Highly Branched Polycaprolactone/Glycidol Copolymeric GreenPlasticizer by One-Pot Solvent-Free PolymerizationKyu Won Lee,† Jae Woo Chung,*,‡ and Seung-Yeop Kwak*,†

†Department of Materials Science and Engineering, and Research Institute of Advanced Materials (RIAM), Seoul National University,1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea‡Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Korea

*S Supporting Information

ABSTRACT: This study aims to develop a simple, low-costmethod for synthesis of highly branched polycaprolactone(hbPCL) for use as effective “green” plasticizers for poly(vinylchloride) (PVC). We demonstrate the facile synthesis ofhbPCL with tunable molecular architecture using glycidol as abranching monomer. A series of hbPCLs is prepared via one-pot, solvent-free copolymerization of ε-caprolactone andglycidol, wherein the molecular architecture is readilycontrolled by varying the molar ratio of glycidol to ε-caprolactone. Further, studying the kinetics of copolymeriza-tion reveals the preferential reaction of glycidol over ε-caprolactone, resulting in a multiarm star-like copolymer afterthe ring-opening of the two monomers. The crystallization ability of hbPCL is found to gradually weaken with the introductionof the branching structure, and its molecular mobility is improved substantially by esterification with butyric anhydride, followingwhich a maximum mobility is realized at an intermediate level of branching. The butyl-esterified hbPCL (hbPCL-C4) is misciblewith PVC, and their mixtures have excellent flexibility comparable to that of PVC/bis(2-ethylhexyl) phthalate (DEHP). Inparticular, the stretchability of PVC/hbPCL-C4 is superior to that of PVC/DEHP, owing to its better structural homogeneity.Furthermore, PVC/hbPCL-C4 shows outstanding migration stability with the weight loss after extraction being >85% lower thanthat of PVC/DEHP.

KEYWORDS: Highly branched polymer, Polycaprolactone, Glycidol, Copolymerization, Green plasticizer

■ INTRODUCTION

Over the past 20 years, polycaprolactone (PCL) has attractedconsiderable attention owing to its excellent biocompatibilityand biodegradability, low melting/glass transition temperatures,high miscibility with a variety of commercial polymers, andpotential to be synthesized from a monomer derived fromsustainable resources.1−5 Owing to these unique properties,PCL has been widely investigated in both academic andindustrial fields for various applications, e.g., as a drug deliverysystem,6,7 scaffold in tissue engineering,8,9 food packagingmaterial,10 and thermoplastic elastomer.11 One of the mostattractive and valuable applications of PCL is as an eco-friendlyplasticizer replacing phthalate, a low-molecular-weight organiccompound primarily used as a plasticizer in poly(vinyl chloride)(PVC).12−14 Phthalate easily migrates out of the matrix duringprocessing or use, thereby causing severe deterioration of thephysical properties of PVC products, and potentially acts as anenvironmental hormone for a human health hazard.15,16 Severalstudies have demonstrated that flexible PCL segments showplasticizing effects in PVC, and PCL plasticizers have bettermigration stability than phthalate owing to their high molecularweights. However, the plasticizing efficiency of PCL is much

lower than that of phthalate, which is a challenge to beovercome for the practical application of PCL plasticizers.Highly branched polymers have the characteristics of low

melting point, low crystallinity, low melt viscosity, low chainentanglement, high solubility, and high end-group concen-tration compared to linear polymers.17−20 In this regard, weanticipated that introducing a branching structure into the PCLbackbone could enhance its molecular mobility, and henceprovide sufficient flexibility to PVC. Indeed, our grouppreviously reported that hyperbranched poly(ε-caprolactone)(HPCL) exhibits a high plasticizing efficiency concurrent withits excellent migration stability.21 However, despite thedistinguishing performance of HPCL, its complicated manu-facturing processes, including the synthesis of macromonomersvia protection−polymerization−deprotection steps and poly-condensation of the macromonomers, impede their large-scaleproduction at a low cost. Therefore, a simple synthetic methodshould be developed for the highly branched PCL to becommercially available.

Received: March 26, 2018Revised: May 16, 2018Published: May 29, 2018

Research Article

pubs.acs.org/journal/ascecgCite This: ACS Sustainable Chem. Eng. 2018, 6, 9006−9017

© 2018 American Chemical Society 9006 DOI: 10.1021/acssuschemeng.8b01356ACS Sustainable Chem. Eng. 2018, 6, 9006−9017

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To date, several synthetic strategies have been reported forthe preparation of highly branched PCLs. These strategies canbe mainly classified into two approaches: (i) step growth and(ii) chain growth polymerization. The former approach is basedon self-condensing polymerization of intrinsically branchedAB2-type macromonomers, as described above.22−25 Mean-while, the latter approach involves multibranching copolymer-ization of ε-caprolactone (CL) and branching AB2 monomer.For example, Frey et al. reported the synthesis of hyper-branched copolyesters through combined ring-opening poly-merization/AB2 polycondensation of CL and 2,2-bis-(hydroxymethyl)butyric acid.26 Although this method isconveniently accessible in one pot, it still requires the use ofa toxic solvent, expensive catalysts, and a continuous waterremoval system during the polymerization. Recently, Irvine etal. successfully prepared highly branched PCLs using 4,4′-bioxepanyl-7,7′-dione (BOD) as a branching agent.27 In thiswork, PCLs with a well-defined branching structure wereachieved via one-pot bulk copolymerization of CL and BOD inthe presence of commercially relevant tin 2-ethylhexanoate(Sn(Oct)2) catalyst. However, this strategy required a separateprocess to synthesize BOD, and the thermal/physical proper-ties of highly branched PCLs were very close to those of linearPCL as a result of the extremely low content of the branchingBOD unit in the final product.Glycidol, one of the most versatile derivatives of renewable

glycerol, has been widely used as a raw material in the chemicalsynthesis of polyglycerols, glycidyl ethers, and polyurethanes, aswell as in perfumes, cosmetics, surface coatings, detergents, andpaints.28−30 In particular, since glycidol is converted to theglycerol branching unit after anionic/cationic ring-openingpolymerization, a number of studies have demonstrated theavailability of glycidol to produce highly branched polymers viacopolymerization with other monomers.31−33 From thisperspective, we hypothesized that glycidol would be a suitablebranching monomer for PCL, if it can be ring-opened underthe typical conditions used for synthesizing PCL, i.e., a one-pot,solvent-free, and Sn(Oct)2-catalyzed polymerization. However,to date, there have been only a few reports describing the ring-opening of glycidol in the presence of Sn(Oct)2.

34,35

Furthermore, to the best of our knowledge, there are noreports on the ring-opening multibranching copolymerizationof CL with glycidol.In this work, we present the facile synthesis of PCL with

tunable molecular architecture using glycidol as a novelbranching agent. The highly branched PCLs (hbPCLs) weresynthesized via Sn(Oct)2-catalyzed bulk copolymerization ofCL and glycidol in a single pot. We achieved hbPCLs withdistinctly different branching structures by varying the feedmolar ratio of glycidol with respect to the core material, and wesystematically investigated the structure−property relationshipof the obtained hbPCLs to gain an in-depth understanding oftheir plasticization behaviors. According to our study, thehbPCLs could prove to be promising candidates for thedevelopment of sustainable, safe, and feasible plasticizers.

