Article pubs.acs.org/Macromolecules
Carbon Dioxide-Derived Immortal Brush Macromolecules with Poly(propylene carbonate) Side Chains Satoshi Honda* and Hiroshi Sugimoto* Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, 12-1 Ichigaya-Funagawara-machi, Shinjuku-ku, Tokyo 162-0826, Japan S Supporting Information *
ABSTRACT: Brush macromolecules (BMs) are unique because of their distinct properties from linear or other nonlinear polymers, relying on side chain steric repulsions and backbone stiffness. BMs with densely grafted side chains have been synthesized either by cationic, anionic, or controlled radical polymerization, yet CO2−epoxide immortal alternating copolymerization has rarely been applied for the synthesis of BMs via the “grafting-from” approach. Here we report synthesis of BMs by alternating copolymerization of CO2 and propylene oxide (PO) initiated from poly(acrylic acid) as a multifunctional macroinitiator. The copolymerization afforded ultrahighmolecular-weight BMs with poly(propylene carbonate) (PPC) side chains (molecular weight >106), and the side chain length was tunable via further CO2−PO alternating copolymerization initiated from hydroxy end groups of the resulting side chain PPCs. These BMs were directly observed by atomic force microscopy, demonstrating that the BMs have ellipsoidal morphologies with 20−50 nm. Furthermore, the BMs were thermally decomposable at around 240 °C.
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INTRODUCTION Polymers obviously contribute to the prosperity of humankind, yet considerable kinds of them have a negative effect on resource, energy, and environment problems. Development of an alternative eco-friendly material is imperative in modern chemistry. Synthesis of useful organic materials based on the utilization of carbon dioxide (CO2) as a source material is arguably challenging.1 Among them, immortal alternating copolymerization of CO2 and epoxide to produce polycarbonates (PCs) has been studied worldwide from its discovery by Inoue and co-workers.2,3 One of the ongoing challenges in this area is to manipulate their macromolecular architectures. Commendable researches for synthesizing nonlinear PCs have emerged.4−8 Particularly, Wang and co-workers reported threearmed star-shaped copolymers synthesized by CO2−propylene oxide (PO) copolymerization initiated from hydroxy-terminated trifunctional oligo(PO).5,6 We have also attained the synthesis of CO2-derived star-shaped,9 H-shaped,10 and cyclic11 poly(propylene carbonate)s (PPCs). Further synthesis of CO2derived PCs with complex macromolecular architectures is an interesting issue for designing future innovative materials. While polymers, especially artificial plastics, are mainly used as structural materials, thermally decomposable CO2-derived PCs are opening an attractive opportunity such as selective degradation of a microdomain with several tens of nanometers in block copolymer thin films.12 To expand applications of the thermal decomposition of CO2-derived PCs, we focused on a single molecularly nanosized brush macromolecule (BM), © XXXX American Chemical Society
expecting application for a novel mesoporous material by ordering BMs and selective removal of them after filling a second phase. A brushlike architecture, which is defined by their multitudes of branch points and chain ends, found in nature has greatly inspired chemists because it plays an important role to maintain biological activity as seen in proteoglycans.13 Accordingly, many synthetic polymers with tailored brushlike architectures have been reported to date.14−16 In particular, a grafting-from approachpolymerization of monomers from side chain initiating groups of a backbone polymerhas an