+] (mol L−1)

t (h)

f MBL (mol %)

FMBLd (mol %)

Mn(NMR)d × 103 (g mol−1)

Mn(GPC) × 103 (g mol−1)

Đ

0.07 0.08

0.06 (TfA) 0.04 (TfA)

0.08 0.07 0.09 0.08 0.08

0.05 (TfA) 0.06 (TfA) 0.04 (TfA)b 0.08 (DPP) 0.08 (DPP)

0.05

0.06 (DPP)b

72 21 24 24 24 96 24 24 21 24 96

28 66 39 50 69 78 50 84 82 79 70

4 11 4 NDe ND ND 4 7 11 8 4

10.2 3.27 6.45 ND ND ND 1.88 1.82 3.97 4.25 11.0

17.9 3.28 7.73 ND ND ND 2.98 2.49 7.48 7.93 5.85

1.3 2.2 1.8 ND ND ND 1.4 1.5 1.5 1.6 1.7

a

[M]0 and [MBL]0/[CL]0 stay for total feed monomer concentration and relative ratio of feed MBL and CL monomers concentrations, respectively. Reaction conditions: room temperature except the polymerization 5C and 8C, where the temperature of −78 °C was applied. cCopolymerization conducted without solvent, otherwise DCM was used. dDetermined according to 1H NMR spectra collected in CDCl3. eND = not determined, since in addition to ROP, also PMBLVA homopolymer and partial cross-linking was observed. b

Scheme 2. Cationic ROP Copolymerization of CL with MBL According to the Activated-Monomer Mechanism

of GPC traces to a lower retention volume (higher molar mass, Mn(GPC) = 7.73 kg mol−1) region was observed for the copolymer obtained after the second stage, and dispersity became narrower (Đ = 1.8) (Figure S4). During cationic copolymerization conducted without a solvent (Table 2, entries 3C and 4C) and the presence of TfA as a catalyst, the turbidity and gelation of the reaction mixture were observed, indicating the formation of undesired PMBLVA and/or a cross-linked structure. According to recent work,44 three types of MBL homopolymer can be formed depending on the type of propagation, such as ring-opening (PMBLROP), vinyl addition (PMBLVA), or cross-propagation (PMBLcross). Although limited vinyl addition was observed and cross-propagation did not occur under coordination−insertion ROP using Al(OiPr)3, it was observed that TfA as strong acid can catalyze these processes even at −78 °C (Table 2, entry 5C). Moreover, vinyl addition was also significant in DCM, using a very active lanthanide complex employing coordination−insertion mechanism.47 The comparison of both mechanisms shows that the initiating system in cationic ROP is more active because in the copolymerization performed with access to CL (MBL/CL = 1.2/3.0), the copolymer contains 4 mol % of MBL, whereas in the case of a comparable feed composition employing coordination−insertion ROP, the amount of incorporated MBL was less than 1 mol %. The obtained polyester with a molar mass of 17.9 kg mol−1 exhibited a relatively narrow dispersity (Đ = 1.3), showing good control over the ROP process (Table 2, entry 1C). Additionally, reactions with higher MBL/CL feed ratios, while they still did not exceed a MBL/CL feed ratio of 1

various compositions of copolymerization mixture. Depending on the conditions, trifluoromethanesulfonic acid (TfA) or less reactive diphenyl phosphate (DPP) was used as a Brönsted acid, which activated the cyclic esters for reaction with the alcohol initiator and subsequently with the hydroxyl polymer chain end to provide polyester formation. Considering the mechanism of the polymerization as well as the low reactivity of the butyrolactone ring, in most cases CL was introduced into the reactive mixture dropwise to maintain a low instantaneous concentration and to enhance incorporation of MBL into the copolymer chain. Initially, the isopropanol/triflic acid (i-PrOH/TfA) initiating system in DCM at room temperature was tested, as successfully applied earlier in the polymerization of other cyclic esters.61 For the copolymerization conducted under the continuous addition of CL over 4 h and with 40 mol % of final feed content of MBL (Table 2, entry 1C), a copolyester with a molar mass of 17.9 kg mol−1 was obtained, as determined by the GPC and MBL content of 4 mol %. In cationic polymerization according to an activated monomer mechanism, the growth of a polymer chain proceeds via the addition of the protonated monomer molecule to the hydroxyl group from the initiator and subsequently with −OH terminal groups of the growing chain, as presented in Scheme 2. To support the living nature of the triflic acid-catalyzed ROP of CL with MBL, the chain extension experiment was performed (Table 2, entry 2C). The P(MBL-co-CL) copolymer with Mn(GPC) = 3.28 kg mol−1 and Đ = 2.2 was prepared in the first stage. Then, the chain extension was performed by the subsequent addition of CL (9.65 × 10−3 mol), and the shifting H


