DSpace at VNU: Synthesis of Allyl End-Block Functionalized Poly(epsilon-Caprolactone)s and Their Facile Post-Functionalization via Thiol-Ene Reaction

ARTICLE WWW.POLYMERCHEMISTRY.ORG JOURNAL OF POLYMER SCIENCE Synthesis of Allyl End-Block Functionalized Poly(e-Caprolactone)s and Their Facile Post-Functionalization via Thiol–Ene Reaction Thuy Thu Truong,1 Son Hong Thai,1 Ha Tran Nguyen,1,2 Vinh-Dat Vuong,2 Le-Thu T Nguyen1 Faculty of Materials Technology, Ho Chi Minh city University of Technology, Vietnam National University (VNU–HCM), 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam Materials Technology Key Laboratory (Mtlab), Ho Chi Minh City University of Technology, Vietnam National University (VNU–HCM), 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam Correspondence to: L.-T.T Nguyen (E-mail: nguyenthilethu@hcmut.edu.vn) Received 13 July 2016; accepted November 2016; published online in Wiley Online Library DOI: 10.1002/pola.28454 ABSTRACT: A simple and facile strategy for the functionalization of commercial poly(e-caprolactone) diols (PCLs) with pendant functionalities at the polymer chain termini is described Well-defined allyl-functionalized PCLs with varying numbers of allyl end-block side-groups were synthesized by cationic ring-opening polymerization of allyl glycidyl ether using PCL diols as macroinitiators Further functionalization of the allyl-functionalized PCLs was realized via the UV-initiated radical addition of a furan-functionalized thiol to the pendant allyl functional groups, showing the suitability for post-modification of the PCL materials Changes in polymer structure as a result of varying the number of pendant functional units at C 2017 Wiley Periodicals, the PCL chain termini were demonstrated V Inc J Polym Sci., Part A: Polym Chem 2017, 55, 928–939 INTRODUCTION Poly(e-caprolactone)s (PCLs) are an essential class of synthetic polymers widely used as biomaterials for medical devices, drug delivery and tissue engineering, as shape-memory polymers, adhesives, and soft segments for polyurethane synthesis as well as in packaging and microelectronics.1 Such extensive application of PCLs is possible as a result of their tailorable mechanical properties, miscibility with a large range of other polymers and biodegradability.2 PCLs are hydrophobic, semi-crystalline and have low melting points (typically 60 8C) The typical synthesis route to PCLs is either free radical polymerization of 2-methylene-1-3dioxepane3 or ionic ring-opening polymerization of ecaprolactone (CL)4 using a variety of enzymatic,5–7 organic,8–12 and metal catalyst systems.13–15 They are usually obtained by metal-alcoholate initiators, producing hydroxyl end-functionalized polymers.16–20 The use of diols affords a,x–hydroxytelechelic PCLs.4,16,17 Alternatively, the use of metal carboxylate initiators for CL polymerization has also been well studied.20–22 along PCL chains consists of the synthesis and polymerization of functional CL substituted at the a- or c-position by a large series of functional groups such as halogen, acid, acyloxy, alcohol, amine, allyl, or an ATRP initiating group.2,23–32 Usually a multi-step process involving protection and deprotection of functional groups before and after polymerization was necessary, since the functional groups may coordinate with metal catalysts.27 Alternatively, CL has been ringopening copolymerized with other functionalized monomers.33–36 A different post-polymerization route was the grafting of functional groups or macromolecular chains onto PCLs through the a-position of the carbonyl, although chain degradations were hardly avoidable.37–40 Functional groups have often been added to render PCLs more hydrophilic, adhesive, or biocompatible to diversify their applications, or for subsequent coupling or crosslinking reactions toward different macromolecular structures The methodology for introduction of pendant functional groups KEYWORDS: activated monomer; allyl; functionalization; poly(e- caprolactone); thiol–ene A strategy of choice to obtain PCLs functionalized at one end is the polymerization of CL initiated by functional alcohols, such as 2-hydroxyethyl methacrylate,41 propargyl alcohol,42,43 2-mercaptoethanol,44 or a-(2,4-dinitrophenylthio)ethanol.45 Hedfors et al.44 has also employed functional terminators in an enzyme-catalyzed polymerization process of CL to end–cap PCLs with the thiol functionality On the other hand, end-functionalization of pre-formed PCLs represents a convenient alternative The hydroxyl end groups of PCLs can be chemically modified by coupling reactions of hydroxyl groups with carboxylic acid, anhydride, acyl halide, Additional Supporting Information may be found in the online version of this article C 2017 Wiley Periodicals, Inc V 928 JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG isocyanate, or carboimidazole.46–51 As such, functional groups, such as methacrylate,42,43,52,53 acrylate,46,54,55 isocyanate,49 furan and maleimide,47 amine,56 bromide,57 carboimidazole,50 allyl,51 and thiol45,58 