Synthesis of high molar mass poly(n-butyl acrylate) and poly(2-ethylhexyl acrylate) by SET-LRP in mixtures of fluorinated alcohols with DMSO

Abstract

SET-LRP of n-butyl acrylate (nBA) and 2-ethylhexyl acrylate (EHA) initiated with bis(2-bromopropionyl)ethane (BPE) to synthesize high molar mass poly(nBA) and poly(EHA) was carried out in binary mixtures of 2,2,2-trifluoroethanol (TFE) or 2,2,3,3-tetrafluoropropanol (TFP) with DMSO at 50 °C. Using a solvent mixture of TFP containing 30% DMSO led for the first time to the synthesis of poly(nBA) with M_n = 527 700, M_w/M_n = 1.21 in 12 h, and poly(EHA) with M_n = 913 100, M_w/M_n = 1.20 in 15 h via SET-LRP. Although these two fluorinated alcohols provide complete solubilization of the hydrophobic monomers, nBA and EHA, and of the resulting polymers in addition to an efficient disproportionation of Cu(I)Br and subsequent stabilization of the Cu(II)Br₂/L complex, SET-LRP targeting high molar mass poly(nBA) and poly(EHA) in these solvents alone resulted in a relatively slow polymerization with limited conversion. By contrast, DMSO, in spite of being the preferred solvent for SET-LRP with ability to disproportionate Cu(I)Br, stabilizes the “nascent” Cu(0) nanoparticle and provides an efficient SET process, generating a non-living polymerization for these hydrophobic monomers due to the insolubility of the resulting polymers. Remarkably, by a cooperative and synergistic effect, the binary mixtures of TFE or TFP with DMSO provide excellent reaction media for the synthesis of high molar mass poly(nBA) and poly(EHA) by SET-LRP.

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Introduction

The development of Cu(0) mediated Single-Electron Transfer Living Radical Polymerization (SET-LRP)^1–4 emerged as a robust technique for the synthesis of polymers^3–25 with well-defined chain-end functionality even at complete monomer conversion.^6,7,26–29 However, polymerization of hydrophobic monomers by SET-LRP is rather challenging since the most crucial step of SET-LRP, the disproportionation^{1–4,10,20,29–35} of the in situ generated Cu(I)X into a Cu(0) activator and a Cu(II)X₂ deactivator in the presence of N-containing ligands (L)^1–3,36 and subsequent stabilization of the Cu(II)X₂/L complex is achieved in polar solvents, such as H₂O,^8,20,37–40 dimethyl sulfoxide (DMSO),^{6,7,10,14,17,20,29,41–43} dimethyl formamide (DMF),^38,44 dimethyl acetamide (DMAC),^38,44 ethylene and propylene carbonate,^38,44 alcohols,^15,41,45 and their binary mixtures,^38,46 some of them exhibiting cooperative and synergistic effects.^38,46 By contrast, non-polar solvents, such as toluene which mediates the disproportionation of CuBr in the presence of Me₆-TREN⁴⁴ but exhibits a limited solubility of the CuBr₂/Me₆-TREN complex gave rise to loss of chain-end functionality during SET-LRP of methyl acrylate (MA).^44,47–49 Hence solvent choices for SET-LRP of monomers containing long hydrophobic chains have gained significant attention in recent years. When DMSO was employed as a solvent for a series of hydrophobic acrylates, such as n-butyl acrylate (nBA), tert-butyl acrylate (tBA), lauryl acrylate (LA), 2-ethylhexyl acrylate (EHA), phase separation of the polymer from the reaction mixture resulted in polymers with uncontrollable high molecular weight and broad molecular weight distribution as the hydrophobicity of the polymer increases.⁵⁰ However, in spite of phase separation during SET-LRP of lauryl acrylate in isopropyl alcohol (IPA) living polymerization leading to polymers with narrow molecular weight distribution and high chain-end functionality resulted.²² In addition to this monomer–solvent system, binary mixtures of toluene with MeOH, and toluene with isopropyl alcohol (IPA) gave rise to excellent SET-LRP conditions for acrylates containing long hydrocarbon chains (C₁₂–C₁₈).²² In recent publications^45,51,52 we have demonstrated that two fluorinated alcohols, TFE and TFP, which promote disproportionation of Cu(I)X and subsequent stabilization of the resultant Cu(0) species generated by disproportionation, provide suitable reaction media for SET-LRP of various hydrophobic and hydrophilic acrylates,^45,51 including MA, nBA, tBA, EHA, and 2-hydroxylethyl acrylate and hydrophobic methacrylates,⁵² methyl methacrylate, ethyl methacrylate and n-butyl methacrylate. In both TFE and TFP, SET-LRP proceeds in a single phase without any visual sign of polymer insolubility until complete monomer conversion.^45,51,52 A detailed study on solvent choices for SET-LRP has revealed the cooperative and synergistic solvent effect, which demonstrates that the addition of a solvent that promotes a higher extent of disproportionation in the reaction mixture results in a faster rate of polymerization.^38,46 Therefore, we considered that it would be important to find a second solvent which would accelerate the rate of SET-LRP performed in the fluorinated alcohols, TFE and TFP that would enable us to synthesize high molar mass polymers from hydrophobic monomers. In this regard, DMSO is a unique solvent, which promotes fast disproportionation of Cu(I)Br and provides stable “nascent” Cu(0) nanoparticles and provides ultrafast polymerization of a range of vinyl monomers, such as acrylates, methacrylates, vinyl chloride in addition to the synthesis of ultra-high molar mass poly(MA),³ and even of hydrophilic poly(2-hydroxyethyl methacrylate)¹⁷ without the need for protection of the hydroxyl moiety. Hence we selected the binary mixtures of TFE and TFP with DMSO at various ratios for the synthesis of a high molar mass polymer of two hydrophobic monomers, nBA up to DP = 6220 and EHA up to DP = 10 000 using bis(2-bromopropionyl)ethane (BPE) as a bifunctional initiator, Me₆-TREN as a ligand, and Cu(0) wire as a catalyst. In this context it is noteworthy that the Atom Transfer Radical Polymerization (ATRP) conducted at an extremely high pressure (5 kbar) resulted in the synthesis of high molar mass poly(nBA) with M_n = 808 000, M_w/M_n = 1.51 at Cu(I)/initiator = 1 : 1 in 4 h and M_n = 612 000, M_w/M_n = 1.25 at Cu(I)/initiator = 1 : 4 in 6 h in MeCN at 80 °C.⁵³ Nevertheless, the results reported in this publication demonstrate that the binary mixtures of TFE and TFP with DMSO provide suitable reaction media for SET-LRP of hydrophobic monomers, such as nBA and EHA mediating the synthesis of high molar mass polymers under mild reaction conditions such as 50 °C at atmospheric pressure (Scheme 1).