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on the α-carbon atom in the protonated (thus activated) monomer molecule, the resulting polymer chain contains end groups coming from the used initiator and two kinds of aliphatic esters repeating units in their main chain (Scheme 2). Incorporation of both comonomers into the polymer chain at copolymerization conducted in the presence of isopropanol as an initiator was confirmed by a typical 1H NMR spectrum of purified copolymerization product collected in CDCl3 (Figure S5, top). Signals of both comonomer units were present with characteristic chemical shifts in the range that is typical for unsaturated copolyesters P(MBL-co-CL), as was shown in the case of coordination−insertion ROP above and reported previously in the literature.47 The analysis of 1H NMR spectra fully confirmed the desired structure of P(MBL-co-CL) copolyester containing unsaturated pendant groups, an isopropyl head group, and an −OH tail group. NMR spectra further proved that in cationic ROP more reactive CL is incorporated preferentially into the copolymer. In addition, the signals of head group coming from the alcohol moiety were observable in the range of 4.99−5.01 ppm [(CH3)2CHO−] and 1.22 ppm [(CH3)2CHO−]. Because of the lower Mn of polyesters prepared by cationic ROP, these signals were clearly visible and enabled determination of DPn and thus the molar mass of the obtained copolymers (Tables 2 and 3). Similarly, signals with comparable intensity at approximately 3.5 and 3.6 ppm were assigned to protons of the MBL and CL methine groups in the penultimate unit of the copolymer at the chain end terminated with hydroxyl group. Comparing the molar masses determined from GPC and NMR, the overestimation of molar masses in GPC analysis was confirmed similar to what was expected and discussed in the section dedicated to coordination−insertion ROP. Additionally, the structure of the final product obtained in the cationic copolymerization of CL with MBL initiated with isopropyl alcohol was analyzed using the MALDI-TOF technique (Figure 8). It was found that in the typical MALDI-TOF spectrum of the product each of the signals can be assigned to a macromolecule composed of one end group derived from the alcohol that was used: x CL units and y MBL units. An expansion of the spectrum reveals a complex nature in which a series of signals represent a copolymer chain with the same total degree of copolymerization but different contributions of comonomers. The results indicate that if monofunctional alcohol is used as an initiator, growing macromolecules contain end groups from the alcohol, and linear, unsaturated copolyesters with a C3H7O− head group and an −OH tail group are formed. Application of polyols as initiators can have an advantage in the formation of the multiarm copolymers containing hydroxyl end groups as it was demonstrated for other lactones.3,66 Multihydroxyl (star shaped or branched) polyesters attract

(also with higher [MBL]0), they led to higher incorporated MBL contents compared to coordination−insertion ROP but provided polyesters with a wider dispersity as a consequence of lower selectivity with possible higher extent of transesterification. The attempt to further increase MBL also led to a cross-linked P(MBL-co-CL) structure, which limits this approach for the preparation of copolyesters with a higher concentration of functional double bonds along the polyester chain. For the above-mentioned reasons, diphenyl phosphate (DPP), as a less reactive catalyst, was applied for further studies. DPP was previously used as an efficient acidic catalyst for the synthesis of polycarbonates and polylactides.64,65 It was found that the application of DPP provided polymerization that proceeded exclusively according to the ring-opening mechanism, affording the formation of linear copolyesters bearing pendant vinyl groups (Table 2, entry 6C). The absence of the PMBLVA homopolymer was also confirmed by 1H NMR spectra collected in DMSO-d6 where a signal at 2.07 ppm, which is typical for the −CH2− group formed by vinyl addition, did not appear (Figure S5, bottom). Applying DPP as an acid catalyst, a series of P(MBL-co-CL) copolymers were also prepared using the chain extension experiment conducted in three steps (Table 2, entry 7C) to verify the livingness of the system. After each stage of the copolymerization, a shifting of GPC traces to a lower retention volume indicated a continuous increase of molar mass of the copolyester (Figure 7).

Figure 7. GPC traces of purified products obtained in cationic ROP of CL and MBL in the presence of isopropanol as an initiator and DPP as a catalyst conducted in three stages (Table 2, entry 7C). Copolymers 2 and 3 (Mn(GPC) = 7.48 and 7.93 kg mol−1, respectively) obtained by chain extension from precursor 1 (Mn(GPC) = 2.49 kg mol−1).

