Nonmigratory Poly(vinyl chloride)-block-polycaprolactone Plasticizers

Feb 11, 2019 - The hydroxy end-group of the PVC macroRAFT agent allowed its use as a ROP initiator in forming a series of PVC-b-PCL with different PCL...
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Nonmigratory Poly(vinyl chloride)-block-polycaprolactone Plasticizers and Compatibilizers Prepared by Sequential RAFT and Ring-Opening Polymerization (RAFT-T̵ -ROP) Zhonghe Sun,†,‡ Bonnie Choi,† Anchao Feng,*,† Graeme Moad,*,‡ and San H. Thang*,†,‡,§

Macromolecules Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 02/12/19. For personal use only.

†

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ Commonwealth Scientific and Industrial Research Organization (CSIRO) Manufacturing, Clayton, Victoria 3168, Australia § School of Chemistry, Monash University, Clayton Campus, Victoria 3800, Australia S Supporting Information *

ABSTRACT: Well-defined nonmigrating polymeric plasticizers, poly(vinyl chloride)-block-polycaprolactone (PVC-b-PCL), were synthesized by sequential reversible addition−fragmentation chain transfer (RAFT) polymerization and ring-opening polymerization (ROP) with 2-hydroxyethyl 2-(ethoxycarbonothioylthio)propanoate (HECP) as RAFT agent and incipient initiator for ROP of ε-caprolactone (CL, oxepan-2-one). Kinetic experiments demonstrated that HECP provided good control in RAFT polymerization of vinyl chloride (VC) with dispersity (Đ) ∼ 1.2 for 3400 < Mn < 11000. Chain extension experiments with VC and vinyl acetate proved the high end-group fidelity of the macroRAFT agent formed. The hydroxy end-group of the PVC macroRAFT agent allowed its use as a ROP initiator in forming a series of PVC-b-PCL with different PCL block lengths by RAFT-T̵ -ROP. Characterization by wide-angle X-ray diffraction (WAXD), polarized light microscopy (PLM), and differential scanning calorimetry (DSC) indicates that there is enhanced chain entanglement for PVC-b-PCL block copolymers when compared to PCL homopolymers in PVC blends, which accounts for PVC-b-PCL being able to provide permanent plasticization. The PVC-b-PCL copolymers are also effective as polymeric compatibilizers in PVC/PCL blends where they suppress the migration of PCL. PVC blends plasticized with PVC-b-PCL show similar or better ductility than PVC containing the archetypical PVC plasticizer dioctyl phthalate (DOP) for the same level of plasticizer. Most importantly, the PVC-b-PCL polymeric plasticizers are nonleaching and do not migrate under conditions where DOP is readily extractable.

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INTRODUCTION Worldwide, poly(vinyl chloride) (PVC) is one of the most widely produced thermoplastics.1,2 Flexible plasticized PVC (P-PVC) is extensively used in applications which range from packaging materials to biomedical materials, including disposable blood-contact devices.3−5 Pure PVC has inherently low ductility, and plasticizers must be added in amounts of up to 50% of the total formulation weight to enhance its flexibility. Historically, these plasticizers have taken the form of phthalate esters, in particular, bis(2-ethylhexyl) phthalate (also known as dioctyl phthalate, DOP). The phthalate plasticizers are inexpensive and perform well in improving ductility. However, over time they can leach out of the PVC matrix into the surrounding medium where they present health and environmental issues. These concerns have caused many countries to impose strict regulations on the use of PVC containing phthalate plasticizers in direct food contact applications, medical devices, childcare equipment, toys, and so forth.6−8 Substituting phthalates with polymeric plasticizers has proven to be a promising way of obtaining a more environmentally acceptable, plasticized PVC (P-PVC). Poly© XXXX American Chemical Society

caprolactone (PCL) has been shown to be nontoxic and biocompatible and has been proposed as an alternative to phthalate plasticizers for more than a decade.9,10 Rusu et al.11 investigated the total/partial replacement of DOP with PCL in P-PVC medical devices. They11 found that PCL and PCLDOP blends are better plasticizers than DOP alone and provide a lower leaching risk in different media. However, even with PCL there is evidence of some plasticizer loss. Kwak and co-workers12 reported on hyperbranched PCL (HPCL) and unentangled star-PCL (UESPCL)13 as new types of plasticizer for PVC. Their formulations showed improved plasticization, but the synthetic route to the production of HPCL is challenging. Maric’s group14 synthesized a series of plasticizers for PVC based on PCL with terminal octanoate and benzoate functionality. However, these plasticizers showed poor migration resistance with the degree of leaching being high, even relative to DOP. Breslau and co-workers15 reported on Received: October 6, 2018 Revised: December 23, 2018