have been incorporated in PCLs as terminal groups Alternatively, polymers bearing end-block triplebond, benzaldehyde, naphthoyl chloride, benzyl chloroformate, and iodomethane groups, can be obtained using macroinitiators with functional end groups to initiate either cationic activated monomer (AM)59–63 or anionic polymerization64,65 of functional cyclic monomers Aliphatic PCLs containing double bond substituents are of particular interest owing to the derivatization of the double bond into a large variety of functional groups and the exploitation of its reactivity to obtain polymeric networks The synthesis of PCLs end-capped with acrylate and methacrylate groups via chemical modification of hydroxyl end groups, followed by further derivatization of these functional groups or subsequent free-radical crosslinking of polymer chains, have been reported.42,43,46,52–55,66–68 Recently, Boire et al.32 copolymerized CL with a-allyl carboxylate e-caprolactone to synthesize PCLs bearing pendant allyl groups, which were photo-crosslinked via free-radical polymerization of allyl groups Taha and co-workers51 obtained multi-branched poly(ester urethane)s bearing PCL segments and allyl end functions via two steps, including the synthesis of isocyanate terminated prepolymers by reacting a hydroxyl-terminated PCL and glycerol with an excess of a diisocyanate and the subsequent reaction of the isocyanate terminated prepolymers with allyl amine These multiallyl-functionalized PCLbased polymers were then free-radical photo-copolymerized with 2-hydroxyethylmethacrylate During the last decades, the polymer functionalization strategy via “click” and coupling reactions has already proved to be an efficient and straightforward pathway.69–72 Among a wide range of metal-free “click” reactions,73 the photoinduced and thermally induced thiol–ene radical addition reactions have been widely recognized as a fast and robust tool for polymer synthesis and modification.74–77 Although in many cases, radical-mediated thiol–ene reactions not fulfill the “click” criteria and can only be considered as coupling reactions, they have still proved to be a promising approach to readily attach functional groups, particularly biomolecules like proteins and peptides commonly bearing free thiols.71,78,79 In this respect, there are few examples where PCLs containing either thiol side groups or (meth)acrylate end groups have been post-modified via thiol–ene coupling reactions.38,42,54,55,80 Nottelet et al.38 synthesized a PCL grafted with pendant thiol groups and crosslinked the obtained thiol-functionalized polymer via the thermally initiated thiol–ene coupling reaction with a tri-allyl compound The group of Mather and co-workers54,55 conducted the photo-crosslinking of terminally di-acrylated PCLs by the thiol–ene reaction with a tetrathiol compound The radical-initiated thiol–methacrylate addition reaction has been applied on methacrylated PCLs to attach an amino acid or cholesterol moiety for imparting desired bioactivity.42,80 WWW.MATERIALSVIEWS.COM ARTICLE In this study, we present a straightforward approach to obtain PCLs decorated with a variable number of functional side groups situated at the chain termini This strategy involves consecutive additions of protonated allyl glycidyl ether ring-molecules to hydroxyl-ended PCL macromolecules, proceeding according to the AM mechanism.59,61,62,81 While PCL-based polymers consisting of either pendant allyl functions randomly distributed along the backbone or multibranches terminated by allyl groups have been created,32,51 this is the first study of linear PCL materials fitted with multiple pendant non–activated double bonds at the same chain ends, which can be facilely prepared via a one-stage reaction Finally, the use of allyl side groups as modification sites for further functionalization by a photoinduced radical thiol–ene coupling reaction with furfuryl mercaptan has been demonstrated The use of the “food-grade” furfuryl mercaptan compound as a model thiol has the advantages of non-toxicity and that the 1H NMR furan signals are clearly separated from all others and thus can be integrated without interference Although not within the scope of this study, we believe that the possibility to localize as well as vary the number of functional groups may result in final materials with diversified thermal and physical properties, and that the resulting furan-functionalized PCLs with variable content of furan pendants are expected to find broad applications, such as in the fabrication of drug carrier materials,82,83 self-healing and thermoreversible polymeric materials exploiting the diene role of furan in Diels–Alder reactions,84 or biocompatible materials with enhanced fire-resistance imparted by furan entities.85–89 EXPERIMENTAL Materials Poly(e-caprolactone) diol (Mn 2000 g mol21, manufactured with diethylene glycol as initiator, Mn 1H NMR 2112 g mol21, Acros) and poly(e-caprolactone) diol (CAPA 2403D, manufactured with butanediol as initiator, Mn 4000 g mol21, Mn 1H 