advantage in synthesizing BMs because steric hindrance around reactive sites, which often limits conditions of grafting-onto and -through approaches, is kept to the minimum by gradual growth of side chains. Although cationic and anionic polymerizations have historically been applied for grafting-from synthesis of BMs, increase in the initiating groups generally leads to severe difficulty in concurrent initiation from all of them and also cannot avoid termination reactions. Controlled radical polymerization can avoid such difficulty to some extent owing to the low concentration of active radical species throughout the polymerization,13 yet further exploration for available polymerization technique for grafting-from approach remains a challenge. Received: July 13, 2016 Revised: August 26, 2016
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DOI: 10.1021/acs.macromol.6b01505 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
stored under a nitrogen atmosphere. Commercial CO2 was used without further purification. DMAP was recrystallized from toluene. Tetraphenylporphyrinatocobalt(III) chloride ((TPP)CoCl)22 was synthesized according to the procedure reported in the literature. Other reagents were used as received. ATRP of tert-Butyl Acrylate from Methyl α-Bromoisobutylate. Into a test tube equipped with three-way stopcock were placed CuBr (18 mg, 0.125 mmol), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) (26 μL, 0.125 mmol), tBA (1.5 mL, 10 mmol), and methyl α-bromoisobutylate (17 μL, 0.125 mmol), and the mixture was degassed by three “freeze−pump−thaw” cycles. The test tube was placed in an oil bath to proceed the reaction with stirring at 110 °C for 1 h under vacuum. The reaction was then quenched by cooling with liquid nitrogen. After being allowed to warm to room temperature, CHCl3 was added to the mixture, and the resulting solution was subjected to alumina column chromatography. The eluent was concentrated and dried under reduced pressure to give PtBA80. The yield was 1.29 g. Mn(RI) = 10 000, Mw(RI) = 13 000, PDI = 1.36. 1H NMR (400 MHz, CDCl3) δ ppm 1.14 (s, −C(CH3)2−), 1.20−1.67 (−CH2CH(COOC(CH3)3)−, −C(CH3)3), 1.69−1.96 (−CH2CH(COOC(CH 3 ) 3 )−), 2.07−2.40 (−CH 2 CH(COOC(CH 3 ) 3 )−). PtBA670 was likewise synthesized. Synthesis of PAA. Deprotection reaction of the tert-butyl group was carried out according to a modified procedure reported in the literature.23 Thus, trifluoroacetic acid (5.8 mL, 78.2 mmol) was added to the chloroform solution (20 mL) of PtBA80 (500 mg, 3.91 mmol for tert-butyl groups), and the mixture was stirred at room temperature for 13 h under nitrogen. The resulting suspension was filtered, and the white precipitates were washed with chloroform. The precipitates were dried under reduced pressure to give PAA80. The yield was 307 mg (74%). 1H NMR (400 MHz, D2O) δ ppm 1.14 (s, −C(CH3)2−), 1.20−1.67 (−CH 2 CH(COOH)−), 1.69−1.96 (−CH 2 CH(COOH)−), 2.07−2.40 (−CH2CH(COOH)−). PAA670 was likewise synthesized. Alternating Copolymerization of CO2 and Propylene Oxide Initiated from PAA. The general procedure is described: A stainless steel autoclave (150 mL) containing (TPP)CoCl (71 mg, 0.10 mmol) and DMAP (9 mg, 0.10 mmol) was dried in vacuo and purged with nitrogen. A DMF (7 mL) solution of PAA80 (30 mg, 0.40 mmol for carboxyl groups) and PO (14 mL, 200 mmol) was added to the autoclave, and the copolymerization was initiated by pressurizing the resulting solution with CO2 up to 50 atm at 40 °C. After 40 h, the autoclave was cooled, and unreacted CO2 was discharged. The remained DMF solution was then poured into an excess amount of MeOH to afford a reddish-purple solid. The crude polymer product was purified by precipitation into MeOH twice to afford a slightly yellow solid (4.72 g). A weighed amount of the solid (1.4 g) containing three copolymers initiated from (TPP)CoCl-derived chloride anion, water, and PAA80 was then subjected to preparative HPLC to collect the polymer fraction corresponding to a brush macromolecule. The yield was 515 mg. 1H NMR (CDCl3): δ ppm 1.05−1.38 (d, J = 6.5 Hz, −CH2CH−, −CH3), 1.75−2.05 (m, −CH2CH−), 2.40 (m, 8H, −COCH2CH2−), 4.01−4.40 (m, −OCH2CH−), 4.91−5.13 (m, −CH(CH3)O−). Mw(RI) = 1.9 × 105, Mw(RALLS) = 13 × 105, Mw/Mn = 1.20. 