■ EXPERIMENTAL SECTIONMaterials. Glycidol, 1,4-benzenedimethanol (BDM), Sn(Oct)2,

and bis(2-ethylhexyl) phthalate (DEHP) were purchased from Sigma-Aldrich Ltd., Korea. ε-Caprolactone and butyric anhydride waspurchased from Tokyo Chemical Industry Co., Ltd., Japan. Dichloro-methane (DCM), tetrahydrofuran (THF), n-hexane, sodium bicar-bonate (NaHCO3), and magnesium sulfate (MgSO4) were sourced

from Daejung Chem., Korea. PVC resin LS100 was provided by LGChem. Ltd., Korea. The thermal stabilizer SONGSTAB CZ-400 (Ca/Zn complex) was supplied by Songwon Co., Ltd., Korea. All chemicalswere used as received without purification.

Synthesis of Highly Branched Polycaprolactone (hbPCL) viaCopolymerization of CL and Glycidol. The typical procedure usedfor the synthesis of hbPCL is as follows: 0.415 g of BDM (3 mmol)and 13.697 g of CL (0.12 mol, 40 equiv) were added to a 250 mLthree-necked, round-bottom flask, and different quantities of glycidol(0.444, 1.111, 2.222, and 3.334 g, which are 2, 5, 10, and 15 equiv withrespect to BDM) were added to the reaction mixture to control thebranched architecture of hbPCLs. The mixture was stirred vigorouslyunder heating for 30 min to obtain a homogeneous mixture. Then, themixture was heated in an oil bath at 130 °C, and a catalytic amount(0.2 equiv) of Sn(Oct)2 was added. The reaction mixture was allowedto react at the same temperature for 16 h under argon atmosphere, andthen quenched with 100 mL of n-hexane. This sample is referred to ashbPCLx, where x indicates the average length of branched segments ofthe hbPCLs. The same procedure was used for the synthesis of thelinear PCL (LPCL) without the addition of glycidol into the reactionmixture. To investigate the kinetics of copolymerization, aliquots ofthe reaction mixture were retrieved at different time intervals anddissolved in DMSO-d6 for NMR analysis. In the kinetic experiments,DMSO-d6 was used as a solvent instead of chloroform-d since theintermediates are polar and did not dissolve in chloroform-d.

Esterification of Free Hydroxyl End Groups of hbPCL. Aknown quantity of hbPCL (e.g., 10 g) and an excess of butyricanhydride (1.5 equiv corresponding to the hydroxyl groups in hbPCL)were allowed react at 120 °C for 3 h under argon atmosphere. Thereaction mixture was dissolved in DCM, washed several times with 1M NaHCO3, and then dried with MgSO4. The solution wassubsequently filtered and concentrated under reduced pressure. Thepolymer product was dried for 24 h at 60 °C under vacuum to obtainbutyl-esterified hbPCL (hbPCLx-C4).

Preparation of PVC Films. PVC resin (3.0 g, 100 parts perhundred resin (phr)), plasticizer (1.8 g, 60 phr), and thermal stabilizer(0.06 g, 2 phr) were added to 50 mL of THF and vigorously stirred for2 h at room temperature. The mixture was subsequently poured into aglass Petri dish and dried in an oven at room temperature for 24 h andthen at 40 °C for 24 h to completely remove the residual solvent. Afterdrying, flexible PVC films with a thickness of approximately 0.20 mmwere obtained.

Characterization. 1H NMR and 13C NMR spectra were recordedin chloroform-d at 600 MHz on a Bruker Avance 600 spectrometer.Size exclusion chromatography (SEC) was carried out at 35 °C inTHF (flow rate = 1.0 mL min−1) on a Waters Alliance e2695 equippedwith a refractive index detector and three Waters Styragel columns.Number-average molecular weight (Mn) and molecular weightdistribution (Mw/Mn) were calculated by calibrating with polystyrenestandards. Differential scanning calorimetry (DSC; Netzsch DSC 200F3) was performed under a nitrogen atmosphere over the temperaturerange from −100 to 140 °C at a heating rate of 10 °C min−1. X-raydiffraction (XRD; New D8 Advance, Bruker) patterns were collectedat room temperature using Cu Kα radiation (λ = 1.541 Å), at a voltageof 40 kV and current of 40 mA, over the Bragg angle (2θ) range of15−30° with scan rate of 2° min−1. Steady state viscosity measure-ments were carried out using a stress-controlled rheometer (RS-1,Thermo Fisher Scientific, Germany) in the temperature range of 60−100 °C over the shear rate range of 1−500 s−1. Thermogravimetricanalysis (TGA) was carried out under flowing nitrogen up to 700 °Cand a heating rate of 10 °C min−1. Fourier transform infrared (FTIR)spectra were acquired using a Thermo Scientific Nicolet 6700.Dynamic mechanical analysis (DMA; TA Instruments DMA 2980)was conducted in tension mode at a frequency of 1 Hz at −60 to 110°C with a heating rate of 3 °C min−1. The oscillatory amplitude andstatic force were 15 μm and 0.01 N, respectively. The mechanicalproperties of the film samples were evaluated using an Instron-5543universal testing machine (UTM) at a strain rate of 20 mm min−1 witha 50 N static load cell. UV−vis spectra were recorded at ambienttemperature using a PerkinElmer Lambda25 spectrophotometer over

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the wavelength range of 400−900 nm. The migration stability wasdetermined by estimating the amount of plasticizer that migrated outof samples placed under harsh conditions. The specimens for testingwere prepared with the dimensions of 20 × 20 × 0.20 mm3. Theextraction test was carried out based on ASTM D5227; the specimenswere immersed in 1 L of n-hexane and stirred at 50 °C for 2 h. Thedegree of migration was evaluated by gravimetric analysis.

■ RESULTS AND DISCUSSION

Synthesis of Highly Branched Polycaprolactone. Aseries of hbPCLs containing glycerol branching units was

prepared through Sn(Oct)2-catalyzed bulk copolymerization ofCL and glycidol, as depicted in Scheme 1. To accuratelycharacterize the chemical structure of the obtained copolymers,1,4-benzenedimethanol (BDM) bearing an aromatic ring wasselected as the core material. To achieve hbPCLs with differentmolecular architectures, the feed molar ratios of glycidol to thecore BDM were adjusted to 2, 5, 10, and 15. LPCL wasprepared as the linear counterpart serving as a control. Figure1a presents the representative 1H NMR spectrum of hbPCLwith the assignment of characteristic proton peaks of PCL. Thebroad peak observed in the range of 4.69−3.27 ppm is

Scheme 1. Synthetic Scheme for Sn(Oct)2-Catalyzed Bulk Copolymerization of ε-Caprolactone and Glycidol

Figure 1. (a) Representative 1H NMR spectrum of hbPCL with proton assignments, (b) 1H NMR spectra of LPCL and hbPCLs with differentbranch structures, and (c) SEC traces of synthesized hbPCLs and LPCL.

