Synthesis of Hyperbranched Polymers with High Molecular Weight in

Article pubs.acs.org/Macromolecules

Synthesis of Hyperbranched Polymers with High Molecular Weight in the Homopolymerization of Polymerizable Trithiocarbonate Transfer Agent without Thermal Initiator Xiaofeng Wang, Yi Shi, Robert W. Graff, Xiaosong Cao, and Haifeng Gao* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556-5670, United States S Supporting Information *

ABSTRACT: This article presents the first synthesis of high-molecular-weight hyperbranched polymers (over half a million) in the homopolymerization of a polymerizable trithiocarbonate chain transfer agent (i.e., transmer). Traditional reversible addition−fragmentation chain transfer (RAFT) homopolymerization of transmers that used thermal initiator as radical source has been reported to only produce hyperbranched polymers with relatively low molecular weights (≤104). The first part of this study extensively varied the experimental parameters in RAFT polymerization but could only marginally improve the polymer molecular weights. It was found that the vinyl focal groups were gradually consumed during the polymerization and the radical termination reactions were mainly happening between propagating radicals and primary radicals from the thermal initiator, which failed to increase the polymer molecular weight. In the second part, a new strategy was explored that used copper catalyst to activate the alkyl trithiocarbonate to generate radicals without the use of thermal initiator. This new initiation system eliminated the presence of primary radicals and ensured radical termination reactions only happening between propagating radicals, resulting in the production of hyperbranched polymers with very high molecular weight. When a small amount of atom transfer radical polymerization (ATRP) inimer was added, the concurrent ATRP/RAFT homopolymerization of transmer achieved faster polymerization rate and produced hyperbranched polymers with both high molecular weight and high degree of branching.

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INTRODUCTION In the past few decades, both dendrimers and hyperbranched polymers are popularly studied and proposed as promising types of nanostructured polymers due to their attractive structural features, including compact three-dimensional structures, cavernous interior, and large number of peripheral groups.1−8 Unlike dendrimers, hyperbranched polymers can be easily synthesized in a one-pot polymerization following several different techniques: polymerization of ABm (m ≥ 2) monomers (with or without the use of multifunctional Bf core),9−28 copolymerization of An and Bm monomers (e.g., A2 + B3),29−32 polymerization of divinyl or multivinyl cross-linkers (in the presence or absence of monovinyl monomers),33−41 and self-condensing vinyl polymerization (SCVP) of polymerizable initiators, often known as inimers. The last technique requires the use of controlled polymerization methods, such as controlled radical polymerization (CRP),42−52 living ionic polymerization,53−57 ring-opening metathesis polymerization (ROMP),58 and group transfer polymerization.59 Among the various CRP methods, atom transfer radical polymerization (ATRP)43,60−63 and nitroxide-mediated polymerization (NMP)42,64−66 have been first applied for the homopolymerization of inimers or copolymerization with various functional monovinyl monomers to produce hyperbranched and branched polymers. It is important to note that © XXXX American Chemical Society

the use of large excess of monovinyl monomers in the copolymerization with inimers is expected to decrease the degree of branching (DB) of the polymers and produce branched (instead of hyperbranched) polymers. In contrast, fewer reports have been published on the reversible addition− fragmentation chain transfer (RAFT) polymerization of polymerizable chain transfer agents (i.e., transmers), and even fewer studies have investigated the homopolymerization of transmers to produce high-DB hyperbranched polymers.48−50,67−84 Different from AB* inimers that could generate propagating radicals under proper initiation conditions, the transmers in the RAFT polymerization typically require an external radical source, such as 2,2′-azobisisobutyronitrile (AIBN), for initiation and polymerization.52 The presence of primary radicals throughout the polymerization system has a profound effect on the radical−vinyl reactions and radical−radical termination reactions, which have a significant effect on the structures and the molecular weights of the hyperbranched polymers.84 A thorough literature research within the best of our knowledge indicates that RAFT homopolymerization of transmers when Received: May 12, 2016 Revised: August 9, 2016