21 , Perstorp) were azeotropically dried with NMR 4090 g mol toluene before use Allyl glycidyl ether (AGE, 991%, TCI Chemicals–Japan) was dried over molecular sieves and distilled under vacuum before use Dichloromethane (99.9%, Fisher Chemicals) was dried over CaH2 and distilled Tetrafluoroboric acid diethyl ether complex (HBF4ÁEt2O, 901%, Sigma-Aldrich), furfuryl mercaptan (FM, 971%, Sigma-Aldrich), 2,2-dimethoxy-2-phenylacetophenone (DMPA, 99%, Sigma-Aldrich), calcium oxide (96%, Fisher Chemicals), triphenylphosphine (TPP, 99%, Sigma-Aldrich), thioglycerol (TG, 98%, Evans Chemetics/Bruno Bock), 1,10 -(methylenedi-4,1-phenylene) bismaleimide (95%, Sigma-Aldrich), ethyl acetate (99%, Fisher Chemicals), methanol (99%, Fisher Chemicals), and n-heptane (99%, Fisher Chemicals) were used as received Measurements H NMR spectra were recorded in deuterated chloroform (CDCl3) with TMS as an internal reference, on a Bruker Avance 300 at 300 MHz Transmission Fourier transform infrared (FT-IR) spectra, collected as the average of 128 scans with a resolution of JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 929 ARTICLE JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG TABLE Characterization of Allyl-Functionalized PCLs Starting PCL-Diol Structurea Mn (g mol21)b HOA(CL)18AOH 2112 HOA(CL)35AOH 4090 Allyl-Functionalized PCL Mn (g mol21)e intensity Ratio of Observed-to-Theoretical Terminal AGE (a0 ) Signal Structurea Entry HOA(AGE)yA(CL)18A (AGE)zAOH 5.8 5.0 2683 0.96 3.7 3.6 2523 1.20 3.1 3.0 2455 1.12 5.0 4.0 4547 1.09 10.0 8.8 5094 1.23 20.0 16.1 5928 1.05 30.0 17.7 6110 1.09 HOA(AGE)yA(CL)35A (AGE)zAOH a Additional unit corresponding to initiator used in the synthesis of the commercial PCL-diol present in PCL is not shown b As estimated from 1H NMR spectra of the starting PCL-diol (Figs S1 and S2, Supporting Information) c Calculated on the basis of the Mn (NMR) of the starting PCL-diol and the feeding amount of AGE (for complete AGE conversion) d Calculated on the basis of 1H NMR analysis of the purified product [Fig 1(a) and Figs S4–S9, Supporting Information] e On the basis of 1H NMR analysis, Mn Mn (starting PCL) Mn (AGE) (y z) cm21, were recorded from KBr disk on an FT–IR Bruker Tensor 27 Attenuated total reflectance (ATR) FT-IR spectra were collected as the average of 128 scans with a resolution of cm21 on a FT-IR Tensor 27 spectrometer equipped with a Pike MIRacle ATR accessory with a diamond/ZnSe element Gel permeation chromatography (GPC) measurements were performed on a Polymer PL-GPC 50 gel permeation chromatograph system equipped with an RI detector, with THF as the eluent at a flow rate of 1.0 mL/ Molecular weight and molecular weight distribution (Ð) were calculated with reference to polyethylene glycol standards Thermogravimetric analysis (TGA) measurements were performed under nitrogen flow using a NETZSCH STA 409 PC Instruments with a heating rate of 10 8C/min from ambient temperature to 800 8C Differential scanning calorimetry (DSC) measurements were carried out with a DSC Q20 V24.4 Build 116 calorimeter under nitrogen flow, from 240 to 170 8C at a heating rate of 10 8C/min and cooling rate of 50 8C/min vacuum line Yields: 78–85% for Entries 1–6, Table and 56% for Entry Table Synthesis of Allyl-Functionalized PCLs A typical reaction procedure for the synthesis of allylfunctionalized PCLs is described: Commercial PCL diol (CAPA 2403D) with Mn 4090 g mol21 (3.45 g, 1.69 mmol of –OH groups) was dissolved in 17.5 mL of dichloromethane in a round–bottom flask To this solution, 0.017 mL (0.12 mmol) of HBF4ÁEt2O was added Then, a nitrogen flow was passed over the mixture and the flask was closed with a rubber septum 0.5 mL (4.22 mmol) of AGE was slowly introduced with a syringe during h The reaction mixture was kept at room temperature for 24 h, and, after that, the acid catalyst was neutralized with solid CaO After filtration of CaO, the solution was concentrated and the product was isolated by precipitation three times in methanol (dichloromethane/methanol 1/5, v/ v), washed three times with methanol, filtered and dried on 930 y z, 1H NMRd y z, Theor.c JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 Thiol–Ene Addition Reaction of Allyl-Functionalized PCLs with Furfuryl Mercaptan As an example, the reaction of an allyl-functionalized PCL (Entry 4, Table 1, PCL4000–4ene, HOA(AGE)yA(CL)35A(AGE)z–OH) with y z 4, Mn 4546.6 g mol21, containing in average allyl groups per chain) with furfuryl mercaptan is described In a flask containing a stirring bar and closed with a rubber septum, 2.82 g (2.48 mmol of allyl groups) of PCL4000–4ene, in a round-bottom flasked dipped in an oil bath, was melted at 55 8C under stirring After stopping heating, PCL4000–4ene remained as a clear liquid and a minimum amount of tetrahydrofuran was added to maintain the polymer in the liquid state at room temperature Then, 64 mg (10 mol% with respect to allyl groups) of DMPA and 0.5 mL (4.96 mmol of thiol groups) of furfuryl mercaptan were added in succession In the case of using