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attributed to the protons in the glycerol branching unitsproduced after the ring-opening of glycidol.36 As shown inFigure 1b, the relative peak intensities of terminal (f) to inner(e) units increased with increasing molar ratio of glycidol,indicating a high end-group concentration, which is character-istic of highly branched polymers. The degrees of polymer-ization of CL (DPCL) and glycidol (DPglycidol) were calculatedfrom the ratios of the protons corresponding to the CLsegment (2.30 ppm) and the integral of all the protons in theglycerol unit to the protons in the core (7.33 ppm). Asdisplayed in Table 1, DPCL and DPglycidol are in good agreement

with the target values, implying that the ring-opening reactionof glycidol was highly quantitative under Sn(Oct)2 catalyst. Themolar fraction of glycidol ( fglycidol) was calculated from thecomposition, as determined using 1H NMR spectroscopy. Theaverage length of branched segments (⟨NCL⟩) of hbPCLs wasestimated using the equation

= +N fDP /( DP )CL CL core glycidol (1)

where fcore is the functionality of the core. The ⟨NCL⟩ valuesdecreased gradually from 19.8 to 2.3 with increasing DPglycidol,indicating a more compact and globular structure of theobtained PCLs (Table 1). Figure 1c shows the SEC traces of allthe PCL samples, exhibiting unimodal peaks with moderatemolecular weight distributions. The number-average molecular

weight (Mn) of the PCL samples decreased steadily from 5362to 1400 g mol−1 as the DPglycidol increased (Table 1), which isconsistent with the descending trend of ⟨NCL⟩. The obtainedPCLs can be regarded as oligomers considering their Mn’s.However, note that the actual molecular weights of PCLs maybe larger than the values estimated by SEC analysis, becausehighly branched polymers are generally smaller than linearpolymers with the same molecular weight. Therefore, highlybranched PCLs with controlled molecular structures weresuccessfully obtained by multibranching copolymerization ofCL and glycidol.To elucidate the macromolecular architectures of the

synthesized hbPCLs, we studied the kinetics of copolymeriza-tion on the basis of time-dependent NMR measurements.Figure 2a shows the 1H NMR spectra of samples retrieved atdifferent time intervals during the copolymerization of CL andglycidol. The NMR spectra exhibit individual signals corre-sponding to the protons of methine (a), methylene (b and c) ofglycidol, and methylene (d) of CL, with a decrease in theintensities of the monomer peaks and simultaneous increase inthe characteristic peak (d′) of the PCL segments over thecourse of the copolymerization. For quantitative analysis, theconversion of each monomer is plotted as a function of thereaction time (Figure 2b). The conversion rate of glycidol isobserved to be significantly faster than that of CL. This higherreactivity of glycidol may be ascribed to the strong ring strain inthe epoxide moiety of glycidol. Thus, the branch points formedmainly during the early stage of polymerization, resulting inmultiarm star-like PCL, as illustrated in Figure 3c.39 Thecopolymerization with glycidol induced variations in themolecular architecture of PCL, which would in turn affectvarious properties of PCL, including its ability to crystallize, itsmolecular mobility, and its plasticizing performance whenincorporated into the PVC matrix.

13C NMR spectroscopy was used to investigate the chemicalstructures of hbPCLs in detail. The glycidol monomer producesa glycerol branching unit with one primary and one secondaryhydroxyl group after the ring-opening reaction. As thesecondary hydroxyl group is known to have lower reactivity

Table 1. Molecular Characterization of Highly BranchedPolycaprolactone (hbPCL) and Linear Polycaprolactone(LPCL)

samplea DPCL DPglycidol fglycidol ⟨NCL⟩ Mn (g mol−1) Mw/Mn

LPCL19.8 39.5 0 0 19.8 5362 1.65hbPCL9.6 39.3 2.1 0.05 9.6 2986 1.47hbPCL5.8 40.1 5.0 0.11 5.8 1961 1.38hbPCL3.3 39.5 10.1 0.20 3.3 1610 1.28hbPCL2.3 39.4 15.1 0.28 2.3 1400 1.17

aIn the sample notation hbPCLx and LPCLx, x indicates the averagelength of branched segments (⟨NCL⟩).

Figure 2. (a) 1H NMR spectra of samples retrieved after different time intervals during the copolymerization of CL and glycidol with a feed molarratio of [CL]:[glycidol]:[BDM] = 40:5:1, (b) plots of monomer conversion versus time, and (c) schematic illustration of molecular architecture ofthe resulting copolymer on the basis of reactivity ratios of monomers.

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than the primary hydroxyl group to the initiation of CLmonomer, the reaction of the secondary hydroxyl group isnecessary to confirm whether glycidol indeed acts as abranching agent.37,38 Figure 3a shows the 13C NMR spectraof LPCL and hbPCLs corresponding to the carbons of the estergroups, in which LPCL shows only one peak at 173.5 ppm,whereas the hbPCL samples show an additional peak at 173.7ppm due to the esters generated from the secondary hydroxylgroup. This could be rationalized by the fact that the reactiontemperature in our experiments was higher than the temper-ature typically used for polymerizing PCL, which mightpromote the reactivity of secondary hydroxyl groups in the

glycerol branching unit to initiate CL. On one hand, thehbPCLs exhibit individual signals arising from carbon atoms inthe core, glycerol unit, and CL repeating unit, as shown inFigure 3b,c. It is clear that the peak intensities corresponding tothe terminal PCL segment (1′, 2′, and 3′) increased inproportion to the content of the branching unit in the hbPCL.This further confirms that highly branched structures wereintroduced into the PCL main chain owing to copolymerizationwith glycidol.The DEPT 135 and 13C NMR spectra were recorded to

accurately assign peaks of the glycerol units (Figure S2). Thepeaks corresponding to the dendritic (D), linear (L13, L14), and

Figure 3. 13C NMR spectra of LPCL19.8 (black line), hbPCL5.8 (blue line), and hbPCL2.3 (red line) corresponding to carbon atoms of (a) estergroups; (b) BDM, glycerol unit, and terminal CL; and (c) inner CL.

Figure 4. Crystallization characteristics of hbPCLs and LPCL. (a) Second DSC heating curves, (b) maximum endothermic temperatures as afunction of end group concentration, (c) DSC cooling curves, (d) XRD patterns, and (e) phases of hbPCLs and LPCL.

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terminal (T) units of glycerol were observed at 75−60 ppm,and the peak corresponding to the dendritic units (DCL1 andDCL2) connected to the caprolactone unit were also observed at67.6, 65.2 ppm and 66.5, 64.9 ppm, respectively. These resultsindicate that not only the homopolyglycerol moiety but also theglycerol unit exists as a branching point between linearoligocaprolactone segments in the molecular structure ofhbPCL. We measured inverse-gated (IG) 13C NMR forquantitative analysis of hbPCLs. Unfortunately, the content ofglycerol could not be obtained due to the very low intensity ofthe peaks of the glycerol unit as well as the partial overlap ofsignals and the noise level inherent to IG NMR (Figure S3).However, a notable finding is that, as the content of glycerolincreases, the proportion of DCL2 relative to DCL1 increases,indicating that the dendritic glycerol units present in the linearoligocaprolactone segments are relatively dominant. In otherwords, as the ratio of glycerol in hbPCL increases, it becomescloser to an ideal highly branched polymer in which branchingpoints are randomly distributed throughout the molecularstructure.Crystallization Behavior and Molecular Dynamics of

hbPCL. DSC was used to investigate the effect of branchedarchitecture on the crystallization behavior of hbPCLs. Figure4a shows the second DSC heating traces of hbPCLs and LPCL.LPCL shows a sharp and intense endothermic peak at 52.9 °Csimilar to that of a typical PCL, whereas hbPCLs show broaderpeaks with relatively lower intensities. Additionally, as plottedin Figure 4b, the maximum endothermic (melting) temperatureof the PCL samples decreased inversely with an increase in theend-group concentration (i.e., as the branching level increased).Similar to the observation for the melting temperatures (Tm’s),the crystallization temperatures (Tc’s) of hbPCLs shift towardlower temperature, and concurrently, the degree of crystallinity(Xc) decreased with increasing branching level (Figure 4c).