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DOI: 10.1021/acs.macromol.8b02146 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules nonmigratory PVC plasticizers which comprise phthalate ester functionality grafted to a PVC backbone through pendant triazole linkages formed by copper-free azide−alkyne thermal cycloaddition. To improve the miscibility in PVC blends, PCL copolymers have been studied. Ferruti et al.16 reported the use of PCL− PEG copolymers as plasticizers. Hakkarainen et al.17 focused on polycaprolactone−polycarbonate (PCL−PC) copolymers. Their results indicate that PCL−PC degradation products and other low molar mass compounds can migrate from the PVC blend during aging. Ritter et al.18 investigated various copolyesters of ε-caprolactone (CL) and trimethylcaprolactone (TMCL), synthesized via ring-opening polymerization (ROP), for plasticizing PVC, but they did not describe leaching behavior. These studies demonstrate that PCL-based plasticizers have promise. However, a viable system that is both synthetically viable and effective in commercial application is yet to be described. In this paper, we propose that PCL-based polymers comprising covalently bound, RAFT-synthesized VC blocks may provide an answer to the long sought after need for nonmigrating permanent PVC plasticizers. Techniques for reversible deactivation radical polymerization (RDRP),19 such as nitroxide-mediated polymerization (NMP), 2 0 − 2 5 atom transfer radical polymerization (ATRP),26−32 and reversible addition−fragmentation chain transfer (RAFT)33−40 polymerization, provide powerful tools for the synthesis of polymers with controlled structure, chainend fidelity, and narrow molar mass distribution. RDRP of vinyl chloride (VC) is challenging because VC is a lessactivated monomer (LAM), and its polymerization is complicated by a range of side reactions.41,42 To date, the most successful RDRP of VC was reported by Percec and coworkers,43−46 who applied what they called single electron transfer−living radical polymerization (SET-LRP or SETRDRP). Breslau et al.47 used SET-RDRP to make the triblock poly(butyl acrylate)-block-PVC-block-poly(butyl acrylate) as a potential plasticizer for PVC. The uses of SET-RDRP and other RDRP methods in forming VC-based polymers are summarized in recent reviews.41,42,48 Vinyl chloride is mentioned in the earliest patents on RAFT polymerization49−51 where it is pointed out49 that it should be grouped with LAMs, such as VAc and NVP, for which dithiocarbamates and xanthates are the most effective RAFT agents.52 However, the first experiments on RAFT polymerization of VC did not appear until 2012 when Abreu et al.53 used S-cyanomethyl N-methyl-N-phenylcarbamodithioate (CMPCD) as a RAFT agent to successfully synthesize a relatively low dispersity PVC (best result Mn 6000, Đ = 1.43). In a subsequent report, they used cyclopentyl methyl ether as a green polymerization solvent. They found higher rates of polymerization but obtained slightly reduced control (e.g., Mn 4200−17300, Đ ∼ 1.5).54 Bao’s group55,56 used a fluorinated xanthate (TDFCP) for RAFT polymerization of VC and found better control than CMPCD (Đ ∼ 1.28, Mn 7750, 55% conversion). Examples of further functionalization of PVC with other polymers are limited.57 Even though the RAFT polymerization and copolymerization of VC have been studied, there are few reports on block copolymer synthesis and none involving RDRP transformation processes such as sequential RAFT polymerization and ring-opening polymerization (RAFT-T̵ ROP).58,59 Herein we describe the synthesis of well-defined PVC-b-PCL block copolymers using a hydroxyl-functionalized

Figure 1. Structures of RAFT agents CMPCD, HECP, and TDFCP used in mediating synthesis of PVC.

xanthate, namely, 2-hydroxyethyl 2-(ethoxycarbonothioylthio)propanoate (HECP), as a dual RAFT agent and ROP initiator for RAFT-T̵ -ROP (Scheme 1). HECP has previously been used to prepare poly(N-vinylprrolidone)-block-polycaprolactone54 or polycaprolactone-block-poly(vinyl acetate) by RAFTT̵ -ROP.60,61 Scheme 1. Synthesis of PCL-b-PVC by Sequential RAFT Polymerization and Ring-Opening Polymerization with HECP

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EXPERIMENTAL SECTION

Materials. Vinyl chloride (VC, Dalian Special Gases Co., 99%), 1,4-dioxane (purchased from J&K Chemical, 98%), oxepan-2-one (CL, ε-caprolactone J&K Chemical, 98%), diphenyl phosphate (DPP, TCI, 99%), 3-phenyl-1-propanol (PPA, J&K Chemical, 98%), tetrahydrofuran (THF, J&K Chemical, anhydrous, 98%), ethylene glycol (J&K Chemical, 99.5%), 2-bromopropionyl bromide (Aldrich, 97%), potassium O-ethylcarbonodithioate (J&K Chemical, 95%), pyridine (J&K Chemical, 99%), vinyl acetate (VAc, J&K Chemical, 99%), dichloromethane (DCM, J&K Chemical, 99%), dioctyl phthalate (DOP, J&K Chemical, 98%), and 1,6-hexamethylene diisocyanate (J&K Chemical, 99%) were used as received. 2,2′Azobis(2,4-dimethylpentanenitrile) (ABVN, J&K Chemical, 95%) and 2,2′-azobis(2-methylpropanenitrile) (AIBN, J&K Chemical, 95%) were recrystallized from methanol and kept in a refrigerator before use. PVC powder (SG-5, Xinjiang Tianye Co.) was used for preparing PVC/DOP and PVC/PCL blends. The Mn and Đ of SG-5 were determined to be 87000 (PMMA equivalents) and 1.87, respectively, by GPC. NMR spectra were recorded on a Bruker Avance III (400 MHz) spectrometer. For 1H NMR spectra, chemical shifts (δ) are quoted in parts per million (ppm), using residual protons in the deuterated solvent as an internal standard (CDCl3 at 7.26 ppm). Abbreviations used in the description of resonances are s (singlet), d (doublet), t (triplet), q (quartet), and br (broad). Coupling constants (J) are quoted to the nearest 0.1 Hz. For 13C NMR spectra, chemical shifts (δ) are quoted in parts per million (ppm) with CDCl3 at 77.0 ppm as internal standard. The molar mass was analyzed by using a Shimadzu B