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DOI: 10.1021/acs.macromol.6b00994 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules using thermal initiators as radical sources could only produce hyperbranched polymers with low molecular weight. Several groups have reported results for RAFT homopolymerization of transmers, including Zhao’s group with the highest molecular weight as Mn = 24 700,71 Sumerlin’s group with Mn = 8740,70 and Poly’s group with Mn ∼ 1000.84 Many groups simply used more monovinyl monomers, i.e., high ratios of γ = [monomer] 0 /[transmer] 0 , to increase the molecular weights.48−50,67−84 On the other hand, Ishizu and Tanaka reported the photopolymerization of dithiocarbamate-containing styrenyl monomer without using thermal initiator and produced hyperbranched polymers with molecular weight above 100K,85,86 although no detailed polymerization kinetics and comparison to thermal RAFT system were studied. With the intention to increase the molecular weights of hyperbranched polymers in the homopolymerization of transmers, we in this contribution systematically studied the evolution of the polymer molecular weights and the polymer structures when thermal initiator was used as the radical source. Based on the results, a new initiation system was applied to effectively produce high-molecular-weight hyperbranched polymers, including the concurrent ATRP/RAFT of transmers using copper catalyst and the ATRP inimers.

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RESULTS AND DISCUSSION Within our best knowledge, traditional RAFT homopolymerization of transmers when using thermal initiator as radical source failed to produce high-molecular-weight hyperbranched polymers. To understand the reason and track the evolution of polymer structure in nuclear magnetic resonance (NMR) spectroscopy, a poly(ethylene glycol) (PEG)-labeled thermal azo initiator (termed as PEG-azo) was synthesized via esterification between 4,4′-azobis(4-cyanovaleric acid) and PEG methyl ether (MeO-PEG-OH, Mn = 550) with detailed synthesis in the Supporting Information. The RAFT homopolymerization of transmer 1 was conducted using [transmer 1]0/[PEG-azo]0 = 50/1 with toluene as solvent at 65 °C and [transmer 1]0 = 0.75 mol L−1. Samples were taken at timed intervals for 1H NMR and tetrahydrofuran (THF) size exclusion chromatography (SEC) measurements of vinyl conversions and polymer molecular weights, respectively. At 48 h, the polymerization reached over 98% vinyl conversion (Figure 1A), and the molecular weight stopped increasing at Mn,RI = 5600 (Figures 1B and 1C), determined by THF SEC based on linear poly(methyl methacrylate) (PMMA) standards. As comparison, the number-average molecular weights of the hyperbranched polymers determined by multiangle laser light scattering (MALLS) detector (using measured dn/dc = 0.11 mL/g) only became reliable at high conversions. The value of Mn,MALLS was higher than the Mn,RI, confirming the compact structure of the final hyperbranched polymer (Table 1, entry 3). The series of polymer samples at various conversions, including the final polymer, were thoroughly purified, i.e., precipitation into methanol for four times to completely remove the unreacted transmer and unincorporated PEG (Figure S2 in the Supporting Information). Figure 2A shows the stacked 1H NMR spectra of the purified polymers, identifying the evolution of four key groups in the polymer structure: the trithiocarbonate (TTC) group, the focal vinyl group (Vfoc), the dangling PEG group, and the vinyl group from disproportionation reaction (Vdisp). It should be noted that the complicated structure of hyperbranched polymers has posted a great challenge to obtain accurate integration of NMR peaks.

Figure 1. (A) Vinyl conversion as a function of polymerization time, (B) overlaid THF SEC chromatograms of hyperbranched polymers during the polymerization, and (C) dependence of molecular weights as a function of vinyl conversion in the RAFT homopolymerization with [transmer 1]0/[PEG-azo]0 = 50/1, 65 °C, [transmer 1]0 = 0.75 mol L−1. Mn,RI (based on linear PMMA standards) and Mn,MALLS were determined from THF SEC coupled with RI and MALLS detectors, respectively.