These results indicate that the crystallization ability of hbPCLswas weakened by the introduction of the branching structure.The branch points in hbPCLs would act like structural defects,and hence disturb the molecular packing and long-rangeordering of the hbPCLs. In particular, hbPCL3.3 and hbPCL2.3showed much lower Xc values of 26.0 and 16.1%, suggestingthat the length of the linear backbone segments decreased to aneffective level to hinder the crystallization of hbPCL. Apartfrom DSC, XRD was used to determine the crystal structure ofhbPCLs at room temperature. As shown in Figure 4d, the XRDpatterns of LPCL19.8, hbPCL9.6, and hbPCL5.8 exhibit thecharacteristic diffraction peaks of the PCL crystal, correspond-ing to its (110), (111), and (220) planes.40 When the branchedsegments are smaller, the characteristic peaks became lessintense and shifted to higher angles, suggesting the decrease inthe degree of crystallization and the interplanar distance of thepolymers, respectively. In contrast, the XRD patterns ofhbPCL3.3 and hbPCL2.3 have no peaks, indicating a fullyamorphous phase because of their Tm’s lower than roomtemperature. In fact, LPCL19.8 and hbPCL9.6 are powders andhbPCL5.8 exists in a waxy state, whereas hbPCL3.3 andhbPCL2.3 are gel-like materials, as shown in Figure 4e.Considering that the molecular mobility of a plasticizer is

closely related to its plasticization efficiency, it is necessary toinvestigate the molecular dynamics of hbPCLs in order tounderstand their plasticizing behaviors in PVC. The glasstransition temperature (Tg) refers to the temperature at whichthe amorphous polymer chains change from a glassy to arubbery state. At temperatures above Tg, the polymer chainsbecome flexible with a free volume; therefore, Tg is generallyconsidered to be an indicator of the cooperative motion ofpolymer segments. We therefore measured and compared Tg’sof hbPCLs to evaluate their molecular mobility with respect tothe branching structure. As the branching level in hbPCL

Figure 5. (a) DSC thermograms of hbPCLs and LPCL (upper) and those of hbPCL-C4s and LPCL-C4 (lower) and (b) steady shear viscosity as afunction of shear rate for hbPCL-C4s and LPCL-C4. (c) Dependence of glass transition temperature and steady shear viscosity on linear backbonelength. (d) Variation in length of linear segments and end-group concentration of hbPCL-C4s and LPCL-C4.

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increases, however, the number of hydroxyl end groupsincreases and, as a consequence, the segmental motion ofhbPCL is hindered by inter- and intramolecular hydrogenbonds.41 Accordingly, we modified the hydroxyl end groups ofthe polymers to butyl ester groups to eliminate the hydrogen-bonding effect (Figure S4).42 As shown in Figure 5a,unmodified hbPCL has a single Tg at approximately −60 °Csimilar to that of LPCL regardless of their ⟨NCL⟩. Meanwhile,the butyl-esterified hbPCLs (hbPCL-C4s) have significantlylower Tg’s than hbPCLs, reflecting their enhanced molecularmobility owing to the removal of hydrogen bonds. The Tg’s ofhbPCL-C4s decreased gradually as ⟨NCL⟩ decreased until ⟨NCL⟩became 3.3 and slightly increased thereafter (Figure 5a). Inother words, the Tg’s showed a minimum at the intermediatebranching level. This result suggests that the branching point inhbPCL contributed to the enhancement in the molecularmobility at the initial stage of introduction, but it acted as anobstacle to the segmental motions along the polymer beyond acertain level. However, it should be noted that the molecular

mobility of semicrystalline polymers could be decreased due tothe crystalline confinement.43,44 LPCL19.8-C4, hbPCL9.6-C4,and hbPCL5.8-C4 still exhibited sharp melting peaks in theirDSC curves (Figure S5), indicating that the polymer chains inthe amorphous region would be constrained by the crystallineregion. Furthermore, given the fact that the plasticizer isuniformly distributed among the PVC chains to form ahomogeneous mixture in the plastic material, it is necessary tostudy the molecular dynamics of hbPCL-C4s in the absence ofexternal factors such as crystallization, aggregation, and chainentanglements.According to the lubricity theory of the plasticization

mechanism, a plasticizer serves as a lubricant for PVC, thusproviding softness and ductility to the rigid PVC backbone.45,46

Therefore, the rheological property of a plasticizer is consideredto be the most direct and reliable predictor of its plasticizingperformance. In this study, the steady shear viscosity wasevaluated to investigate the molecular dynamics of hbPCL-C4sand LPCL-C4. Figure 5b shows the shear rate dependence of

Figure 6. (a) FTIR spectra of plasticized PVCs. FTIR spectra showing carbonyl regions of (b) LPCL19.8-C4, (c) hbPCL9.6-C4, (d) hbPCL5.8-C4,(e) hbPCL3.3-C4, (f) hbPCL2.3-C4, (g) DEHP, and corresponding PVC mixtures. FTIR spectra of plasticized PVC for (h) methylene deformationand (i) C−Cl stretching regions. (j) Ratios of absorbance intensities of amorphous and crystalline bands of plasticized PVCs.

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the steady shear viscosity (η) for hbPCL-C4s and LPCL-C4 intheir melt states. For all samples, the measured viscosity wasfound to be independent of the shear rate, indicating aNewtonian fluid behavior.47 This suggests that none of thesamples have any aggregation or entanglement of chains in thenetwork. The η values at 60 °C were found to decrease in theorder LPCL-C4 (2.29 Pa·s) > hbPCL9.6-C4 (0.55 Pa·s) >hbPCL2.3-C4 (0.45 Pa·s) > hbPCL3.3-C4 (0.29 Pa·s) >hbPCL5.8-C4 (0.24 Pa·s). The η values for hbPCL-C4s andLPCL-C4 showed the same order at other temperatures (70−100 °C) (Figure S6). This descending trend of η is similar tothat of Tg, with a difference that the lowest value was observedfor hbPCL5.8-C4 (Figure 5c). In other words, hbPCL5.8-C4had the most enhanced molecular mobility among the PCLsamples. This can be attributed to the result of excluding therestricted molecular motion due to the crystallization of thelinear segments.In our copolymer, increasing the molar ratio of glycidol

resulted in a large number of end groups as well as a morecompact branched structure of the hbPCL-C4 (Figure 5d). Theformer provides more free volume to the polymer chains, thusenhancing the molecular mobility.48 In contrast, the latterreduces the length of the linear backbone segments, therebydecreasing the flexibility of the chain.49 We speculate that themolecular mobility of hbPCL-C4 is significantly affected by theincrease in the end-group concentration at the initial stage ofthe introduction of the branching structure; however, thedecrease in the flexible linear PCL segments begins to dominateafter the critical branching point is passed. As a result, hbPCL-C4 with a moderately branched architecture had the highestmolecular mobility among the PCLs of various architectures.Therefore, through detailed analysis of the correlation betweenthe branching structure of hbPCL-C4s and their dynamics,hbPCL5.8-C4 is expected to exhibit the best plasticizingperformance in the PVC.We performed TGA and compared the thermal stabilities of

hbPCL-C4s with that of DEHP. As shown in Figure S7,

hbPCL-C4s degraded at a higher temperature than DEHP,indicating that hbPCL-C4s have better thermal stability thanDEHP. Note that the 5% weight-loss temperatures (Td5) ofhbPCL-C4s were between 310 and 375 °C, which is above theextrusion temperature range of PVC (180−200 °C). Mean-while, the Td5 of DEHP was 185 °C, suggesting that someDEHP may be lost during processing.