TPP, TPP was added at the same time as thiol The reaction mixture was degassed and was purged with nitrogen through a needle using vacuum/nitrogen line After an overnight exposure to UV light (wavelength of 365 nm, with twelve lamps of W circularly oriented), the product was collected by precipitation three times in diethyl ether and was further dried under vacuum (1023 torr) at 60 8C to remove any left unreacted furfuryl mercaptan Finally, colu Mn chromatography of the product (ethylacetate: n-heptane, 1: 2) was performed to eliminate traces, if any, of DMPA species (Rf 0.6–0.7) and furfuryl mercaptan (Rf 0.82) (Fig S12, Supporting Information) The purified polymer product was remained in the silica colu Mn (Rf in ethylacetate: n-heptane, 1: 2) and finally eluted by ethylacetate (Rf 0.94, Fig S13, Supporting Information) JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ARTICLE SCHEME The reaction pathway to allyl-functionalized PCLs and subsequent functionalization via thiol–ene chemistry Crosslinking of Furan-Functionalized PCLs (Demonstration Test) A mixture of a furan-functionalized PCL and 1,10 -(methylenedi-4,1-phenylene)bismaleimide in a 1:1 furan to maleimide equivalent ratio in tetrahydrofuran was injected in molds and cured at 30 8C for 48 h After opening the molds, the samples were washed by a Soxhlet extraction in acetone at 60 8C to eliminate un-crosslinked materials and solvent RESULTS AND DISCUSSION The straightforward and up-scalable synthetic pathway to allyl-functionalized PCLs and subsequent functionalization via thiol–ene reaction is described in Scheme The incorporation of allyl groups to PCL chain ends was carried out by the addition of an allyl glycidyl ether (AGE) unit (proceeded by earlier activation of AGE by the protic acid HBF4ÁEt2O) in the presence of the polymeric diol Two commercial PCLdiols with average Mn values given by the suppliers of 2000 and 4000 g mol21 were used as macroinitiators for cationic polymerization of AGE The accurate Mn values of the PCLdiol macroinitiators were determined by 1H NMR (Figs S1 and S2, Supporting Information) In all cases, AGE was slowly introduced to the system containing macroinitiator and HBF4ÁEt2O as catalyst An addition of the protonated AGE to the terminal hydroxyl groups gives rise to the modified PCLdiol containing multiple repeating AGE units, located at both polymer chain ends The characteristics of the PCL-diol macroinitiators and the obtained modified PCLs are presented in Table WWW.MATERIALSVIEWS.COM The allyl-functionalized PCLs were characterized by 1H NMR analysis Taking into consideration on one hand the Mn values of the PCL-diol macroinitiators previously determined, and on the other hand the molar ratio between AGE and CL units determined from 1H NMR spectra, the average number of allyl groups and Mn values of the obtained products were calculated Figure 1(a) shows a representative 1H NMR spectrum of the obtained AGE-functionalized PCL (entry 1, Table 1) The spectra of all other samples can be found in the Supporting Information (Figs S4–S9) In the spectra, all expected signals corresponding to CL and AGE monomer units are present By comparing the intensity of signals corresponding to the alkene group (signal f at 5.27–5.04 ppm) with that of the separate signal assigned to repeating CL units (signal m at 2.55–2.05 ppm) and taking into account the degree of polymerization of the starting PCL-diols, the total number of attached AGE units per polymer chain could be calculated as shown in Table An additional 1H NMR analysis of the reaction mixture before purification via precipitation showed that the number ratio of AGE and CL units was in good agreement with the feed amount ratio of AGE and PCL-diol (Fig S3, Supporting Information) It is worth noting that side reactions such as the polymerization of AGE proceeded via the activated chain end mechanism or a reaction between an activated AGE and moisture, resulting in homopoly(AGE) byproduct, might not be completely excluded.62 Unreacted AGE and poly(AGE) byproduct are soluble in methanol,90–93 and thereby could be removed by multiple precipitation in methanol Accordingly, for a relatively low feed AGE to JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 931 ARTICLE WWW.POLYMERCHEMISTRY.ORG JOURNAL OF POLYMER SCIENCE FIGURE 1H NMR spectra in CDCl3 of an allyl-functionalized PCL (Entry 1, Table 1) before (a) and after (b) thiol–ene reaction with furfuryl mercaptan (Entry 4, Table 2) The signal denoted as a0 corresponds to the methine protons of HOACH(R)A groups of terminal HO–AGE units The signal denoted as i0 corresponds to the methylene protons of A(AGE)yACH2(R)A groups of CL units next to AGE units [Color figure can be viewed at wileyonlinelibrary.com] PCL-diol molar ratio (3.1–20.0), 1H NMR analysis of the purified product indicated that the number of allyl groups incorporated into a PCL chain was slightly lower than the theoretical value (Entries 1–6, Table 1) In these cases, the average composition of the products corresponds to 