1 H NMR Measurements. 1H NMR spectra were recorded on a Bruker DPX-400 spectrometer operating at 400 MHz. CDCl3 and D2O were used as the solvent, and chemical shifts were reported relative to tetramethylsilane (TMS) (δ = 0.00 ppm) or solvent residual signals. GPC Measurements. GPC measurements were performed using a Tosoh model HLC-8220 high-speed liquid chromatograph equipped with a refractive index (RI) detector. Two series-connected TSKgel SuperMultiporeHZ-H columns were employed with tetrahydrofuran (THF) as the eluent at a flow rate of 0.35 mL/min at 40 °C. The molecular weight calibration curve was obtained with standard polystyrenes (TSK standard polystyrene from Tosoh Co.). By using a Viscotek model TDA302 with triple detector, RI increment (dn/dc), weight-average molecular weight (Mw(RALLS)), hydrodynamic radius (Rh), and radius of gyration (Rg) were measured. For the dn/dc
BMs with backbone CO2-derived PCs have also been synthesized. Zhang et al. have reported the synthesis of BMs with backbone PCs through the copolymerization of CO2 and 4-vinyl-cyclohexene-1,2-epoxide, functionalization of side chain vinyl groups with β-mercaptoethanol, and subsequent ringopening polymerization (ROP) of ε-caprolactone from the side chain hydroxy groups.17 Frey et al. reported a similar strategy for synthesizing BMs via the ROP of L-lactide from backbone PCs with hydroxy side chains.18 More recently, Lu et al. reported another strategy for synthesizing BMs via the ROP of 19 L-lactide from a backbone PC having hydroxy groups. Conversely, BMs with CO2-derived PC side chains have rarely been reported; however, Coates et al. reported the graftingthrough synthesis of brush macromolecules by ring-opening metathesis polymerization of norbornene-terminated poly(cyclohexene carbonate)s.20 Although we can recognize one example of applying CO2−PO copolymerization for the grafting-from approach, in which an aspect of catalytic activity is mainly discussed,21 solution properties, morphologies, and potential applications based on their brushlike architectures have not been explored. Herein, we report the synthesis of BMs with a backbone poly(acrylic acid) (PAA) and side chain CO2-derived PPCs. The backbone PAAs were first synthesized by atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBA) followed by deprotection of tert-butyl groups using trifluoroacetic acid (TFA). The immortal alternating copolymerization of CO2 and PO initiated from the resulting PAAs effectively afforded BMs with side chain CO2-derived PPCs. Remarkably, it is disclosed that the synthesized BM works as multifunctional macroinitiator for further CO2−PO copolymerization, meaning the BM is immortal (Figure 1). The size and morphology of the
Figure 1. Immortal BMs with tunable CO2-derived side chain polymer length.
BMs were directly observed by atomic force microscopy (AFM), demonstrating that the BMs take ellipsoidal single molecular morphologies. Furthermore, thermal decomposition temperature (Td) determined based on a thermogravimetric analyzer (TGA) revealed the BMs uniformly and quickly decompose at 230−240 °C, opening a new avenue for constructing degradable nanostructures made from CO2.