Plasticizing Effects of hbPCL on PVC. Plasticized PVCfilms containing 60 phr (∼38 wt %) hbPCL-C4s and LPCL-C4were fabricated individually via solution blending, and a PVCfilm with 60 phr DEHP was also prepared in the same manner.As shown in Figure 6a, all the FTIR spectra of plasticized PVCshave characteristic bands at 2940, 1730, 1430, and 625 cm−1

corresponding to the C−H stretching, CO stretching, C−Hbending (methylene), and C−Cl stretching, respectively. PVC/hbPCL-C4s and PVC/LPCL-C4 show an additional IR band at1165 cm−1 corresponding to C−O stretch of the ester group,whereas the C−O stretching band of PVC/DEHP shifted to1273 cm−1 due to the aromatic group in DEHP. Figure 6b−gshows the FTIR spectra of the plasticizers and thecorresponding PVC mixtures in the carbonyl region.Apparently, all the plasticizers showed a significantly shiftedCO band after mixing with PVC, implying that the COgroups of the plasticizer strongly interact with the polarmoieties (C−Cl bonds) of PVC, and thus, these componentsare compatible with each other.50 In addition to the shift of thecarbonyl bands, the ratio of the amorphous and crystallinebands of plasticized PVC indicates the compatibility betweenthe plasticizer and PVC. As the plasticizer can only solvate theamorphous regions of PVC, the relative intensities of theamorphous bands decrease when the plasticizer is morecompatible with PVC.51 Parts h and i of Figure 6 show theFTIR spectra of plasticized PVCs in the wavenumber ranges of1440−1420 and 660−580 cm−1, corresponding to themethylene deformation and C−Cl stretching, respectively. Inall cases, the amorphous (1434 and 610 cm−1) and crystalline(1426 and 636 cm−1) bands are observed in these regions.

Figure 7. (a) DSC thermograms of neat PVC and plasticized PVCs. (b) Temperature dependence of storage modulus and loss tangent of neat PVCand plasticized PVCs. (c) Comparison of plasticization efficiencies and structural homogeneities of plasticized PVC samples.

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Further, the ratios of absorbance intensities of the amorphousand crystalline bands (A1434/1426 and A610/636) were estimated.Despite a slight difference in the order of the two ratios, it isobvious that the ratios of PVC/hbPCL3.3-C4 and PVC/hbPCL2.3-C4 are significantly higher than those of PVC/LPCL19.8-C4, PVC/hbPCL9.6-C4, and PVC/hbPCL5.8-C4(Figure 6j). This indicates that hbPCL3.3-C4 and hbPCL2.3-C4 are less compatible with PVC. This might be due to thehigh concentration of the glycerol branching unit, whichrenders the molecules more polar and restricted. The ratios ofthe absorbance intensities of PVC/DEHP were found to be ashigh as those of PVC/hbPCL3.3-C4 and PVC/hbPCL2.3-C4.Consequently, all the plasticized PVCs are considered to bemiscible mixtures; however, hbPCL3.3-C4, hbPCL2.3-C4, andDEHP are relatively less compatible with PVC than LPCL19.8-C4, hbPCL9.6-C4, and hbPCL5.8-C4.The addition of a plasticizer imparts free volume to PVC,

thereby lowering its Tg. Therefore, the plasticizing ability of theplasticizer can be evaluated by comparing the reduction in Tg.As shown in the DSC thermograms, all the PVC mixtures havea single Tg at much lower temperature than that of neat PVC,indicating the homogeneity of all the mixtures and adequateplasticization of PVC (Figure 7a). Remarkably, the Tg values ofPVC/hbPCL-C4s were found to be comparable to that ofPVC/DEHP, despite the much bulkier molecular structures ofhbPCL-C4s relative to that of DEHP (Figure 7c). This can berationalized by the synergistic effect of the flexible linear PCLsegments and a large number of the highly mobile end groupsof hbPCL-C4. From the perspective of the free volume theoryof the plasticizing mechanism, free-moving end chains inaddition to the flexible linear segments effectively create freevolume in the PVC, resulting in a further reduction in thesoftening temperature. Although the plasticization efficiency ofhbPCL-C4s was slightly lower than that of low-molecular-weight DEHP, it was significantly higher than that of LPCL-C4,confirming that hbPCL-C4s are sufficient as alternativeplasticizers. In particular, hbPCL5.8-C4 showed superiorplasticizing ability among the hbPCL-C4s, which correlateswell with the results of the molecular dynamics of hbPCL-C4s.Therefore, the proper branching structure could promote themolecular mobility of hbPCL-C4s, thus increasing the degree ofplasticization of the PVC mixtures. The distribution in the glasstransition for neat PVC and PVC mixtures was determinedbased on the difference between the onset temperature (Tonset)and the end temperature (Tend). As shown in Figure 7c, PVC/hbPCL3.3-C4, PVC/hbPCL2.3-C4, and PVC/DEHP exhibitedbroad glass transition temperatures compared to othermixtures, which is relevant to high fluctuation in the localconcentration of hbPCL-C4 in miscible blends.52,53 Thisimplies that these mixtures had poorer structural homogeneitythan others, which is related to lower compatibility between theplasticizer and PVC, as determined by FTIR analysis.The viscoelastic properties of PVC/hbPCL-C4 were also

characterized to evaluate the plasticizing ability of hbPCL-C4.Figure 7b shows the storage modulus (E′) and loss tangent (tanδ) of neat PVC and plasticized PVC as a function oftemperature. The E′ decreases steadily as the temperatureincreases, indicating the dissipation of energy. The E′ curves forplasticized PVC are located below and to the left of that of neatPVC, implying that the stiffness and rigidity of PVC decreasedwith the addition of the plasticizer. Similarly, the tan δ peaks ofplasticized PVCs shift to a lower temperature than that of neatPVC (Figure 7b). The α relaxation temperature (Tα relaxation) of