DOI: 10.1021/acs.macromol.8b02146 Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Dependence of Monomer Conversion, Mn, and Đ of PVC-MacroRAFT Agent on Time for HECP-Mediated RAFT Polymerization with [VC]:[CTA]:[ABVN] = 160:1:0.15 at 45 °C

gel permeation chromatography (GPC) system equipped with a SIL20A autosampler, a refractive index detector, three Shodex KF-805L columns (8 × 300 mm, 10 μm, 5000 Å), and one Shodex KF-801 column (8 × 300 mm2, 6 μm, 50 Å) using N,N-dimethylacetamide (containing 2.1 g L−1 lithium chloride) as the eluent at 80 °C with a flow rate of 1 mL min−1. The GPC calibration was accomplished with a series of low-dispersity poly(methyl methacrylate) (PMMA) standards with Mp in the range 800−600000 g mol−1. Glass transition temperatures (Tg) were measured on a TA Instruments differential scanning calorimeter (DSC) Q2000 using a heating rate of 15 °C/ min over the temperature range −90 to 90 °C under a nitrogen atmosphere. Wide-angle X-ray diffraction (WAXD) patterns of PVC/ PCL blends and PVC-b-PCL copolymers were recorded at room temperature by a Rigaku Ultima IV X-ray unit at a scan speed of 0.5 deg min−1 between 5 and 90 °C. Infrared spectra were recorded on a Thermo Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer to determine the extraction of plasticizers. An Instron universal tensiometer (INSTRON 5967) at a cross-head speed of 30 mm min−1 was used to examine the stress−strain relationship of DOP or PVC-b-PCL plasticized PVC. The dumbbell-shaped mini-tensile bars were cut from 1.0 mm films, which were cast from THF solutions of DOP/PVC or PVC-b-PCL/PVC mixtures; the THF was allowed to evaporate under ambient conditions (24 h), and the residual THF was evaporated in a vacuum oven at 25 °C (0.7 kPa). A ZEISS polarizing optical microscope (Axio Scope A1) with crossed polarizers was used to observe the morphology of solution-cast films through a polarized light microscope (PLM). Synthesis of HECP. HECP was prepared with minor modification of previously reported procedure.62 A solution of 2-bromopropionyl bromide (10 mL, 95.5 mmol) in dry tetrahydrofuran (50 mL) was slowly added to a cooled (ice bath), stirred solution of ethylene glycol (301 g, 4.84 mol) and pyridine (8.1 mL, 100 mmol) in anhydrous THF (150 mL) over 1 h. The mixture was then stirred at room temperature for 16 h. The solvent was then removed under vacuum, aqueous hydrochloric acid (0.1 M, pH 2, 600 mL) was added, and the mixture was extracted with DCM (4 × 100 mL). The organic extracts were washed with water, dried over anhydrous MgSO4, and evaporated to dryness to afford 2-hydroxyethyl 2-bromopropionate (14.4 g, 76.7%) as a colorless liquid. A solution of the above 2-hydroxyethyl 2-bromopropionate (4 g, 20.3 mmol) in acetone (15 mL) was added dropwise to potassium Oethylcarbonodithioate (potassium xanthate, 3.65 g, 22.7 mmol) in acetone (15 mL) at room temperature over 30 min, and stirring was continued for 12 h. The product was collected by filtration, washed with water (75 mL), and dried under reduced pressure. The crude yellow viscous liquid was dissolved in DCM (100 mL) and dried over anhydrous MgSO4, and the solvent was removed under reduced pressure to give HECP as a viscous yellow oil (4.56 g, 94.3%). 1H NMR (400 MHz, CDCl3, Figure S1): δ 4.57 (2H, q, J = 7.1 Hz), 4.34 (1H, q, J = 7.4 Hz), 4.20−4.17 (2H, m), 3.78−3.73 (2H, m), 1.52 (3H, d, J = 7.4 Hz), 1.35 (3H, t, J = 7.1 Hz). 13C NMR (100.6 MHz, CDCl3): δ 212.0, 171.6, 70.4, 67.1, 60.6, 47.0, 16.7, 13.6. The NMR data are consistent with the previous literature.62 RAFT Polymerization of VC. RAFT polymerizations of VC were performed in a 100 mL stainless steel high-pressure reactor (YanZheng, YZQR-100). For example, HECP (0.13 g, 0.53 mmol), ABVN (0.02 g, 0.08 mmol), and 1,4-dioxane (15 mL) were added into the reactor to provide a mole ratio molar ratio of [M]:[CTA]:[I] of 160:1:0.15. The reactor headspace was flushed with N2 three times, then the solution was degassed through a single freeze−pump−thaw cycle, and the reactor was sealed under N2. Precondensed VC (5.5 g, 88 mmol) was then introduced into the reactor. The reactor was heated with stirring (500 rpm) at 45 °C for the times indicated in Table 1. On completion of each reaction, the excess VC was distilled from the reactor, and the residual reaction mixture was precipitated into methanol (300 mL). The polymer was separated by centrifugation and dried in a vacuum oven (25 °C, 0.7 kPa) to constant mass. Details of the polymers produced are collected in Table 1. A control experiment carried out under similar conditions

entry

polymerization time (h)

conva (%)

Mn,thb (kDa)