Three independent NMR measurements of each sample were taken to average the integration areas with error bar included. By using the ethylene linker in each transmer unit as internal reference, the molar ratios of these four groups to the C2H4 linker were recorded as the function of transmer conversion (Figure 2B). The molar fraction of the Vfoc group, the peak at δ = 6.12 ppm, decreased rapidly with conversion and ultimately became undetectable. Since the molecular weight of the final hyperbranched polymer was merely Mn,MALLS = 10 100 (DP ∼ 30), the trend of Vfoc vs conversion indicates that many focal vinyl groups were consumed but helped little to the increase of polymer molecular weights, probably through the intramolecular cyclization reactions. Meanwhile, the molar ratio of TTC group decreased with conversion, confirming the occurrence of radical termination reactions. By the end of polymerization, about 4−5% of TTC groups were lost via several possible termination reaction pathways, including B


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Macromolecules Table 1. Hyperbranched Polymers Produced by Homopolymerization of Transmer 1 entry

initiation

feed ratioa

temp (°C)

Mn,RIb

Mw/Mnb (RI)

Mn,MALLSc

Mw/Mnc (MALLS)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

PEG-azo PEG-azo PEG-azo PEG-azo PEG-azo PEG-azo PEG-azo PEG-azo V-40 CuBr/(dNbpy)2 CuBr/(dNbpy)2 CuBr/(dNbpy)2 CuBr/(dNbpy)2 CuBr/(dNbpy)2 + BIEM

10/1 25/1 50/1 100/1 500/1 2000/1 50/1 50/(1 × 5) 50/1 10/1 25/1 50/1 200/1 50/1

65 65 65 65 65 65 50 65 65 65 65 65 65 65

4700 5100 5600 5200 3900 3800 5700 6200 5900 32 500 41 300 48 100 50 700 51 900

1.37 1.47 1.35 1.48 1.43 1.39 1.39 1.28 1.40 18.60 21.23 26.93 23.57 38.23

9300 9500 10100 9700 7200 6200 11700 14300 12400 327800 389300 434400 478900 635500

1.19 1.18 1.18 1.17 1.30 1.49 1.26 1.28 1.35 5.29 5.65 5.17 5.23 5.68

DBd

0.32

0.30 0.31

a

[Transmer 1]0:[PEG-azo]0 for entries 1−7, [transmer 1]0:[PEG-azo]0 = 50:(1 + 1 + 1 + 1 + 1) for entry 8, in which the PEG-azo initiator was added in five batches at 0, 48, 72, 96, and 120 h; [transmer 1]0:[V-40]0 for entry 9, [transmer 1]0:[CuBr/(dNbpy)2]0 for entries 10−14, [BIEM]0 = [CuBr/(dNbpy)2]0 in entry 14. All experiments were conducted at fixed concentration [transmer 1]0 = 0.75 mol L−1. bApparent number-average molecular weight and molecular weight distribution measured by THF SEC with RI detector, based on linear PMMA standards. cNumber-average molecular weight and molecular weight distribution measured by THF SEC with MALLS detector (dn/dc = 0.11 mL/g). dDegree of branching (DB) of the hyperbranched polymers, determined by inverse gated decoupled quantitative 13C NMR spectroscopy.

Figure 2. (A) Overlaid 1H NMR spectra of purified hyperbranched polymers and (B) molar ratio of each structural groups to ethylene reference unit (X/C2H4) in the polymers as a function of vinyl conversion, the RAFT homopolymerization: [transmer 1]0/[PEG-azo]0 = 50/1, 65 °C, [transmer 1]0 = 0.75 mol L−1.