As in the AM mechanism, propagation proceeds by the nucleophilic attack of the oxygen atom of the hydroxyl group

Table 3. Reaction Conditions and Results of MBL/CL ROP Cationic Copolymerizations According to the Activated Monomer Mechanism Using Either Difunctional or Tetrafunctional Alcohol as an Initiator entry

[M]0b (mol L−1)

[MBL]0/[CL]0b

[I] (mol L−1)

[DPP] (mol L−1)

t (h)

f MBL (mol %)

FMBL (mol %)

Mn (NMR)c × 103 (g mol−1)

Đd (GPC)

9C 10C 11C

9.1 9.6 10.6

0.82 1 1

0.22 (EG) 0.11 (DTMP) 0.06 (DTMP)

0.08 0.17 0.09

72 24 72

45 50 50

8.8 4.3 5.3

6.20 5.99 8.94

1.30 1.14 1.61

a

Reaction conditions: bulk polymerization, room temperature. b[M]0 and [MBL]0/[CL]0 stay for total feed monomer concentration and relative ratio of feed MBL and CL monomers concentrations, respectively. cDetermined according to 1H NMR spectra performed in CDCl3. dPerformed in DCM as eluent and determined according to PS standards. I


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Figure 8. Part of the MALDI TOF spectra of P(MBL-co-CL) obtained by cationic ROP initiated with isopropanol (Tables 2 and 3C). The numbers represent a z-mer of polyester with the same DPn but different compositions and expanded series of signals coming from macromolecules at the 28mer and 34-mer.

Scheme 3. Cationic ROP of CL with MBL According to the Activated Monomer Mechanism in the Presence of Protic Acid as a Catalyst and Either Difunctional (EG) or Tetrafunctional (DTMP) Alcohol as an Initiator

increasing attention due to their unique thermomechanical and solution properties compared to their linear counterparts.67 Recently, star-shaped biodegradable polyesters and the polymer networks made from end-functionalized star polymers have been synthesized as environmentally benign materials with tailor-made thermal, mechanical, and biodegradable properties.68 Multiarm polycaprolactone-block-polylactide was also used for generating transparent PLLA-based resin with enhanced properties.69 Here, the possibility to prepare functional telechelic and star copolyesters was investigated using the cationic copolymerization of MBL with CL conducted in the presence of ethylene glycol (EG) and di(trimethylolpropane) (DTMP), respectively, as initiators and DPP as a catalyst (Scheme 3). The reaction conditions and the analytical results are shown in Table 3. The structures and compositions of the telechelic and star-shaped P(MBL-co-CL) were confirmed by NMR spectra of purified products (Figures SI6 and SI7) as well as by the MALDI-TOF technique (Figure S8). Similar to copolyesters prepared from isopropanol, also for telechelic and star copolyesters, the content of MBL units incorporated into the polymer chains was in the range of 4−8

mol % depending on the feed MBL/CL and monomers/ initiator ratios. The results obtained from cationic copolymerizations of CL with MBL conducted in the presence of mono-, di-, and multifunctional alcohols demonstrated that a successful synthesis was performed under mild conditions, leading to functionalized medium-molar-mass copolyesters capable of further postfunctionalization either at hydroxyl chain ends or pendant vinyl groups. Additionally, the presented synthesis of functional polymers in “metal-free” polymerization enables the tailoring of polymer structures without contamination of the final products by metallic impurities. Thermal Properties. The MBL incorporation in the P(MBL-co-CL) is expected to affect final copolyester properties. Therefore, the effects of the content of MBL units in the copolyesters on thermal properties were investigated. DSC measurements were performed to analyze the dependence of the melting temperatures on the composition and structure of P(MBL-co-CL) copolyesters obtained by coordination−insertion and cationic ROP. The DSC curves of selected copolyesters obtained in the second heating scans are shown J


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Flory theory of crystallization in copolymers that consist of a crystallizable (CL) and noncrystallizable (MBL) component.75 In this case, both transitions are shifting to lower temperatures, and thus two types of crystals containing both CL and MBL units are considered. The lower peak represents a proportion of material that is associated with a different structure and/or morphology. The DSC curves of copolyesters obtained by cationic copolymerization exhibited essentially the same character with double melting peaks located at lower temperatures compared to PCL, indicating successful incorporation of MBL into the polyester chain. The DSC traces of selected linear and starshaped copolymers are compared in Figure 10. While melting

in Figure 9. A single or double melting endotherm with peaks at lower temperatures was observed for P(MBL-co-CL) copo-