3–16.1 AGE units per PCL chain Nevertheless, the oligomerization of AGE initiated by PCL-diol was less efficient with further increasing the feed amount ratio of AGE and PCL-diol As 932 JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 demonstrated by Entries and in Table 1, the average number of AGE end units only slightly increased from 16.1 to 17.7 despite a large increase in the feed AGE to PCL-diol molar ratio from 20 to 30 The decreased efficiency of the AGE oligomerization via the AM mechanism is attributed to the increase in the ratio of the dropwise-added AGE and the hydroxyl polymer end groups over reaction time Consequently, more competing side reactions could occur This JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG explanation can also be applied to the observation that the AGE oligomerization was generally more efficient when initiated with the PCL-diol of lower Mn We have been interested to introduce multiple allyl groups to PCL chain termini However, although not within the scope of this study, we anticipate that it is still possible to incorporate one or two AGE units by the present approach, since the concentration ratio between the AGE and the hydroxyl end groups in the reaction medium could be more easily maintained low In addition, analysis of 1H NMR spectra could reveal whether AGE units were attached at both ends of PCL chains In the spectra of AGE-functionalized PCLs, the methine resonance of HOACH(R)A groups of terminal HO–AGE units (signal denoted as a0 ) may be identified As an example shown in Figure 1(a), although signal a0 partially overlaps with others (i.e., signals i and d), the total integral value of peaks i, d and a0 in the range of 4.15–3.88 ppm was obtained separately Thus, by taking into consideration the integrals of peak i (calculated from the intensity of signal m and the intensity ratio of peak i and m previously obtained from the spectrum of the starting PCL-diol in Figure S1, Supporting Information) and peak d (equal to the integral of peak f), an estimation of the integral value of signal a0 was possible Assuming that each PCL chain has two AGE end groups, from the degree of polymerization of the starting PCL-diol and the intensity of the 1H NMR signal corresponding to repeating CL units (signal m), the theoretical integral value of the terminal HOACH(R) A group signal (a0 ) can be estimated and is compared with that obtained from the spectra (Table 1) The calculation of intensity ratio of observed-to-theoretical terminal a0 signal can be expressed as follows:   Int: a0 obs: Int: ½4:15–3:88 ppmŠ Int: i Int: f 2 52DP diol PCL2 Int: m Int: m Int: m Int: a0 theor: (1) where Int is the abbreviation for integral, Int: i=Int: m equals to 0.877, as estimated from the 1H NMR spectrum of the starting PCL-diol in Supporting Information Figure S1, and -DPPCL2 diol is the degree of polymerization of the starting PCL-diol In principle, the observed-to-theoretical intensity ratio of signal a0 is for PCL chains bearing two AGE end groups and is 0.5 for those with one AGE terminal group For all samples, an observed-to-theoretical intensity ratio of signal a0 equal to approximately confirms the structure of PCL chains with both AGE end groups Transmission FT–IR spectra of the allyl-functionalized PCLs additionally confirmed the incorporation of allyl groups by the appearance of the alkene @CAH stretching vibration at 3070 cm21 (Fig S10, Supporting Information) The addition of AGE units to PCL chains was also indicated by GPC analysis (Fig S11, Supporting Information) of the allylfunctionalized PCL revealing a shift to a higher molecular weight (Mn 5970 g mol21; Ð 1.26), as compared with WWW.MATERIALSVIEWS.COM ARTICLE that of the corresponding starting PCL-diol (Mn 4750 g mol21; Ð 1.21) PCLs with pendant allyl groups were used for the UVinitiated thiol–ene reaction with furfuryl mercaptan as a model thiol (Scheme 1) Therefore, 1.1–5 equivalents of furfuryl mercaptan with respect to one allyl group were used in the coupling reactions with allyl-functionalized PCLs It should be noted that the unreacted thiol after coupling reactions and initiator species were removed mainly by precipitation and evaporation Moreover, any trace of these remaining molecules was eliminated completely by colu Mn chromatography purification, as evidenced by TLC analysis (Fig S12, Supporting Information) Representative 1H NMR spectra of an allyl-functionalized PCL before and after thiol– ene reaction with furfuryl mercaptan (Entry 4, Table 2) are shown in Figure 1; spectra for other thiol–ene reactions are shown in Supporting Information The occurrence of thiol–ene reactions on the allylfunctionalized PCLs was evidenced by a decrease in the 1H NMR signal of the alkene protons (signal e at 5.92–5.71 ppm and f at 5.27–5.04 ppm, Fig 1) and the concurrent appearance of new signals corresponding to the coupled thiol molecule, that is signals of furan group at 7.29, 6.23 and 6.11 ppm [signals s, r, q in Fig 1(b), respectively], methylene group of the formed