coupling and disproportionation reactions with either another propagating radical or a PEG-based primary radical (Scheme 1A). Among these several pathways, the coupling termination with PEG primary radicals was confirmed by the increased molar fraction of PEG, the peak at δ = 3.64 ppm, with conversion (Figure 2B). On the other hand, the peak at δ = 6.20−6.25 ppm, the protons in Vdisp groups produced in the disproportionation reaction, showed increased fraction with conversion. If we take the knowledge that the ratio of disproportionation/combination in methacrylate radicals was about 3/1 to 4/1,87−90 the loss of 4% of TTC by the end of polymerization indicates ca. 1.5% Vdisp was generated in total. Since the detected amount of Vdisp at the end of polymerization was less than 1%, it is speculated that some of the Vdisp groups could be consumed in the polymerization via reaction with radicals, although its contribution to the increase of polymer molecular weight was very limited (Figure 1B). Furthermore, the marginal increase of molecular weight also confirmed that the coupling termination reaction between two propagating radicals was either rare or only limited within intramolecular reaction. Radical termination reactions played an important role in the RAFT homopolymerization of transmers to incorporate Vdisp

groups and primary radical fragments into the polymer structure, although its effect on the increase of molecular weight was marginal. Since the decomposition half time (t1/2) of the used PEG-azo thermal initiator at 65 °C is around t1/2 = 10 h,91 97% of the initially added PEG-azo had decomposed after 48 h (five t1/2 cycles). The lack of primary radicals in the RAFT system stopped the polymerization, producing a hyperbranched polymer with Mn,MALLS = 10 100. To further increase the polymer molecular weight, several experimental parameters were varied to increase the radical concentration or extend its presence time in the RAFT system. The first attempt was to adjust the initial molar ratios of transmer to azo initiator, and the results (Table 1, entries 1−6) show a mixed influence on the molecular weights. More PEGazo initiator caused faster and complete consumption of vinyl groups but also introduced more PEG fragments into the polymer. Within the investigated feed ratios of [transmer 1]0/ [PEG-azo]0 = 10/1, 25/1, 50/1, 100/1, 500/1, and 2000/1, the highest molecular weight produced was Mn,RI = 5600 when [transmer 1]0/[PEG-azo]0 = 50/1 (Figure 3). High ratios of [transmer 1]0/[PEG-azo]0 = 500/1 and 2000/1 generated low radical concentration and resulted in incomplete vinyl conversions at the end of polymerization as 78% and 63%, C


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Macromolecules

Scheme 1. Schematic Illustration of Homopolymerization of Transmer (1) Using (A) RAFT with PEG-azo Thermal Initiator and (B) Concurrent ATRP/RAFT without Thermal Initiator

we decided to use ATRP copper catalyst for activation of the alkyl TTC and provided radicals for the RAFT polymerization with no use of thermal initiators. It has been reported that dithioester and dithiocarbamate chain transfer agents could be effectively activated using CuBr/ligand to conduct a concurrent ATRP/RAFT.93−95 However, the activation of TTC-based transmer for synthesis of hyperbranched polymers has not been reported. The transmer 1 was first tested for homopolymerization using 2 mol % of CuBr at [transmer 1]0/[CuBr(dNbpy)2]0 = 50/1 at 65 °C (dNbpy: 4,4′-di-5-nonyl-2,2′-bipyridine). During the polymerization, the SEC traces of hyperbranched polymers indicated a slow and steady increase of molecular weights over a 2 week reaction time (Figures 4A and 4C). The molecular weight increased slowly before 96% vinyl conversion followed by an eruptive increase of molecular weight and polydispersity at very late stage (Figure 4B). Hyperbranched polymers at 350 h with vinyl conversion ∼99.9% represented the last sample that could be separated by the SEC columns (Figure 4C), which showed a number-average molecular weight Mn,MALLS = 434.4K (Table 1, entry 12). The polymerization continued after 350 h although the system became too viscous to be magnetically stirred. After thorough purification, a series of polymer samples at different conversions were characterized in 1H NMR spectroscopy (Figure S4). The molar ratios of the TTC group, Vfoc group, and Vdisp group versus the internal reference C2H4 group were monitored as a function of transmer conversions. By the end of polymerization, the molecular weight reached Mn,MALLS = 434.4K, much higher than that in the PEG-azo initiated system. It was noticed that ca. 18% of TTC groups were lost from the hyperbranched polymers, which was higher than the 2 mol % of CuBr catalyst initially added (Figure 4D). A control experiment by mixing a structurally similar TTC chain transfer agent, 2-(((butylthio)carbonothioyl)thio)-2-methylpropanoate (TTC-CTA, Supporting Information) with 2 mol % of CuBr(dNbpy)2 under similar conditions confirmed that extended heating at 65 °C caused about 20% loss of TTC