Mn,GPCc × 103

Đ

1 2 3 4 5 6 7

8 16 24 36 48 60 72

15.6 20.1 28.7 35.4 42.2 52.5 53.6

3.4 4.3 6.0 7.3 8.7 10.7 10.9

(3.8) (4.2) 6.4 7.8 8.9 10.2 11.0

(1.18) (1.23) 1.24 1.23 1.21 1.21 1.20

a

The conversion was based on the mass of isolated PVC macroRAFT agent. bThe theoretical molar mass Mn,th. = MCTA + (conv × [VC] × MVC)/[CTA], where MCTA and MVC are the molar mass of the RAFT agent HECP and vinyl chloride monomer, respectively. cMolar mass in poly(methyl methacrylate) equivalents determined by GPC (see the Experimental Section). Mn,GPC values and Đ values in parentheses will be significantly higher and lower than actual values because of interference of the salt peak in the GPC trace.

but with no RAFT agent gave 80% VC conversion after 72 h. The PVC had Mn=76500, Đ=2.31 (see Supporting Information). Chain Extension of PVC-OH with VC. PVC-OH (2.0 g, 0.2 mmol), ABVN (0.02 g, 0.08 mmol), and 1,4-dioxane (15 mL) were added to a 100 mL stainless steel high-pressure reactor. The solution was degassed by flushing the reactor headspace with N2 three times followed by a freeze−pump−thaw cycle. Precondensed VC (2.0 g, 32 mmol) was introduced into the reactor. The reactor was heated to 45 °C with stirring (500 rpm) for 60 h before the excess VC was distilled from the reactor, and the remaining reaction mixture was precipitated into methanol. The polymer was separated by centrifugation and dried in a vacuum oven (25 °C, 0.7 kPa) to constant mass. Details of the chain extended PVC prepared are provided in Figure 3. Chain Extension of PVC-OH of VAc (PVC-b-PVAc). VAc (3.0 g, 34.8 mmol), PVC-OH (2.0 g, 0.2 mmol), AIBN (0.02 g, 0.12 mmol) and 1,4-dioxane (15 mL) were placed in a 25 mL Schlenk flask. The solution was degassed using three freeze−pump−thaw cycles, and the Schlenk flask was sealed under vacuum and then immered in a preheated oil bath at 70 °C. Heating with stirring (500 rpm) was maintained for 6 h. The mixtures were precipitated in methanol, and polymer was separated by centrifugation and dried in a vacuum oven (25 °C, 0.7 kPa) to constant mass. Details of the PVC-b-PVAc are provided in Figure 3 and in the Supporting Information. Synthesis of PCL and PVC-b-PCL. PCL and PVC-b-PCL were synthesized by DPP-catalyzed ROP.63 A typical procedure for the polymerization of CL is as follows: CL (2.5 mL, 21.3 mmol) was added to a 25 mL Schlenk flask with solution of PPA (0.03 mL, 0.22 mmol) and DPP (0.02 g, 0.08 mmol) in 1,4-dioxane (5 mL), and the solution degassed by three freeze−pump−thaw cycles. The Schlenk flask was sealed under vacuum (∼10 Pa) and immersed in a preheated oil bath at 45 °C for 6 h. The mixtures were precipitated in methanol, and PCL was separated by centrifugation and dried in a vacuum oven (25 °C, 0.7 kPa) to constant mass. Details of the polymers prepared are in Table 2. A typical procedure for ROP polymerization to form PVC-b-PCL is as follows: CL (1.4 mL, 12.3 mmol), PVC-OH (2.0 g, 0.2 mmol), DPP (0.01 g, 0.04 mmol), and 1,4-dioxane (10 mL) were added to a 25 mL Schlenk flask. The solution was degassed by three freeze− pump−thaw cycles, and then the Schlenk flask was sealed under vacuum and immersed in a preheated oil bath at 70 °C for 6 h. The mixtures were precipitated in methanol, and polymers were separated by centrifugation and dried in a vacuum oven (at 25 °C, 0.7 kPa) to constant mass. Details of the polymers are in Table 2 and Figure 4. Migration Resistance Test of the Flexible PVC. The plasticized PVC specimens with plasticizers (DOP or PVC-b-PCL) C

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Macromolecules Table 2. Characteristics of PVC-b-PCL Block Copolymers samplea

convb (%)

PVC10k PVC10k-b-PCL2.6k PVC10k-b-PCL7.1k PVC10k-b-PCL14.7k PVC10k-b-PCL40.3k PVC10k-b-PCL89.5k PCL10k

52.5 85.3 86.1 85.6 83.5 80.2 90.1

Mn,NMRc (kDa)

Mn,GPCd × 103

Đ

CL contentc (%)

Tg (°C)