plasticized PVCs increased in the order PVC/DEHP (25.0 °C)< PVC/hbPCL5.8-C4 (28.5 °C) < PVC/hbPCL9.6-C4 (30.0°C) < PVC/hbPCL3.3-C4 (32.1 °C) < PVC/LPCL19.8-C4(35.3 °C) < PVC/hbPCL2.3-C4 (36.3 °C). This resultindicates that the moderately branched architecture ofhbPCL-C4 effectively improved its plasticizing ability, whichis in good agreement with DSC results. The full width at half-maximum (fwhm) of the tan δ peak was estimated to assess thestructural homogeneity of the plasticized PVC. As shown inFigure 7c, the obtained fwhm values are consistent with thedistribution in the glass transition temperatures, confirming therelatively lower uniformity of PVC/hbPCL3.3-C4, PVC/hbPCL2.3-C4, and PVC/DEHP.Next, we carried out tensile tests to investigate the impact of

the plasticization efficiency and structural homogeneity ofPVC/hbPCL-C4s on their mechanical properties. All theplasticized PVCs display typical stress−strain characteristics offlexible PVC (Figure 8a). The plasticized PVCs simultaneously

exhibit much smaller stress and much larger strain during theirdeformation compared to neat PVC, because the plasticizerweakened the intermolecular friction forces between the PVCchains. The tensile strength and elongation at break of eachplasticized PVC sample were determined to compare theirmechanical properties in greater detail (Figure 8b). Asexpected, PVC/hbPCL5.8-C4 has the lowest tensile strengthamong the PVC/hbPCL-C4s with the strength being slightlyhigher than that of PVC/DEHP. Notably, in case of the

Figure 8. (a) Representative stress−strain curves of neat PVC andplasticized PVCs. (b) Variation in tensile strength and elongation atbreak of neat PVC and plasticized PVCs.

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elongation at break, PVC/hbPCL5.8-C4 exhibited the largestvalue of 397%, which is even greater than that of PVC/DEHPby 21%. Such exceptional stretchability of PVC/hbPCL5.8-C4can be attributed to its high degree of plasticization concurrentwith the good structural homogeneity. For the plasticized PVCwith poor homogeneity, a high concentration of the plasticizerwould induce structural defects in the system, resulting in rapidcrack propagation and subsequent fracture under the tensileload.54,55 As shown in Figure 8b, the elongation at breakdecreased substantially for PVC/hbPCL3.3-C4 (338%) andPVC/hbPCL2.3-C4 (312%), which supports the aboveassumption. Consequently, it can be concluded that thestructural integrity as well as the high degree of plasticizationplayed crucial roles in enhancing the stretchability of flexiblePVC.The transparency of plasticized PVC is an essential property

for its practical applications. As displayed in Figure 9a, PVC/

hbPCL5.8-C4 and PVC/DEHP are highly transparent, whereasPVC/LPCL19.8-C4 and PVC/hbPCL9.6-C4 contain randomlydistributed impurities on their surfaces. This is likely due to theformation of PCL crystals as a result of the high tendency ofLPCL19.8-C4 and hbPCL9.6-C4 to crystallize in PVC.56 PVC/LPCL19.8-C4 had more pronounced impurities than PVC/hbPCL9.6-C4 due to the much higher crystallinity ofLPCL19.8-C4. In contrast, PVC/hbPCL3.3-C4 and PVC/hbPCL2.3-C4 were observed to be foggy throughout thesurface, which is attributed to the lower uniformity of thesemixtures.57 As observed in the UV−vis spectra (Figure 9b), thetransmittances of PVC/hbPCL5.8-C4 and PVC/DEHP are ashigh as 90% over the visible wavelength range, whereas those of

the other PVC films are considerably lower than those of theformer mixtures. Therefore, both crystallinity and dispersity ofthe plasticizer should be carefully considered to achievetransparent PVC.The migration stability of plasticized PVC was estimated by

leaching tests conducted under harsh experimental conditions(heated in n-hexane at 50 °C for 2 h). When flexible PVC isapplied as a packaging material and comes into contact withoily foods, the plasticizer tends to migrate out from the PVCmatrix. The use of n-hexane as the extraction medium forplasticizer is intended to accelerate the migration of plasticizerin harsher environments, as n-hexane readily swells PVC at hightemperature and has a solubility profile similar to that ofcooking oil. The weight loss of PVC products caused by themigration of the plasticizer must not exceed 5.5% for theirpractical usage.58 As shown in Figure 10, the weight losses of

PVC/hbPCL-C4s ranged from 2.4 to 4.6%, which are belowthe limit of 5.5%, whereas that of PVC/DEHP was found to beextremely high at 36%. Such remarkable migration stability ofPVC/hbPCL-C4 can be attributed to the high molecular weight(i.e., bulky molecular structure) of hbPCL-C4 along withnumerous ester groups in the molecular structure, which canstrongly interact with PVC. Moreover, even if a small amountof hbPCL-C4 migrated out during processing or in use, thePCL-based plasticizer is believed to be harmless to humans andenvironment owing to its well-known biocompatibility andbiodegradability. Thus, our hbPCL-C4s will be applicable invarious PVC products requiring long-term stability andnontoxicity, such as medical devices, interior materials, infanttoys, and food packaging.

■ CONCLUSIONSThe synthesis of hbPCL was accomplished in a straightforwardmanner via the Sn(Oct)2-catalyzed, one-pot, and solvent-freecopolymerization of ε-caprolactone and glycidol. A series ofhbPCLs with different lengths of linear backbone segments wasobtained by simply varying the molar ratio of glycidol. Thestudy of the copolymerization kinetics revealed that theresulting copolymers had multiarm star-like architectures.With the introduction of the branching structure, thecrystallization ability and molecular mobility of hbPCLschanged drastically. The molecular mobility of hbPCL wasconsiderably enhanced by butyl esterification, and the mobilityof the butyl-esterified hbPCL reached a maximum at an

Figure 9. (a) Digital images and (b) UV−vis spectra of plasticizedPVCs.

Figure 10. Weight loss of plasticized PVCs heated in n-hexane at 50°C for 2 h.

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intermediate branching level (for hbPCL5.8-C4). Indeed,hbPCL5.8-C4 was miscible with PVC, and showed excellentplasticizing effect, which is comparable to that of DEHP. Inparticular, the stretchability of PVC/hbPCL5.8-C4 was betterthan that of PVC/DEHP, owing to its better structuralhomogeneity. Furthermore, PVC/hbPCL-C4s showed out-standing migration stability compared to that of PVC/DEHP.Consequently, our hbPCLs are extremely attractive assustainable, safe, and feasible plasticizers for versatileapplications of flexible PVC.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssusche-meng.8b01356.

13C NMR spectra of hbPCLs and LPCL, 1H NMRspectra of hbPCL and hbPCL-C4, DSC thermograms ofhbPCL-C4s and LPCL-C4, temperature dependence ofviscosity for PCL samples, TGA curves for PCL samplesand DEHP (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: sykwak@snu.ac.kr. Tel.: +82-2-880-8365. Fax: +82-2-885-1748 (S.-Y.K.)*E-mail: jwchung@ssu.ac.kr. Tel: +82-2-828-7047. Fax: +82-2-817-8346 (J.W.C.).

ORCIDJae Woo Chung: 0000-0002-2082-9209Seung-Yeop Kwak: 0000-0002-8903-4287NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported in part by the Korean Ministry ofScience and Technology under the National ResearchLaboratory (NRL) Program (RIAM).