thio-ether linkage at 2.50 ppm [signals f0 , Fig 1(b)] and the methylene group next to the thio-ether group at 1.75 ppm [signal e0 in Fig 1(b)] The efficiency of thiol–ene reactions was confirmed by integration of the signals in the 1H NMR spectra of the products of allyl-functionalized PCLs coupled with furfuryl mercaptan By comparing the signal intensities before and after coupling reactions, using a separate signal corresponding to the polymer backbone as the reference, both the conversion of allyl groups and the number of attached thiol molecules per allyl group could be determined For all samples, good agreement between conversion of allyl groups and the amount of attached furfuryl mercaptan was observed [Fig 1(b) and Figs S14–S30, Supporting Information] The conditions and corresponding degree of functionalization by thiol–ene reactions are summarized in Table The results presented in Table show that the applied conditions of the thiol–ene reaction considerably impact the conversion of allyl groups, particularly the content of photoinitiator (DMPA) For instance, a higher functionalization degree was obtained with increasing photoinitiator content between and 25 mol% with respect to allyl groups (comparing Entries 1–4 and Entries 7, 9–12, Table 2) Thus, a sufficient amount of photoinitiator is necessary to achieve a high functionalization degree, as also previously noted.71,94,95 Apparently, the amount of photoinitiator used for the optimal conditions compensates for any inhibition of the photoreaction by oxygen quenching We observed that, with equivalents of thiol, the thiol–ene reaction conversion was optimal at a photoinitiator content of 25 mol% (Entries 4, 7, Table 2) Further increasing photoinitiator content led to no JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 933 ARTICLE JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG TABLE Conditions and Functionalization Degree of Thiol–Ene Reactions on Allyl-Functionalized PCLs 934 Thiola [–SH]/[allyl] in Feed Mol% of DMPAb Functionalization Degree (%)b Entry Structure, y z HOA(AGE)yA(CL)18A(AGE)zAOH, FM 1.1 54 HOA(AGE)y–(CL)18–(AGE)z–OH, FM 1.1 15 62 HOA(AGE)y–(CL)18–(AGE)z–OH, FM 15 71 HOA(AGE)y–(CL)18–(AGE)z–OH, FM 25 76 HOA(AGE)y–(CL)18–(AGE)z–OH, FM 15 62 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 1.1 25 65 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 25 74 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 25 74 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 57 10 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 61 11 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 10 70 12 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 15 70 13 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 35 74 14 HOA(AGE)y–(CL)35–(AGE)z–OH, 8.8 FM 25 70 15 HOA(AGE)y–(CL)18–(AGE)z–OH, FM 25 10 $100 16 HOA(AGE)y–(CL)35–(AGE)z–OH, FM 25 10 $100 17 HOA(AGE)y–(CL)35–(AGE)z–OH, 8.8 FM 25 10 98 18 HOA(AGE)y–(CL)35–(AGE)z–OH, TG 25 10 $100 [TPP]/[-SH] a FM is furfuryl mercaptan and TG is thioglycerol Mol% with respect to allyl groups b Defined as the percentage of allyl groups coupled with thiol, which was estimated based on 1H NMR spectra in Figure 1(b) and Figures S14–S30, Supporting Information further enhancement of functionalization extent (comparing Entry with Entry 13, Table 2) Indeed, a too high initiator concentration could give rise to considerable termination reactions as a result of radical re-combinations71 On the other hand, as side reactions such as disulfide bond formation or thiyl radical combination are inevitable in thiol addition reactions,71,72,94,95 the use of an excess amount of thiol was necessary to increase functionalization efficiency With closeto-equimolar ratios of thiol to allyl, coupling efficiencies of only 54–65% were obtained (Entries 1, and 6, Table 2) Two equivalents of the thiol with respect to allyl groups, in combination with a high initiator concentration, were sufficient to obtain maximum conversions of approximately 75% The use of more than equivalents of the thiol did not further improve the reaction conversion (see Entries and 8, Table 2) It is also worth noted that when the photoinitiator content was not sufficiently high, a highly increased amount of thiol could result in a decrease in coupling efficiency as it decreases the overall alkene and photoinitiator concentration in the system (comparing Entries and 5, Table 2) The fact that a full functionalization was not achieved due to termination reactions and mainly disulfide formation has also been reported earlier for thiol addition reactions involving polymeric species.71,72,94 In an attempt to suppress disulfide formation, triphenylphosphine (TPP) as a tertiary phosphine was added in the reaction mixture as a disulfide reducing agent Such approach has been previously reported to successfully enhance thiol-ene functionalization efficiencies.71 With the use of a ten-fold excess of the reducing agent with respect to thiol, complete reduction of all disulfide bonds formed in the reaction was