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EXPERIMENTAL SECTION
Materials. tert-Butyl acrylate (tBA) was passed through a plug of alumina and bubbled with nitrogen prior to use. Propylene oxide (PO) was dried over a KOH pellet, distilled over CaH2 and KOH, and B
DOI: 10.1021/acs.macromol.6b01505 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules measurement, five THF polymer solutions with different concentrations were subjected to RALLS measurements, and slope of plots of RI area against concentration was calculated with the software built into the instrument using the equation RI area = K (dn/dc)c where K and c are instrument constant and polymer concentration, respectively. Preparative HPLC Fractionation. Preparative HPLC fractionation was performed on a JAI LC-9204 recycling preparative HPLC system. Two series-connected JAIGEL-3H-40 and JAIGEL-4H-40 columns were employed with THF as the eluent at a flow rate of 14 mL/min. AFM Observations. A few drops of a 0.5, 1.0, or 10.0 mg/mL polymer solution were placed onto a freshly cleaved mica surface. After 30 s, the solution was blotted away with a filter paper, and the surface was dried in air. AFM observations of the residual sample on the mica surface were performed on a Hitachi S-image operated in DFM mode using Al-coated microcantilever ( f = 310 kHz, C = 29 N/m). DSC Measurements. Differential scanning calorimetry (DSC) measurements were performed using a Seiko DSC 7020 analyzer at a heating and cooling rate of 10 °C/min. The reported Tg values were determined from the second heating scan. TGA Measurements. Thermal gravimetric analysis (TGA) was performed on a SII Technology TG/DTA 6200 at a heating rate of 20 °C/min. The thermal decomposition temperature (Td) was defined as an onset temperature of weight loss.
Figure 2. GPC chromatograms of (a) PtBA80, (b) copolymerized product initiated from PAA80 (Table 1, entry 1), and (c) after GPC fractionation.
Mn(GPC). To calculate DPn of PtBA670 by end-group analysis with 1H NMR was difficult because of its high degree of polymerization. The PtBAs were then subjected to deprotection reaction using TFA (Scheme 1, top) to afford poly(acrylic acid) (PAA). From 1H NMR, the complete disappearance of the signal derived from tert-butyl protons was confirmed (Figure S1b). Accordingly, PAA80 and PAA670 were obtained from PtBA80 and PtBA670, respectively. The synthesized PAAs were employed as multifunctional initiators for alternating copolymerization of CO2 and PO (Scheme 1, bottom). In immortal polymerization including CO2−epoxide copolymerization, protic molecules work as chain transfer agents.25,26 The copolymerization in the presence of carboxylic acids from the initial stage, they behave as initiators to give polymers therefrom.9,10 In the present study, the copolymerization without using solvent afforded only lowmolecular-weight copolymers corresponding to a mixture of α,ω-dihydroxy and α-chloro-ω-hydroxy PPCs, which are derived from a chloride anion of the catalyst (TPP)CoCl22 and concomitant water, respectively.10,27−29 Since PAA had poor solubility in PO and common organic solvent including CH2Cl2, which is typical solvent for the copolymerization, highly polar but aprotic solvent was required to make the polymerization mixture homogeneous. We previously reported that synthesis of four- and six-armed star-shaped PPCs by CO2−PO alternating copolymerization initiated from the corresponding tetra- and hexafunctional carboxylic acids and found that the copolymerization was well-controlled by using DMF as the solvent.9 As such, the copolymerization was conducted in DMF with the (TPP)CoCl/DMAP catalytic system. For entry 1 in Table 1, the GPC chromatogram of the product after the copolymerization showed the existence of two discrete polymer fractions (Figure 2b). Apparently, the lowermolecular-weight copolymers (Mw(RI) = 7700, Mw/Mn = 1.12) can be assigned to a mixture of aforementioned α,ω-dihydroxy and α-chloro-ω-hydroxy PPCs, and the higher-molecularweight polymer (Mw(RI) = 1.9 × 105) can be assigned to BM. The BM showed remarkably higher molecular weight than that of the corresponding PtBA80 (Mw(RI) = 1.3 × 104), and it is noteworthy that the BM showed narrow molecular weight
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RESULTS AND DISCUSSION Synthesis of Brush Macromolecules with CO2-Derived PPC Side Chains. According to the reported procedure,24 ATRP of tBA initiated from methyl α-bromoisobutylate was performed (Scheme 1, top), and two PtBA samples having Scheme 1. Synthetic Route for the Brush Macromolecule with PAA Backbone and PPC Side Chains
different molecular weights were synthesized and characterized by 1H NMR (Figure S1a) and GPC (Figure 2a) analyses as summarized in Table S1. These PtBAs are hereafter denoted as PtBAx, where x is the degree of polymerization determined by GPC analysis. The present two PtBAs with 80 and 670 repeating units are expressed as PtBA80 and PtBA670. The DPn of PtBA80 was determined by end-group analysis using 1H NMR (Figure S1a), and that of PtBA670 was determined by GPC. Number-averaged molecular weights determined by GPC (Mn(GPC)) generally do not correspond to those determined by 1H NMR (Mn(NMR)) except for polymers that are the same kind as those used for a GPC calibration curve. However, Mn(NMR) and Mn(GPC) of PtBA80 were almost the same (Table S1), which allowed us to calculate DPn of PtBA670 using C
DOI: 10.1021/acs.macromol.6b01505 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Characterization of BMs Synthesized by CO2−PO Alternating Copolymerization Mwc × 10−5 entry 1 2 3 4 5 6
initiator PAA80a PAA80a PAA670a PAA670a PAA670a PAA670-graf t-PPC17
DPn
time (h)
conv
b
RI
RALLS
Mw/Mnd
PAAe
PPCf
formulag
20 40 15 24 3 14
38 61 14 27 NDh NDi
1.9 2.6 6.8 7.9 2.7 6.9
13 16 130 140 12 150
1.20 1.20 1.46 2.18 1.33 2.23
80 80 670 670 670 670
160 200 180 210 17 220
PAA80-graf t-PPC160 PAA80-graf t-PPC200 PAA670-graf t-PPC180 PAA670-graf t-PPC210 PAA670-graf t-PPC17 PAA670-graf t-PPC220
a
[COOH]0/[cat.]0/[cocat.]0/[PO]0 = 4/1/0.75/2000. bConversion of PO, determined by 1H NMR. cWeight-average molecular weight, determined by GPC with RI and RALLS detectors, respectively. dMolecular weight distribution, determined by GPC with RI detector. eDPns of PtBAs were filled alternative to those of PAA. fDPn per arm, calculated from Mw(RALLS). gFormula of BM, determined by DPn of backbone PAA and side chain PPC. hNot determined because the concentration of PAA-graf t-PPC in an aliquot extracted was quite low and not enough for the 1H NMR characterization at the polymerization time of 3 h. iNot determined because the main chain 1H NMR signals of PPC before (purified BM from entry 5) and after the copolymerization overwrapped.
of PAA670-graf t-PPC17, which is the purified BM from entry 5 of Table 1, also afforded a BM with higher molecular weight (Table 1, entry 6). Similarly to entries 3 and 4, the GPC chromatogram of the THF-soluble part of the copolymerized product analyzed using RI detector showed a shift of the trace from the former (Figure 3a) toward significantly highermolecular-weight region (Figure 3b). Remarkably, the Mw(LS) of the BM was more than 10-fold higher than that PAA670-graf tPPC17 (Table 1, entry 6).