12.7 17.3 24.7 50.6 99.3 10.6

9.8 13.1 17.6 25.0 40.8 85.6 11.0

1.23 1.30 1.29 1.32 1.33 1.32 1.20

0 20.6 41.5 59.5 80.1 90.0 100

63.7 50.5 31.4 −3.4 −14.3 −34.8 −62.5

a

The subscript of each block copolymer and PVC as well as PCL homopolymers denote the designed number-average molar mass. PVC10k is the hydroxyl-end-functional macroRAFT agent prepared using HECP. bThe conversion of VC monomer was calculated from the mass of monomer in feed and mass of PVC obtained. The conversion of CL monomer was calculated from integration of the 1H NMR spectrum. cCalculated from 1H NMR. dMn in PMMA equivalents as determined by GPC (see the Experimental Section). were prepared first. Commercial PVC (SG-5, 0.5 g) powder and DOP or PVC-b-PC plasticizer (0.2 g) were dissolved in THF (30 mL). The THF was then evaporated and further dried under low vacuum (0.7 kPa for ∼2 days). The obtained plasticized PVC specimen (0.2 g) was weighed, transferred into a flask, and filled with n-hexane (5 mL). 1,6Hexamethylene diisocyanate (0.02 g) was added as internal standard. The flask was stirred with a magnetic bar for a predetermined time. Two droplets of the solution were placed onto a KBr window, the hexane allowed to evaporate, and the disk the window placed inthe holder for FT-IR measurement, and the amount of plasticizers extracted was determined using a calibration curve shown in Figure S6. The results appear in Figure 10. Migration Resistance Test of the PVC/PCL with or without PVC-b-PCL. The PVC/PCL blends were prepared first. Commercial PVC (SG-5, 0.7 g) powder and PCL 10K(0.3 g, 0.03 mmol) with or without PVC10k-b-PCL89.5k (0.006 g, 0.0005 mmol) were dissolved in THF (30 mL), and THF was evaporated and further dried under low vacuum. Then PVC/PCL blends with or without PVC-b-PCL samples (0.2 g) were weighed, transferred into a flask and filled with n-hexane (5 mL), and then immersed in a preheated oil bath at 55 °C for 6 h. 1,6-Hexamethylene diisocyanate (0.02 g) was added as an internal standard. Two droplets of the solution were placed onto a KBr window, the hexane allowed to evaporate, and the window placed in the holder for FT-IR measurement. The results appear in Figure 11. Sample Preparation for Polarized Microscopy (PLM). Glass slides (2.5 × 2.5 cm2) were used as the substrates for the spin-coating process. The surfaces of the glass slides were treated with a H2SO4/ H2O2 (piranha solution) mixture to remove any organic residues. The PVC/PCL or PVC-b-PCL was dissolved in THF to give a concentration of 0.05 g mL−1. Films were prepared by spin-coating a 40 μL portion of PVC/PCL or PVC-b-PCL solutions on cleaned glass at 1500 rpm for 30 s with the acceleration set to “immediate”. After most of the THF had been evaporated under ambient conditions (24 h) samples were placed in a vacuum oven at 25 °C (0.7 kPa) for 72 h. The morphologies of PVC/PCL or PVC-b-PCL were observed for samples that had been heated at 60 °C for 2 h, then quenched to 30 °C, and maintained at 30 °C for 6 h.

and 1,4-dioxne are good solvents for RDRP of VC.45 Abreu et al.45 found THF to be a better solvent for RAFT polymerization of VC than either cyclohexanone and dichloromethane. We used 1,4-dioxane solvent which had been successfully used for RAFT polymerization of VC with TDFCP by Bao’s group.55 Studies of SET-RDRP of VC suggested that maximum polymerization temperature enabling synthesis of PVC with minimal structural defects is ∼42−43 °C.64 When the polymerization temperature exceeds 50 °C, chain transfer to VC monomer becomes more likely leading to impairment of control.48 We selected 45 °C as the polymerization temperature to ensure both a fast initiation rate and minimal structural defects. ABVN (10 h half-life in solution at 52 °C) was selected as initiator over the more commonly used AIBN (10 h half-life in solution at 64 °C).65 The dependence of Mn and Đ on polymerization time and monomer conversion are shown in Table 1 and Figure 2b. The Đ of PVC are below 1.25 and reduce slightly with increasing monomer conversion (>30%). The Đ values are significantly lower than those previously reported with CMPCD53 and similar or better than those obtained using the fluorinated xanthate RAFT agent TDFCP.55,56 Kinetic Experiments. The results of kinetic experiments for RAFT polymerization of VC mediated by HECP in dioxane solution are shown in Figure 2. The semilogarithmic kinetic plot displays linear pseudo-first-order kinetic behavior (Figure 2a). We also observed that Mn increases linearly with the monomer conversion and the obtained PVC have Đ < 1.25 (Figure 2b). These data are consistent with a well-controlled RDRP. Chain Extension of PVC MacroRAFT Agent. A reinitiation experiment was performed to confirm the endgroup fidelity the PVC macroRAFT agent and the living character of RAFT polymerization of VC. Xanthates and dithiocarbamates are suitable RAFT agents for controlling the polymerization of less activated monomers, which include vinyl acetate (VAc), N-vinylpyrrolidone (NVP), and VC.51 Thus, chain extension experiments were performed with VAc and VC and HECP-terminated PVC (PVC-OH). The GPC traces of the chain-extended polymers are shown in Figure 3. No shoulder peaks that might be indicative of dead chains in the macroRAFT agent or slow reinitiation were observed. The 1H NMR spectrum of the VAc chain extension product (Figure S2b) shows signals characteristic of both PVC and PVAc. Consistent with successful chain extension, the 1H DOSY spectrum of PVC-b-PVAc (Figure S4b) shows that the PVC and PVAc segments of the block copolymer have the

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RESULTS AND DISCUSSION HECP-Mediated Solution Polymerization of VC. HECP is a dual RAFT agent/ROP initiator. As shown in Scheme 1, the macro-RAFT agent PVC-OH was synthesized first by RAFT polymerization with HECP as the RAFT agent. Subsequently, PVC-b-PCL block copolymers were prepared by DPP-catalyzed ROP of CL initiated by the hydroxyl group of PVC-OH. The RAFT agent HECP was chosen since the 1carboxyethyl group is a good homolytic leaving group relative to the VC propagating radical, and the xanthate group is suitable for controlling polymerization of LAMs such as VC. Solution polymerization of VC was studied to demonstrate the living characteristics of HECP-mediated VC polymerization. Previous work has shown that tetrahydrofuran (THF) D