Figure 3. SEC curves of hyperbranched polymers synthesized with various azo initiators in different amounts, addition modes, and polymerization temperatures.

respectively (Figure S3). Meanwhile, the polymerizations using lower ratios of [transmer 1]0/[PEG-azo]0 = 25/1 and 10/1 reached complete consumption of vinyl groups but produced more hyperbranched polymers with more PEG fragments (Figure S3) and lower molecular weights (Table 1). In addition to varying the molar ratio of transmer 1 to PEGazo initiator, other methods were also tried, including (1) the decrease of polymerization temperature from 65 to 50 °C to extend the presence time of primary radicals (Table 1, entry 7), (2) multiple-batch addition of PEG-azo into the polymerization (Table 1, entry 8), and (3) the use of initiator 1,1′azobis(cyclohexanecarbonitrile) (V-40) that has a longer halfdecomposition time at 65 °C (kd = 6.7 × 10−7 s−1, t1/2 ∼ 300 h at 65 °C,92 Table 1, entry 9). All three strategies worked in the polymerizations but only marginally improved the molecular weight of the hyperbranched polymers to Mn,RI = 5700−6200 (Figure 3). All these efforts by using thermal initiators as radical sources achieved limited success to produce high-molecular-weight hyperbranched polymers. As a new effort in the current study, D


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Figure 4. (A) Vinyl conversions as a function of polymerization time; (B) molecular weight (Mn,RI) and polydispersity (Mw/Mn) of hyperbranched polymers in the polymerizations [transmer 1]0/[CuBr(dNbpy)2]0 = 50/1 and [transmer 1]0/[BIEM]0/[CuBr(dNbpy)2]0 = 50/1/1 at 65 °C, [transmer 1]0 = 0.75 mol L−1; (C) overlaid THF SEC curves; (D) molar ratio of each structural moiety to ethylene reference unit (X/C2H4) in the polymers as a function of vinyl conversion in the polymerization [transmer 1]0/[CuBr(dNbpy)2]0 = 50/1 at 65 °C and [transmer 1]0 = 0.75 mol L−1.

[transmer 1]0/[BIEM]0/[CuBr(dNbpy)2]0 = 50/1/1 at 65 °C in toluene reached 99% conversion within 50 h (Table 1, entry 14), showing a faster polymerization than that without BIEM. Consequently, the polymerization produced high molecular weight (Mn,MALLS = 635.5K) in 5 days instead of 2 weeks (Figure S8). In addition to the high molecular weight, the branching density in hyperbranched polymers, i.e., the DB value, is essential and needs quantitative characterization. Following a recent method developed in our group,61 the structure of the hyperbranched polymers in the current studies was carefully determined using the inverse gated decoupled quantitative 13C NMR spectroscopy.96,97 After careful peak assignments in the 13 C NMR spectra with the assistance of heteronuclear singlequantum coherence (HSQC) and heteronuclear multiple bond correlation (HMBC) spectroscopy (Figure S9), integration of the peaks in Figure 5 was applied to calculate the DB of the hyperbranched polymer (Table 1, entry 12). The number fraction of B* subunit was determined as f B* = NB*/(NB* + Nb) = 0.81, corresponding to the reactivity ratio r = kA*/kB* = (convA + f B* − 1)/(−ln f B* + f B* − 1) = 40.6 and the DB = 0.30. The same calculation method was also applied to the purified hyperbranched polymers produced by RAFT homopolymerization of transmer 1 with PEG-azo initiator (Table 1, entry 3) and the ATRP/RAFT polymerization using both CuBr/(dNbpy)2 and inimer BIEM (Table 1, entry 14), showing similar DB values for all these three systems (Figure S10). Thus, the simple addition of small amounts of CuBr/ (dNbpy)2 catalyst with optional BIEM inimer avoided the use of thermal initiator and significantly improved the molecular