Figure 9. Comparison of DSC curves (second heating scan) of the PCL homopolymer (Mn = 25.0 kg mol−1 linear) and linear P(MBL-coCL) copolyesters with different MBL content obtained by coordination−insertion ROP.

lyesters compared to the melting endotherm of the PCL homopolymer. This decrease in Tm can be attributed to the presence of a noncrystallizable MBL component in the microstructure, which interrupts the linear sequences of PCL and breaks the structural order and causes the melting temperature to decrease.70 As shown in the DSC traces, polyester samples including the PCL homopolymer clearly exhibit a double melting peak with a characteristic Tm1 and Tm2 (Figure 9 and Table 4). The Table 4. Melting Temperatures of the PCL Homopolymer and P(MBL-co-CL) Copolyesters with Different MBL Content Based on DSC Analysis Taken from the Second Heating Scan sample

Mn(GPC) × 103 (g mol−1)

FMBL (mol %)

Tm1 (°C)

Tm2 (°C)

Tc (°C)

PCL 3A 8A 1A 4A 7A 12A

25.0 43.2 87.7 25.0 17.6 4.4 15.5

0 1 3 5 8 14 25

54.5 54.9 52.2 51.0 45.6 39.8 23.9

56.9 NDa ND ND 49.5 45.8 33.9

25.7 30.3 23.1 18.6 12.6 −2.5

Figure 10. DSC curves (second heating scans) of linear and starshaped copolyesters obtained by cationic ROP.

temperatures of the linear copolymer with 11 mol % of MBL lie between those for copolymers with 8 and 14 mol % of MBL (Table 4), the star-shaped copolymer with lower MBL content (4.3 mol %) exhibits even lower melting temperatures. Thermogravimetric analysis was also conducted to analyze the thermal stability of the resulting copolyesters. DTG and TGA curves of P(MBL-co-CL) copolyesters prepared by coordination−insertion ROP and containing various content of MBL units are shown in Figure 11A. The degradation of the copolyesters started at approximately 200 °C regardless of the MBL content. However, with increased MBL content, the degradation region broadened toward higher temperatures with a multistep degradation profile. As shown in Figure 11B, the polyesters obtained by cationic ROP using a metal-free catalyst exhibited a simple degradation profile and a thermal stability that was approximately 100 °C higher compared to those obtained by coordination−insertion ROP in the presence of Al(OiPr)3 as a catalyst. The lower stability of copolyesters obtained by the coordination−insertion ROP process can be explained by the effect of the residual metallic catalyst, which was probably not completely removed even after two purifications by the precipitation process, on the degradation of the copolyester chain. A similar accelerating effect of a Snbased metal catalyst on polylactide thermal degradation was reported by Nishida.76

a

ND = not well separated peaks; the second peak is visible only as a hump of the first peak.

observed Tm were independent of molar mass of the polyesters in the investigated range of molar masses from 4.4 to 88 kg mol−1. The tendency to exhibit double melting peaks was higher for samples with an MBL content over 5 mol %. This indicates the presence of two crystalline regimes in the copolymers. The presence of multimodal melting endotherms is typical for polyesters and is generally attributed to the reorganization processes during the thermal scan.71,72 The characteristics of the crystallization and double-melting behavior can also be explained by the slow rates of crystallization and recrystallization, respectively.73,74 Copolymers with MBL exhibit two sharp overlapping melting transitions, where both are lower than the Tm1 and Tm2 of pure PCL. Melting at lower temperature is in accordance with K


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Figure 11. TGA analysis of copolymers obtained by (A) coordination−insertion and (B) cationic ROP. (A) P(MBL-co-CL) copolyesters with 3 mol % MBL, Mn = 13.0 kg mol−1; 14 mol % MBL Mn = 14.0 kg mol−1; 25 mol % MBL, Mn = 15.5 kg mol−1 (Table 1,entry 12A). (B) HomoPCL, Mn = 8.8 kg mol−1; 8 mol % MBL, linear, Mn = 7.93 kg mol−1 (Table 2, entry 7C), 4.3 mol % MBL, star-shaped, Mn = 5.99 kg mol−1 (Table 3, entry 10C).