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Figure 3. GPC molar mass distributions for PVC-OH (Mn = 6600, Đ = 1.25), PVC-OH extended with VC (Mn = 18700, Đ = 1.21), and VAc (Mn = 11200, Đ = 1.23). The peaks at ∼350 PMMA equivalents correspond to a “salt peak” in the GPC trace.

that can be used as an organocatalyst to control the ROP under mild conditions.63 Three different synthetic routes were considered for the PVC-b-PCL: (i) simultaneous ROP of CL and RAFT polymerization of VC to form PVC-b-PCL block copolymer in one pot; (ii) ROP of CL first, with subsequent RAFT polymerization of VC; (iii) RAFT polymerization of VC first, followed by ROP of CL. As shown in Table 1, the conversion of RAFT of VC mediated by HECP is only 15% after 8 h under our conditions. However, the conversion in ROP of CL can be >95% within 8 h. An attempted simultaneous RAFT and ROP with a [VC]:[CL] feed ratio of 1:1 provided a polymer with a [VC]:[CL] ratio of 0.44:1 as determined by 1H NMR (Figure S5). It is challenging to design the [VC]:[CL] experimental ratio through the feed ratio because of the large difference in rate of polymerization between RAFT polymerization of of VC and ROP of CL. Using route ii, we successfully prepared well-defined PCL-bPVC copolymers. However, synthesis of PCL-b-PVC copolymers with a similar Mn PVC block and a PCL block of increasing Mn proved difficult due to the limited accuracy that could be achieved in addition of the gaseous VC monomer to the reactor. Therefore, the preferred route for preparing the desired PVC-b-PCL samples with a fixed of PVC chain length and differing PCL chain length was to first implement RAFT polymerization of VC and to carry out ROP of CL as a second step. The 1H NMR spectra of PVC-b-PCL and PVC-OH are shown in Figure 4a. The signals associated with the CL repeat unit appear at δ 4.06, 2.31, 1.64, and 1.38 corresponding to protons labeled l, i, j, and k, respectively. The successful incorporation PCL as a block is demonstrated by the 1H DOSY spectrum for PVC-b-PCL shown in Figure 4b. The signals for PVC and PCL segments show the same spin correlation time, which is greater than that of the PVC-OH precursor (see Figure S2). Consistent with their higher molar mass, the GPC peaks for PVC-b-PCL appear at increasingly shorter retention times relative to PVC-OH as the PCL content increases from 20% to 90%. Molar mass distributions for a PVC-b-PCL and that of PVC-OH are shown in Figure 5, and the Mn and Đ values for the PVC-b-PCL are summarized in Table 2. Crystallization Behavior of PVC-b-PCL. The properties of PVC/PCL blends have been previously reported.13 PCL is

Figure 2. (a) Pseudo-first-order kinetics plot, (b) number-average molar mass (Mn) and dispersity (Đ = Mw/Mn) versus polymer conversion, and (c) GPC molecular mass distribution for RAFT polymerization of PVC after different polymerization times. The negative peak at ∼350 PMMA equivalents corresponds to a “salt peak” artifact in the GPC trace.

same spin correlation time, which is different than that of the PVC-OH precursor (Figure S4a). As expected, the signal attributed to the methine adjacent to the xanthate end-group [CH3CH2OC(S)SCHCl−] is absent in the 1H NMR of PVC-b-PVAc (Figure S3b). However, no signals at 6.5 ppm expected for CH3CH2OC(S)SCH(OAc)−66 or for the xanthate methyl CH3CH2OC(S)SCH(OAc)− in PVC-b-PVAc are observed. We do see a signal at 10.0 ppm for an aldehyde chain-end. We conclude that the xanthate chain end of PVC-b-PVAc is lost during work-up. Signals associated with the C(CH3)CHCO2CH2CH2OH end are retained in the 1 H NMR PVC-b-PVAc. Ring-Opening Polymerization of Oxepan-2-one Using PVC-OH. Diphenyl phosphate (DPP) is a weak acid E

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crystallization behavior of PVC-b-PCL and compared this with that of PVC/PCL blend counterparts. The WAXD patterns of PVC-b-PCL copolymers with differing PCL content are presented in Figure 6. Three main diffraction peaks are

Figure 4. (a) From top to bottom: 1H NMR spectra (CDCl3, 400 MHz, 298 K) of PVC-OH (Mn = 6600, Đ = 1.25) and PVC-b-PCL (Mn = 9700, Đ = 1.21) in CDCl3. (b) 1H DOSY spectra (CDCl3, 400 MHz, 298 K) for PVC-b-PCL (Mn = 116400, Đ = 1.43). The high intensity of the signal assigned to 'm' in the DOSY spectrum is attributed to the sharpness of that peak.