achieved, resulting in nearly 100% conversion of thiol–ene coupling reactions (Entries 15–17, Table 2) In addition, applying the same optimal reaction conditions, a full conversion of the thiol–ene coupling of the allyl-functionalized PCL with thioglycerol as an alternative functional thiol was also obtained (Entry 18, Table and Fig S30, Supporting Information) Generally, with a coupling efficiency of 100% with the addition of a tertiary phosphine and around 75% without the use of a disulfide reducing agent, this thiol–ene coupling approach appears very promising to readily attach on-demand functional groups JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 In addition, the coupling of the allyl group with furfuryl mercaptan was also confirmed by transmission FT–IR analysis Figure demonstrates a comparison of the IR spectra of an allylfunctionalized PCL (Entry 1, Table 1) and corresponding products after coupling it with furfuryl mercaptan With increasing the functionalization degree, the intensity of the absorption signal corresponding to double bonds at 3079 cm21 decreases.96 Concurrently, the intensities of the signals corresponding to furan groups at 3146, 3117 va 1011 cm21 increase.97,98 Changes in polymer structure as a result of the introduction of pendant functional units at the PCL chain termini were first demonstrated by DSC analysis As shown from both DSC JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ARTICLE FIGURE Comparison of the transmission FT–IR spectra (normalized to the band area of the C@O stretching vibrations at 1814– 1664 cm21) in the range of 3220–3000 and 1300–650 cm21 of an allyl-functionalized PCL (a, Entry 1, Table 1) and furanfunctionalized PCL products after thiol–ene reactions with furfuryl mercaptan corresponding to functionalization efficiencies of 54 and 71% (b–Entry and c–Entry 3, Table 2, respectively) [Color figure can be viewed at wileyonlinelibrary.com] heating curves in Figure 3, allyl- and furan-functionalized PCLs show no glass transition in the range of 230 to 170 8C, but exhibit multiple melting peaks The multiple melting behavior is attributed to a “melting2recrystallization2melting” process,99 and thus, the total melting enthalpy did not reflect the degree of crystallinity of the polymer samples This suggests that for these samples, perfect crystals were not formed upon quenching, and they recrystallize in the heating process The low endotherm is attributed to a superposition of early melting of secondary crystals with almost simultaneous exothermic recrystallization, and the final endotherm contains contributions from the melting of primary crystals and the melting of FIGURE DSC thermograms of PCL (a, Mn 4090 g mol21) and corresponding allyl-functionalized PCL (b, Entry 4, Table 1) and furan-functionalized PCLs (c and d, corresponding to Entries and 7, Table with thiol–ene coupling efficiencies of 57 and 74%, respectively) The curves are vertically shifted for clarity [Color figure can be viewed at wileyonlinelibrary.com] WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 935 ARTICLE JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG TABLE Thermal Data Obtained Form DSC Analysis of PCL, Allyl- and Furan-Functionalized PCLsa 1st Heating Cooling 2nd Heating Low Endotherm Tm (8C)b High Endotherm Tm (8C)b Tc (8C)c DH (J/g)d Low Endotherm Tm (8C)b PCL (Mn 4090 g mol21) – 57.3 13.8 66.9 – 50.8 Allyl-functionalized PCL (Entry 4, Table 1) 36.0 55.0 10.9 60.6 42.9 48.5 Furan-functionalized PCL (Entry 9, Table 2) 34.1 52.2 4.2 55.4 38.7 47.6 Furan-functionalized PCL (Entry 7, Table 2) 34.1 48.5 2.8 55.8 37.0 44.1 a b c Data obtained from thermograms in Figure Melting temperature Crystallization temperature recrystallized regions formed during heating The melting temperatures and crystallization enthalpy during cooling of PCL and corresponding allyl- and furan-functionalized PCLs are summarized in Table The results show that the incorporation of AGE units to PCL resulted in decreases in melting temperature and crystallization (during cooling) enthalpy The same trend was observed when the allyl-functionalized PCL High Endotherm Tm (8C)b d Crystallization enthalpy, determined via integration of the crystallization signal in the cooling scan was further coupled with furfuryl mercaptan Hence, increasing the number of furan methylene groups (for example comparing the samples with furan functionalization degrees of 57 and 75%) led to a slight decrease in melting temperature The insertion of pendant furan methylene moieties to the polymer chain ends resulted in molecular defects, giving rise to destabilization of the ordered packing Comparative TGA thermograms of a PCL sample before and after functionalization with allyl and furan groups additionally indicated changes in polymer structure (Fig S31, Supporting Information) The functionalization of PCL with furan groups resulted in a slight increase in the char yield The char-yielding behavior is typical of furanic polymers.100 This is ascribed to the formation of char on