■ REFERENCES(1) Labet, M.; Thielemans, W. Synthesis of Polycaprolactone: AReview. Chem. Soc. Rev. 2009, 38, 3484−3504.(2) Woodruff, M. A.; Hutmacher, D. W. The Return of a ForgottenPolymer-Polycaprolactone in the 21st Century. Prog. Polym. Sci. 2010,35, 1217−1256.(3) Averous, L.; Moro, L.; Dole, P.; Fringant, C. Properties ofThermoplastic Blends: Starch-Polycaprolactone. Polymer 2000, 41,4157−4167.(4) Schneiderman, D. K.; Hillmyer, M. A. Aliphatic Polyester BlockPolymer Design. Macromolecules 2016, 49, 2419−2428.(5) Buntara, T.; Noel, S.; Phua, P. H.; Melian-Cabrera, I.; de Vries, J.G.; Heeres, H. J. Caprolactam from Renewable Resources: CatalyticConversion of 5-Hydroxymethylfurfural into Caprolactone. Angew.Chem., Int. Ed. 2011, 50, 7083−7087.(6) Kumari, A.; Yadav, S. K.; Yadav, S. C. Biodegradable PolymericNanoparticles Based Drug Delivery Systems. Colloids Surf., B 2010, 75,1−18.(7) Dash, T. K.; Konkimalla, V. B. Polymeric Modification and ItsImplication in Drug Delivery: Poly-ε-caprolactone (PCL) as a ModelPolymer. Mol. Pharmaceutics 2012, 9, 2365−2379.(8) Williams, J. M.; Adewunmi, A.; Schek, R. M.; Flanagan, C. L.;Krebsbach, P. H.; Feinberg, S. E.; Hollister, S. J.; Das, S. Bone Tissue

Engineering Using Polycaprolactone Scaffolds Fabricated via SelectiveLaser Sintering. Biomaterials 2005, 26, 4817−4827.(9) Hutmacher, D. W.; Schantz, T.; Zein, I.; Ng, K. W.; Teoh, S. H.;Tan, K. C. Mechanical Properties and Cell Cultural Response ofPolycaprolactone Scaffolds Designed and Fabricated via FusedDeposition Modeling. J. Biomed. Mater. Res. 2001, 55, 203−216.(10) Rhim, J.-W.; Park, H.-M.; Ha, C.-S. Bio-Nanocomposites forFood Packaging Applications. Prog. Polym. Sci. 2013, 38, 1629−1652.(11) Schneiderman, D. K.; Hill, E. M.; Martello, M. T.; Hillmyer, M.A. Poly(lactide)-Block-Poly(ε-caprolactone-co-ε-decalactone)-Block-Poly(lactide) Copolymer Elastomers. Polym. Chem. 2015, 6, 3641−3651.(12) Aubin, M.; Prud’homme, R. E. Analysis of the Glass TransitionTemperature of Miscible Polymer Blends. Macromolecules 1988, 21,2945−2949.(13) Ferruti, P.; Mancin, I.; Ranucci, E.; De Felice, C.; Latini, G.;Laus, M. Polycaprolactone−Poly(ethylene glycol) Multiblock Copoly-mers as Potential Substitutes for Di(ethylhexyl) Phthalate in FlexiblePoly(vinyl chloride) Formulations. Biomacromolecules 2003, 4, 181−188.(14) Shi, G.; Cooper, D. G.; Maric, M. Poly(ε-caprolactone)-Based‘Green’ Plasticizers for Poly(vinyl choride). Polym. Degrad. Stab. 2011,96, 1639−1647.(15) Latini, G.; Del Vecchio, A.; Massaro, M.; Verrotti, A.; De Felice,C. Phthalate Exposure and Male Infertility. Toxicology 2006, 226, 90−98.(16) Navarro, R.; Perez Perrino, M.; Gomez Tardajos, M.; Reinecke,H. Phthalate Plasticizers Covalently Bound to PVC: Plasticization withSuppressed Migration. Macromolecules 2010, 43, 2377−2381.(17) Tomalia, D. A.; Frechet, J. M. J. Discovery of Dendrimers andDendritic polymers: A Brief Historical Perspective. J. Polym. Sci., PartA: Polym. Chem. 2002, 40, 2719−2728.(18) Carlmark, A.; Hawker, C.; Hult, A.; Malkoch, M. NewMethodologies in the Construction of Dendritic Materials. Chem.Soc. Rev. 2009, 38, 352−362.(19) Peleshanko, S.; Tsukruk, V. V. The Architectures and SurfaceBehavior of Highly Branched Molecules. Prog. Polym. Sci. 2008, 33,523−580.(20) Hawker, C. J.; Lee, R.; Frechet, J. M. J. One-Step Synthesis ofHyperbranched Dendritic Polyesters. J. Am. Chem. Soc. 1991, 113,4583−4588.(21) Choi, J.; Kwak, S.-Y. Hyperbranched Poly(ε-caprolactone) as aNonmigrating Alternative Plasticizer for Phthalates in Flexible PVC.Environ. Sci. Technol. 2007, 41, 3763−3768.(22) Trollsås, M.; Hedrick, J. L. Hyperbranched Poly(ε-caprolac-tone) Derived from Intrinsically Branched AB2 Macromonomers.Macromolecules 1998, 31, 4390−4395.(23) Choi, J.; Kwak, S.-Y. Synthesis and Characterization ofHyperbranched Poly(ε-caprolactone)s Having Different Lengths ofHomologous Backbone Segments. Macromolecules 2003, 36, 8630−8637.(24) Liu, W.; Dong, C.-M. Versatile Strategy for the Synthesis ofHyperbranched Poly(ε-caprolactone)s and PolypseudorotaxanesThereof. Macromolecules 2010, 43, 8447−8455.(25) Zhang, X.; Cheng, J.; Zhuo, R. Amphiphilic HyperbranchedPolymers with a Biodegradable Hyperbranched Poly(ε-caprolactone)Core Prepared from Homologous AB2 Macromonomer. RSC Adv.2016, 6, 52334−52338.(26) Smet, M.; Gottschalk, C.; Skaria, S.; Frey, H. AliphaticHyperbranched Copolyesters by Combination of ROP and AB2-Polycondensation. Macromol. Chem. Phys. 2005, 206, 2421−2428.(27) Nguyen, N. T.; Thurecht, K. J.; Howdle, S. M.; Irvine, D. J.Facile One-Spot Synthesis of Highly Branched Polycaprolactone.Polym. Chem. 2014, 5, 2997−3008.(28) Isikgor, F. H.; Becer, C. R Lignocellulosic Biomass: ASustainable Platform for the Production of Bio-Based Chemicals andPolymers. Polym. Chem. 2015, 6, 4497−4559.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.8b01356ACS Sustainable Chem. Eng. 2018, 6, 9006−9017