groups from the isobutyryl moieties after 100 h (Figure S5), maybe due to some Cu-catalyzed TTC transformation reactions. Meanwhile, the conversion-dependent evolutions of Vfoc and Vdisp groups in Figure 4D were similar as those in Figure 2B, suggesting their similar reaction behaviors. Therefore, the significant increase of molecular weight in the concurrent ATRP/RAFT system was due to the lack of primary radicals. All termination reactions were between propagating radicals, and the intermolecular coupling termination reactions made major contribution to the increase of molecular weight. It was noticed that the activation of TTC groups using CuBr/(dNbpy)2 catalyst was slow, and the low radical concentration explained the need of 2 weeks to reach very high molecular weight. Further varying the initial feed ratios of [transmer 1]0/[CuBr(dNbpy)2]0 from 10:1 to 200:1, at fixed concentration of [transmer 1]0 = 0.75 mol L−1, could adjust the polymerization rate although all the polymerizations required days to reach complete (>99%) conversion and high molecular weights (Figure S6). A control RAFT polymerization of MMA using the TTC-CTA chain transfer agent was set up in toluene under 65 °C with [MMA]0/[TTC-CTA]0/[CuBr(dNbpy)2]0 = 50/1/1. This polymerization showed no MMA conversion within the first 0.5 h and required 120 h to reach 90% MMA conversion (Figure S7). To increase the polymerization rate, an alternative method was applied to use an ATRP inimer 2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM) in the concurrent ATRP/RAFT system. The inimer functioning as an initiator could provide a high concentration of radicals via activation and chain transfer processes. As shown in Figure 4A, the polymerization under E


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ACKNOWLEDGMENTS The authors thank the ARO YIP award (W911NF-14-1-0227) and ACS Petroleum Research Fund (PRF #54298-DN17) for financial support. Y. Shi thanks the partial financial support from the Center of Sustainable Energy at Notre Dame via the ND Energy Postdoctoral Fellowship Program. H. Gao thanks the support from the University of Notre Dame, the Center for Sustainable Energy at Notre Dame and NDnano.

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Figure 5. Inverse gated decoupled 13C NMR spectrum of hyperbranched polymers (Table 1, entry 12).

weight of the hyperbranched polymers in the homopolymerization of transmer 1.

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CONCLUSIONS We report the first synthesis of hyperbranched polymers with very high molecular weights in the homopolymerization of polymerizable chain transfer agent (i.e., transmer). Traditional RAFT polymerization of transmer using PEG-azo thermal initiators produced relatively low-molecular-weight hyperbranched polymers because the termination reactions mainly occurred between propagating radicals and primary radicals. These termination reactions by losing TTC groups could introduce disproportionation vinyl groups and PEG initiator fragments into polymers but contributed little to the molecular weight of hyperbranched polymers. In contrast, the activation of TTC moieties using CuBr/(dNbpy)2 catalyst avoided the use of any thermal initiator and eliminated the presence of primary radicals in the system. The concurrent ATRP/RAFT polymerization of transmer 1 produced hyperbranched polymers with over half a million molecular weight and high DB = 0.30. Further optimization by adding small amount of ATRP inimer into the system provided a stable radical concentration and resulted in a faster polymerization to produce high-molecular-weight polymers. These results highlight the importance of initiation technique in the homopolymerization of transmers to produce hyperbranched polymers with high molecular weights.

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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.6b00994. Experimental details; Figures S1−S10 and Table S1 (PDF)

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*E-mail [email protected] (H.G.). Notes

The authors declare no competing financial interest. F


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