the upper part of the material, which prevents the formation of volatile compounds from the inner part FIGURE Crosslinking reaction between furan-functionalized PCLs and 1,10 -(methylenedi-4,1-phenylene)bismaleimide, the image of the thermoset T1 obtained from PCL4000–8.8furan (Entry 17, Table 2) without shape-memory (a), and the images of the thermoset T2 obtained from PCL4000–4furan (Entry 16, Table 2) with shape-memory behavior (b: permanent shape; c: programmed shape; shape recovery is shown in Fig S34, Supporting Information) [Color figure can be viewed at wileyonlinelibrary com] 936 JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 55, 928–939 The effect of varying the number of pendant functional units at the PCL chain termini was further demonstrated by comparing the properties of two thermosets crosslinked from two PCL products bearing in average 8.8 and furan groups per chain (samples T1 and T2 corresponding to the furan-functionalized PCLs in Entry 17 and 16, Table 2, respectively) Crosslinking was carried out via Diels–Alder reaction between furan groups and 1,10 -(methylenedi-4,1-phenylene)bismaleimide, with maleimide to furan equimolar ratio (Fig 4), giving samples with high crosslinking contents (97–98%) and insoluble in common solvents at room temperature The occurrence of Diels–Alder reaction was confirmed by the ATR FT-IR result, showing the disappearance of the furan signal at 1011 cm21 and the maleimide signals at 830 and 687 cm21, as well as the increase in intensity of the signal at 863 cm21 attributed to the Diels–Alder linkage (Fig S32, Supporting Information).101 It has been reported that in both physically and thermally crosslinked PCL materials, the crystallized PCL phase with melting and crystallization transitions can act as thermally triggered switching domains, resulting in shape-memory properties For sample T1, the higher crosslinking density as a result of the higher number of pendant furan groups hindered the crystallization of PCL JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG chains Thus, no PCL melting endotherm was found in the DSC heating curve (Fig S33a, Supporting Information), and the materials showed no shape-memory behavior On the other hand, sample T2 showed a PCL melting transition at 44 8C (Fig S33b, Supporting Information), as well as shape-memory behavior Temporary shapes (for example, spiral and stretched strip) of T2 could be programmed by twisting or stretching the samples at temperatures above 50 8C, followed by cooling down to room temperature to fix the temporary shapes (Fig and Fig S34, Supporting Information) The original permanent shapes were recovered by heating the samples at the above deformation temperature (Fig S34, Supporting Information) Besides, the DSC heating curves of the thermosets showed a broad endotherm in the range of 80–170 8C, assigned to the breakage of the Diels–Alder bonds.97 Because the Diels–Alder bonds reform at low temperatures (20–60 8C),97 these thermosets were recyclable (demonstrated in Fig S35, Supporting Information) CONCLUSIONS Activated monomer (AM) oligomerization of AGE using commercially available PCL-diols as macroinitiators in combination with thiol–ene coupling reactions provides a convenient synthetic route for the synthesis of PCLs functionalized with multiple functional side groups at the chain termini A disadvantage of previously reported approaches to end-cap PCLs generally with functionalities and more specifically with furyl groups via coupling reactions of PCL hydroxyl end groups is the impossibility of varying the number of the terminal functional groups Apparently, the strategy used in this study providing PCLs with adjustable, multiple furyl end-block groups as reactive polymer precursors allows for a broader ability to tune the final material structure and properties In general, this synthetic platform is envisaged to be promising for introduction of several various functional groups such as biomolecules or natural compounds containing free thiols to PCLs for on-demand biomaterial applications for example ARTICLE E A Rainbolt, K E Washington, M C Biewer, M C Stefan, Polym Chem 2015, 6, 2369–2381 A Seema, R Liqun, Macromolecules 2009, 42, 1574–1579 M Labet, W Thielemans, Chem Soc Rev 2009, 38, 3484–3504 R T MacDonald, S K Pulapura, Y Y Svirkin, R A Gross, D L Kaplan, J Akkara, G Swift, S Wolk, Macromolecules 1995, 28, 73–78 E Stavila, G O R Alberda van Ekenstein, A J J Woortman, K Loos, Biomacromolecules 2014, 15, 234–241 L A Henderson, Y Y Svirkin, R A Gross, D L Kaplan, G Swift, 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PCLs A typical reaction. .. synthesis of isocyanate terminated prepolymers by reacting a hydroxyl-terminated PCL and glycerol with an excess of a diisocyanate and the subsequent reaction of the isocyanate terminated prepolymers... to allyl- functionalized PCLs and subsequent functionalization via thiol–ene chemistry Crosslinking of Furan -Functionalized PCLs (Demonstration Test) A mixture of a furan -functionalized PCL and