9016

(29) Pagliaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Della Pina,C. From Glycerol to Value-Added Products. Angew. Chem., Int. Ed.2007, 46 (24), 4434−4440.(30) Algoufi, Y. T.; Akpan, U. G.; Asif, M.; Hameed, B. H. One-PotSynthesis of Glycidol from Glycerol and Dimethyl Carbonate overKF/Sepiolite Catalyst. Appl. Catal., A 2014, 487, 181−188.(31) Wilms, D.; Schomer, M.; Wurm, F.; Hermanns, M. I.;Kirkpatrick, C. J.; Frey, H. Hyperbranched PEG by RandomCopolymerization of Ethylene Oxide and Glycidol. Macromol. RapidCommun. 2010, 31, 1811−1815.(32) Schomer, M.; Seiwert, J.; Frey, H. Hyperbranched Poly-(propylene oxide): A Multifunctional Backbone-ThermoresponsivePolyether Polyol Copolymer. ACS Macro Lett. 2012, 1, 888−891.(33) Song, S.; Lee, J.; Kweon, S.; Song, J.; Kim, K.; Kim, B.-S.Hyperbranched Copolymers Based on Glycidol and Amino GlycidylEther: Highly Biocompatible Polyamines Sheathed in Polyglycerols.Biomacromolecules 2016, 17, 3632−3639.(34) Zhou, J.; Wang, W.; Villarroya, S.; Thurecht, K. J.; Howdle, S.M. Epoxy Functionalised Poly(ε-caprolactone): Synthesis andApplication. Chem. Commun. 2008, 0, 5806−5808.(35) Pitet, L. M.; Hait, S. B.; Lanyk, T. J.; Knauss, D. M. Linear andBranched Architectures from the Polymerization of Lactide withGlycidol. Macromolecules 2007, 40, 2327−2334.(36) Sunder, A.; Hanselmann, R.; Frey, H.; Mulhaupt, R. ControlledSynthesis of Hyperbranched Polyglycerols by Ring-opening Multi-branching Polymerization. Macromolecules 1999, 32, 4240−4246.(37) Yu, X.-H.; Feng, J.; Zhuo, R.-X. Preparation of HyperbranchedAliphatic Polyester Derived from Functionalized 1,4-Dioxan-2-one.Macromolecules 2005, 38, 6244−6247.(38) Miao, Y.; Zinck, P. Ring-Opening Polymerization of CyclicEsters Initiated by Cyclodextrins. Polym. Chem. 2012, 3, 1119−1122.(39) Leibig, D.; Seiwert, J.; Liermann, J. C.; Frey, H. Copolymeriza-tion Kinetics of Glycidol and Ethylene Oxide, Propylene Oxide, and1,2-Butylene Oxide: From Hyperbranched to Multiarm Star Topology.Macromolecules 2016, 49, 7767−7776.(40) Song, X.; Yao, W.; Lu, G.; Li, Y.; Huang, X. tBHBMA: A NovelTrifunctional Acrylic Monomer for the Convenient Synthesis of PAA-g-PCL Well-Defined Amphiphilic Graft Copolymer. Polym. Chem.2013, 4, 2864−2875.(41) Sunder, A.; Bauer, T.; Mulhaupt, R.; Frey, H. Synthesis andThermal Behavior of Esterified Aliphatic Hyperbranched PolyetherPolyols. Macromolecules 2000, 33, 1330−1337.(42) Lee, K. W.; Chung, J. W.; Kwak, S.-Y. Synthesis andCharacterization of Bio-Based Alkyl Terminal HyperbranchedPolyglycerols: A Detailed Study of Their Plasticization Effect andMigration Resistance. Green Chem. 2016, 18, 999−1009.(43) Struik, L. C. E. The Mechanical and Physical Ageing ofSemicrystalline Polymers: 1. Polymer 1987, 28, 1521−1533.(44) Mano, J. F.; Gomez Ribelles, J. L.; Alves, N. M.; SalmeronSanchez, M. Glass Transition Dynamics and Structural Relaxation ofPLLA Studied by DSC: Influence of Crystallinity. Polymer 2005, 46,8258−8265.(45) Jia, P.; Hu, L.; Shang, Q.; Wang, R.; Zhang, M.; Zhou, Y. Self-Plasticization of PVC Materials via Chemical Modification of MannichBase of Cardanol Butyl Ether. ACS Sustainable Chem. Eng. 2017, 5,6665−6673.(46) Jia, P.; Hu, L.; Yang, X.; Zhang, M.; Shang, Q.; Zhou, Y.Internally Plasticized PVC Materials via Covalent Attachment ofAminated Tung Oil Methyl Ester. RSC Adv. 2017, 7, 30101−30108.(47) Choi, J.; Kwak, S.-Y. Architectural Effects of Poly(ε-caprolactone)s on the Crystallization Kinetics. Macromolecules 2004,37, 3745−3754.(48) Gong, W.; Mai, Y.; Zhou, Y.; Qi, N.; Wang, B.; Yan, D. Effect ofthe Degree of Branching on Atomic-Scale Free Volume in Hyper-branched Poly[3-ethyl-3-(hydroxymethyl)oxetane]. A Positron Study.Macromolecules 2005, 38, 9644−9649.(49) Zhu, Q.; Wu, J.; Tu, C.; Shi, Y.; He, L.; Wang, R.; Zhu, X.; Yan,D. Role of Branching Architecture on the Glass Transition ofHyperbranched Polyethers. J. Phys. Chem. B 2009, 113, 5777−5780.

(50) Yang, Y.; Huang, J.; Zhang, R.; Zhu, J. Designing Bio-BasedPlasticizers: Effect of Alkyl Chain Length on Plasticization Propertiesof Isosorbide Diesters in PVC Blends. Mater. Des. 2017, 126, 29−36.(51) Tabb, D. L.; Koenig, J. L. Fourier Transform Infrared Study ofPlasticized and Unplasticized Poly(vinyl chloride). Macromolecules1975, 8, 929−934.(52) Shi, P.; Schach, R.; Munch, E.; Montes, H.; Lequeux, F. GlassTransition Distribution in Miscible Polymer Blends: From Calorim-etry to Rheology. Macromolecules 2013, 46, 3611−3620.(53) Evans, C. M.; Torkelson, J. M. Determining MultipleComponent Glass Transition Temperatures in Miscible PolymerBlends: Comparison of Fluorescence Spectroscopy and DifferentialScanning Calorimetry. Polymer 2012, 53, 6118−6124.(54) Bishai, A. M.; Gamil, F. A.; Awni, F. A.; Al-Khayat, B. H. F.Dielectric and Mechanical Properties of Poly(vinyl chloride)−Dioctylphthalate Systems. J. Appl. Polym. Sci. 1985, 30, 2009−2020.(55) Lee, K. W.; Chung, J. W.; Kwak, S.-Y. Structurally EnhancedSelf-Plasticization of Poly (vinyl chloride) via Click Grafting ofHyperbranched Polyglycerol. Macromol. Rapid Commun. 2016, 37,2045−2051.(56) Choi, J.; Chun, S.-W.; Kwak, S.-Y. Influence of Hyperbranchedagainst Linear Architecture on Crystallization Behavior of Poly(ε-caprolactone)s in Binary Blends with Poly(vinyl chloride). J. Polym.Sci., Part B: Polym. Phys. 2007, 45, 577−589.(57) Yin, B.; Aminlashgari, N.; Yang, X.; Hakkarainen, M. GlucoseEsters as Biobased PVC Plasticizers. Eur. Polym. J. 2014, 58, 34−40.(58) Choi, W.; Chung, J. W.; Kwak, S.-Y. Unentangled Star-ShapePoly(ε-caprolactone)s as Phthalate-Free PVC Plasticizers Designed forNon-Toxicity and Improved Migration Resistance. ACS Appl. Mater.Interfaces 2014, 6, 11118−11128.

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