Figure 6. WAXD patterns of (a) PVC/PCL blends and (b) PVC-bPCL copolymers.

observed for neat PCL at 2θ (deg) of 21.3, 21.9, and 23.6, corresponding to (110), (111), and (200) planes, respectively.67 The diffraction peaks do not move from their original position in the blocks. However, their intensity increases with increasing PCL content which is most likely associated with the crystallizability of PCL increasing with increasing PCL block length, and this is similar to PVC/PCL as reported.9 When the PCL block is short (Mn of PCL block is 80% PCL. The diameter of the spherulites decreased for decreased PVC content in the blends, which is due to the enhancement of

Figure 5. GPC molar mass distributions for PVC-OH (Mn = 6600, Đ = 1.25) and PVC-b-PCL (Mn = 116400, Đ = 1.43). The peaks at ∼350 PMMA equivalents correspond to a “salt peak” in the GPC trace.

miscible with PVC and has a low glass transition temperature, which improves the ductility of the blend. We have examined F

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Figure 7. PLM micrographs of PVC/PCL blends: (a) 40/60, (b) 20/ 80, (c) 10/90; (d) neat PCL and PVC-b-PCL block copolymer: (e) PVC10k-b-PCL40.3k; (f) PVC10k-b-PCL89.5k (the scale bar is 10 μm).

nucleation density of PCL spherulites. The earlier literature9,68 reported that PVC/PCL blends with a PCL content of 50− 60% could form a helical structures, but we did not observe this phenomenon. This may be due to the difference sources of PVC (commercial PVC of Mn > 60 kDa vs PVC of Mn ∼ 10 kDa obtained in this work), which mean that the interactions between PCL and PVC for the same blend composition are not the same. The PVC block in PVC-b-PCL is amorphous, and the PCL blocks are potentially able to crystallize. However, there were no spherulites in the visual field with a PCL content 80% (i.e., Mn of PCL block >40.3 kDa), and the space between spherulites comprises a predominantly PVC domain.67 When the length of PCL block is longer and Mn of the PCL blocks are >89.5 kDa, the spherulites appear similar to those for the blends. This indicates that when the PCL content is very high (>90%), the influence of PVC on PCL crystallization is negligible. The Tg’s of PVC-b-PCL copolymers were determined by DSC shown in Figure 8, and results are listed in Table 2. The Mn of PVC-OH obtained by RAFT polymerization is 10 kDa, which is much lower than the Mn of commercial PVC. This accounts for Tg of our PVC-OH being lower than that of commercial PVC, which is also reflected in a low Tg of PVC-bPCL. A monotonic decrease in Tg values is observed with increasing Mn of the PCL block for the PVC-b-PCL block copolymers. When PCL content is >60%, the Tg values of PVC-b-PCL are 90%. However, the molecular motion of the PCL blocks is still slightly restricted in PVC-bPCL when compared with neat PCL. Although the PCL crystallization peak is obvious, the Tg of PVC10k-b-PCL89.5k can still be observed at −34.8 °C. Physical Properties of the Flexible PVCs. The tensile properties of DOP and PVC-b-PCL plasticized PVC were measured on mini-tensile bars; typical stress−strain curves of these samples are shown in Figure 9, and the value of tensile strength, elongation at break, and Young’s modulus are summarized in Table S1. The two PVC/PVC-b-PCL samples exhibited typical ductile stress−strain behavior with shapes that resembled the curve obtained from PVC/DOP. Although the elongation at break value of PVC/PVC10K-b-PCL40.3k is lower than PVC/DOP, PVC/PVC10K-b-PCL89.5K displays a higher tensile strength and a larger elongation at break value than PVC/DOP. These results indicate that PVC-b-PCL block copolymers with a PCL content of >90% can impart better extensibility to the PVC than DOP at a similar mass concentration. The tensile strength values of both PVC/ PVC-b-PCL block copolymers are higher than PVC/DOP. The interaction between PVC and PCL in PVC/PVC-b-PCL is anticipated to be greater than in PVC/PCL blends and to be much stronger than between PVC and DOP, so the tensile G

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Figure 9. Tensile stress−strain curves for PVC/DOP and PVC/ PVC10K-b-PCL40.3K or PVC/PVC10K-b-PCL40.3K block copolymers.

strength in the blends of the PVC/PVC-b-PCL blends is expected to be enhanced relative to both PVC/PCL and PVC/ DOP.9,67 Consequently, PVC plasticized with PVC-b-PCL block copolymer displays both a higher elongation at break and a larger tensile strength. Migration Resistance of PVC-b-PCL Plasticizers. The migration resistance of DOP and PVC-b-PCL block copolymer was tested using procedures for simulating plasticizer migration from food packaging into oily foods. n-Hexane was used as an extraction medium because its solubility profile is similar to cooking oil.13 To ensure a high migration rate of plasticizers, we chose flexible samples with 0.4:1 plasticizer over PVC. The results indicated that the new PVC-b-PCL plasticizer in PVC shows no signs of migrating even after a lengthy extraction time. The results are compared with those for PVC with DOP platiciser in Figure 10a−c. PCL is miscible with PV but still slowly migrates when extracted with n-hexane. When the PVCb-PCL block copolymers are used, the PVC segments interact strongly with PVC, reducing the tendency for migration. Applications as a Polymeric Compatibilizer in PVC/ PCL Blends. PVC and PCL are highly compatible polymers that appear miscible over a wide composition range, indicating strong molecular interactions in PVC/PCL blends such that relatively stable binary blends are formed.71−73 Thus, compatibilizers are not essential in forming PVC/PCL blends. To test the proposal that the migration of PCL in PVC/PCL blends might be suppressed by addition of PVC-b-PCL block copolymers as a polymeric compatibilizer, we compared the migration behavior of PVC/PCL blends with and without added PVC-b-PCL in hexane as shown in Figure 11. The PCL CO stretching band at 1735 cm−1 was not observed for PVC/PCL/PVC-b-PCL blends whereas it is seen for similar PVC/PCL blends. When the mass ratio of PVC-b-PCL and PCL is >1:50, the migration of PCL was completely avoided, which indicates the powerful effect of PVC-b-PCL as compatibilizer in PVC/PCL blends. The block copolymers are thus extremely effective as additives in suppressing migration of PCL plasticizers in PVC/PCL blends.