distribution (Mw/Mn) with retaining unimodal profile (Figure 2b). Prolonged polymerization time led to the production of a BM with longer PPC side chains without causing broadening of Mw/Mn (Table 1, entry 2). The BM fraction was then collected by preparative HPLC fractionation. The GPC chromatogram of the fractionated BM showed similar chromatogram to that before fractionation (Figure 2c). By using GPC with right angle laser light scattering (RALLS) detector, the refractive index increment (dn/dc) was measured to be 0.041 mg/mL (Figure S2), and the absolute molecular weight (Mw(RALLS)) of the BM was determined to be 13 × 105 by using the measured dn/ dc. This remarkable increase in absolute molecular weight compared to the parent PAA80 clearly demonstrates the successful synthesis of the designed BM (Scheme 1). The averaged DPn of PPC per arm was thereby calculated to be 160 (Table 1, entry 1), and the formula of the BM can be denoted as PAA80-graf t-PPC160, by using DPns of PAA and PPC per arm. Likewise, the formula of the BM synthesized in entry 2 of Table 1 was analyzed to have a formula of PAA80-graf t-PPC200. To explore the capability of the present methodology for synthesizing CO2-derived BMs, we next employed PAA670 as the multifunctional macroinitiator. When using PAA670, a mixture after the CO2−PO copolymerization contained a considerable amount of the insoluble part. Although GPC results of the THF-soluble part of the product showed the production of BMs, the calculated Mw/Mn was broad (Table 1, entry 3; Mw(RI) = 6.8 × 105, Mw/Mn = 1.46). Prolonged polymerization time resulted in further broadening of Mw/Mn (Table 1, entry 4; Mw(RI) = 7.9 × 105, Mw/Mn = 2.18). This copolymerization behavior is understood by a difference in copolymerization ability between growing species located inner and outer BMs. As copolymerization proceeds, an outer growing species (carbonate or alcoholate anions) of a longer PPC chain acquires more free volume than an inner one and thus easier to attacks to a next monomer, which likely resulted in broadening of Mw/Mn. In fact, shortened polymerization time (Table 1, entry 5) resulted in the production of BMs with the lower Mw/Mn (Mw(RI) = 2.7 × 105, Mw/Mn = 1.33), and a mixture after the copolymerization did not contain any insoluble part. The formation of insoluble part is therefore explained by extremely high molecular weight of the synthesized BMs in entries 3 and 4 in Table 1. Perhaps, aggregation of BMs occurs when molecular weight reaches a threshold. On another front, we revealed that the synthesized BMs are immortal. Thus, a copolymerization carried out in the presence
Figure 3. GPC chromatograms of (a) the purified BMs from entry 5 of Table 1 and (b) copolymerized product from entry 6 of Table 1.
A series of PAA-graf t-PPCs synthesized in the present study clearly demonstrates that immortal alternating copolymerization of CO2 and epoxide is an effective methodology for synthesizing BMs with the grafting-from approach. Importantly, the immortality of BMs is favorable for the tuning of side chain length by postpolymerization polymerization. Solution Properties of Brush Macromolecules. Based on RALLS, hydrodynamic radius (Rh) and radius of gyration (Rg) of the synthesized BMs were measured. For example, Rh and Rg of PAA80-graft-PPC160 were determined to be 15 and 19 nm, respectively, and thus the Rg/Rh ratio (ρ) was calculated to be 1.27 (Table 2). The calculated ρ was apparently higher than that of hard sphere with uniform density (0.775) but lower than rigid rod (>2),30 indicating PAA80-graf t-PPC160 takes a compact nonsphere shape in THF. The Mw(RI)/Mw(RALLS) ratio (g)g represents the degree of shrinking for polymers having nonlinear architecture in comparison with the corresponding linear analogueof PAA80-graft-PPC160 was calculated to be 0.15, indicating the highly shrunk nature of whole form of the polymer chain in comparison with a linear PPC having identical absolute molecular weight. The extremely decreased apparent D
DOI: 10.1021/acs.macromol.6b01505 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Solution Properties of the Synthesized BMs BMa
Rhb (nm)
Rgc (nm)
ρd
ge
PAA80-graf t-PPC160 PAA80-graf t-PPC200 PAA670-graf t-PPC180 PAA670-graf t-PPC210 PAA670-graf t-PPC17 PAA670-graf t-PPC220
15 16 57 55 19 48
19 21 75 72 25 62
1.27 1.31 1.31 1.31 1.32 1.29
0.15 0.16 0.052 0.056 0.23 0.046
a
Formula of BMs. bHydrodynamic radius, determined by RALLS. Radius of gyration, determined by RALLS. dThe ratio of Rg to Rh. e The ratio of Mw(RI) to Mw(RALLS). c
molecular weight in comparison with the absolute one is again strong indicative of the successful synthesis of the designed BMs. Solution properties of the synthesized BMs are summarized in Table 2. The bigger Rh and Rg of PAA670graf t-PPC180, PAA670-graf t-PPC210, and PAA670-graf t-PPC220 than those of the others are consistent with their higher absolute molecular weight (>1.3 × 107) than the others (