Scheme 4. Thiol−Ene Modification of P(MBL-co-CL) Copolyester Containing 3 mol % of MBL Units with (a) BTXI-SH Using Thermal Initiation and (b) N-Acetylcysteine Using Photoinitiation

Comparing the (co)polyesters prepared by cationic ROP that are not contaminated by a residual metal catalyst (Figure 11B), it is apparent that the thermal stabilities of the P(MBL-co-CL) copolyesters were slightly higher compared to the PCL homopolymer. It is worth noting that the degradation profiles were almost the same for all synthesized P(MBL-co-CL) copolyesters, i.e., with an MBL content up to 11 mol %. The degradation curves also did not deviate significantly for star copolyesters. Postfunctionalization by Thiol−Ene Chemistry. To show the availability of functional polyester for postfunctionalization, the thiol−ene reaction of a methylene double bond with thiol derivatives was applied as a proof of concept according to Scheme 4, in which two different thiol reagents were used. In the first approach, a highly fluorescent probe based on benzothioxanthene fluorophore77 (BTXI-SH) was bonded to P(MBL-co-CL) containing 3 mol % of MBL units using AIBN and thermal initiation. Successful linkage was proven by several methods with results shown in Figure S9. At first glance, color change of several times precipitated polymer either in solution or in dry state was observed. This corresponds to absorption in the range of 430−480 nm, as detected by UV−vis spectroscopy. Additionally, GPC analysis with dual RI and fluorescence detection showed signals in the same elution volume, proving the fluorescence characteristics of the eluted polymer. In the second approach, we functionalized the same P(MBLco-CL) copolyester with 3 mol % of MBL with N-acetylcysteine

in ethyl acetate solvent under photoinitiation. In this case, the successful modification was proven by the disappearance of methylene signals of pendant vinyl groups and the appearance of new signals of N-acetylcysteine in the 1H NMR of the purified polymer (Figure S10).

■

CONCLUSIONS A renewable monomer α-methylene-γ-butyrolactone (MBL), known also as a Tulipalin A, was used as a comonomer in the ROP of ε-caprolactone (CL) under various feed ratios to prepare functional copolyesters containing pendant double bonds in their structures. As compared to previously published work, here, we used (Al(OiPr)3) as a cheap and commonly available catalyst for coordination−insertion ROP. The second advantage of (Al(OiPr)3), compared to previously used highly active lanthanides, is that copolyesters with narrow dispersity and without formation of undesirable byproducts can be obtained, while a high content of double bonds (up to 25 mol % MBL units) is still incorporated into the copolyester chain. It was shown that better control over molecular characteristics and a higher content of double bonds in the polymer chain can be achieved by either decreasing the polymerization temperature down to 0 °C while maintaining a reasonable polymerization time or by the continuous addition of CL into the polymerization mixture, thus maintaining a high excess of MBL comonomer during the polymerization process. It was also shown for the first time that P(MBL-co-CL) copolyesters can also be prepared by cationic ROP using “metal-free” catalysts L


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Macromolecules

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that function under an activated-monomer mechanism. While triflic acid was found to be too active as a catalyst, leading also to the formation of undesirable byproducts through vinyl addition reactions, diphenyl phosphate (DPP) was a less active catalyst and provided polymers exclusively through ringopening of the monomers. The combination of DPP with difunctional or multifunctional alcohols enabled the synthesis of multifunctional telechelic or star copolyesters, respectively, not reported so far for this type of copolyester. The structure of the copolyesters and MBL incorporation into the polymer chain was proven by NMR and MALDI-TOF spectroscopy. Two-dimensional NMR was used to identify the signals that belonged to CL units with neighboring MBL units, and vice versa. Finally, as a proof of concept of availability of the double bonds in the copolyesters for further functionalization, both thermal and photochemical thiol−ene reactions were successfully performed.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00456. Materials, analytical methods, 1H NMR, 13C NMR, UV− vis, FTIR, GPC and MALDI-TOF spectra of some copolymers before and after postfunctionalization (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.B.). *E-mail: [email protected] (J.M.). ORCID

Martin Danko: 0000-0002-6188-0094 Jaroslav Mosnácě k: 0000-0001-9160-590X Author Contributions

M.D. and M.B. contributed equally. Author Contributions

A.D. deceased April 2016. The manuscript was written through contributions of all authors. All authors, except A.D., have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the VEGA Grant Agency for support through project 2/0158/17 and the SRDA Grant Agency through project APVV-15-0528. M.B., M.D., and J.M. thank the Polish Academy of Sciences and Slovak Academy of Sciences for bilateral grant agreements.

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Functional Polyesters with Pendant Double Bonds Prepared by

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