Figure 10. Infrared spectra of extraction liquid from (a) DOP (0.2 g) and (b) PVC-b-PCL (0.2 g) plasticized PVC (0.5 g) after different extraction times as indicated. (c) Extraction of plasticized PVC strips with n-hexane at room temperature (PVC/DOP and PVC/PVC10k-bPCL89.5k). 1,6-Hexamethylene diisocyanate was used as the internal reference.69 The NCO stretching mode for 1,6-hexamethylene diisocyanate can be observed at 2265 cm−1,70 and the dioctyl phthalate CO can be observed at 1735 cm−1 in (a) for times >150 min. No PCL CO peak can be observed for PVC-b-PCL block copolymers in (b).

RAFT polymerization of VC with HECP was confirmed by chain extension experiments with VC and VAc. The hydroxylend-functional PVC was then used as a macroinitiator for ROP in the preparation of a series of PVC-b-PCL differing in the length of the PCL block. The crystallization behavior of PVCb-PCL block copolymers with different PCL block length was compared with PVC/PCL blends of similar composition.

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CONCLUSIONS Well-defined hydroxyl-end-functional PVC was successfully synthesized via RAFT polymerization of VC using the xanthate, HECP, as RAFT agent. The living characteristics of H

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diisocyanate (0.02g) in 10 mL n-hexane solution and calibration curve of the DOP in 10 mL n-hexane solution (Figure S6); tensile properties of PVC/DOP and DOP/PVC-b-PCL block copolymers (Table S1) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.F.). *E-mail: [email protected] (G.M.). *E-mail: [email protected] (S.H.T.). ORCID

Anchao Feng: 0000-0002-8755-4772 Graeme Moad: 0000-0002-4375-5580 San H. Thang: 0000-0003-2629-3895

Figure 11. Infrared spectra of extraction liquid from PVC/PCL (0.2 g, PVC/PCL = 70/30 w/w in 5 mL n-hexane) blends with (red) or without (black) PVC10k-b-PCL89.5k (, PCL/PVC-b-PCL = 50/1 w/w) after extraction for 6 h at 55 °C.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21704001), the Fundamental Research Funds for the Central Universities (buctrc201724), and the Beijing Advanced Innovation Center for Soft Matter Science and Engineering. Z.S. gratefully acknowledges the scholarship from Graduate School of Beijing University of Chemical Technology and the provision of additional supervision and laboratory facilities by CSIRO Manufacturing.

WAXD and PLM measurements indicate that there is greater restriction of the molecular motion for PCL polymer chains in the block copolymer than in the blends. The Tg of PVC-b-PCL block copolymers with a PCL content of >60% is lower than 0 °C, which argues well for their direct application as PVC plasticizers. The physical properties of the flexible PVCs prepared with PVC10k-b-PCL89.5k are better than for a similar composition with DOP, and the block copolymer does not migrate from the PVC even after extended extraction times. In summary, we demonstrated a simple method for the synthesis of PVC-b-PCL block copolymers by RAFT-T̵ -ROP. The PVCb-PCL and blends of PVC-b-PCL with PCL are appropriate alternatives to phthalates and PCL itself as nonmigratory plasticizers for PVC. They offer the potential for a new generation of environmentally friendly flexible PVC products.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02146. 1 H NMR spectrum of HECP in CDCl3 (Figure S1); 1H NMR spectrum of PVC-CTA (Mn = 6600, Đ = 1.25) in CDCl3, 1H NMR spectrum of PVC-b-PVAc (Mn = 11200, Đ = 1.23) from PVC-CTA (Mn = 6600, Đ = 1.25) chain extension polymerization with VAc in CDCl3 and 1H NMR spectrum of PVC-b-PCL (Mn = 9700, Đ = 1.21) in CDCl3 (Figure S2); part of 1H NMR spectrum of PVC-CTA (Mn = 6600, Đ = 1.25) in CDCl3, part of 1H NMR spectrum of PVC-b-PVAc (Mn = 11200, Đ = 1.23) from PVC-CTA (Mn = 6600, Đ = 1.25) chain extension polymerization with VAc in CDCl3 and part of 1H NMR spectrum of PVC-b-PCL (Mn = 9700, Đ = 1.21) in CDCl3 (Figure S3); 1H DOSY spectra (CDCl3, 400 MHz, 298 K) for PVC-OH (Mn = 6600, Đ = 1.25), 1H DOSY spectra (CDCl3, 400 MHz, 298 K) for PVC-b-PVAc (Mn = 11200, Đ = 1.23), and 1 H DOSY spectra (CDCl3, 400 MHz, 298 K) for PVCb-PCL (Mn = 116400, Đ = 1.43) (Figure S4); 1H NMR spectrum of PVC-b-PCL (Mn = 15100, Đ = 1.23) by simultaneous RAFT and ROP in CDCl3 (Figure S5); infrared spectra of DOP and 1,6-hexamethylene I

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DOI: 10.1021/acs.macromol.8b02146 Macromolecules XXXX, XXX, XXX−XXX