Scheme 1 Synthesis of high molar mass poly(nBA) and poly(EHA) in binary mixtures of TFE or TFP with DMSO at 50 °C.

Results and discussion

Synthesis of high molar mass poly(nBA)

We first attempted to conduct SET-LRP of nBA in DMSO at [nBA]₀/[BPE]₀/[Me₆-TREN]₀/[CuBr₂] = 3110/1/1.5/0.5 catalyzed with 1.0 cm of non-activated Cu(0) wire (0.812 mm diameter, 20 gauge) at 50 °C. This resulted in 98% monomer conversion giving rise to a polymer with M_n = 460 100, M_w/M_n = 2.15 and bimodal molecular weight distribution (Table 1, entry 1). The reaction mixture showed phase separation in a similar fashion to the results reported by Boyer et al.⁵⁰ during SET-LRP of nBA in DMSO at a much lower targeted molecular weight ([M]₀/[I]₀ = 39 : 1). By contrast, SET-LRP of nBA at [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1 under similar reaction conditions in either TFP or TFE produced polymers with low monomer conversion but narrow molecular weight distribution (entries 2 and 3, Table 1). However, when a binary mixture of TFP or TFE with DMSO was used as a solvent for SET-LRP of nBA at [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1 no phase separation was observed even at 50% DMSO in TFP and at least up to 30% DMSO in TFE. The kinetic evaluation of the SET-LRP experiments conducted for nBA in TFP containing 30% DMSO and 50% DMSO at [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1 are shown in Fig. 1a–d. While using TFE and TFP as solvents in pure form or as co-solvents with DMSO, non-activated Cu(0) wire was used instead of hydrazine-activated catalyst since these fluorinated alcohols can act as self-activating solvents resulting in comparable k^app_p to that of a hydrazine-activated catalyst.⁵⁴ Remarkably, these mixtures of solvents provided polymerization reactions with significantly higher monomer conversion, a much faster reaction rate compared to only TFE or TFP, and resulted in poly(nBA) with narrow molecular weight distribution (Fig. 2a and b, entries 4–11, Table 1). In both cases the reaction followed first order kinetics up to monomer conversion of almost 80% with an apparent rate constant of propagation (k^app_p) of 0.0038 min⁻¹ for TFP containing 30% DMSO and 0.0041 min⁻¹ for TFP containing 50% DMSO. The number average molecular weight (M_n) of poly(nBA) follows a linear dependence on monomer conversion and narrow molecular weight distribution (Fig. 1b and d) demonstrating that the living nature of the polymerization of nBA in these two solvents results in poly(nBA) with M_n = 315 200, M_w/M_n = 1.19 in TFP containing 30% DMSO and M_n = 335 400, M_w/M_n = 1.17 in TFP containing 50% DMSO in 10 h. In both cases the stirring ceased at around 80% monomer conversion due to the high viscosity of the reaction mixture resulting in almost no additional polymerization even after much longer reaction times. It is noteworthy that when only TFP was used as a solvent under similar reaction conditions, polymerization reached only 58% conversion after 12 h with a k^app_p of 0.0014 min⁻¹ resulting in poly(nBA) with M_n = 225 100, M_w/M_n = 1.21. Hence addition of 50% DMSO to TFP resulted in an almost three fold enhancement of k^app_p without any broadening of the molecular weight distribution. However, addition of 70% DMSO resulted in slight phase separation and exhibited 88% monomer conversion after 9 h resulting in poly(nBA) with M_n = 381 200 and broader molecular weight distribution (M_w/M_n = 1.56). This broadening could be attributed to the insolubility of the resulting polymer at higher monomer conversion in the disproportionating solvent containing the CuBr₂ deactivator. Hence addition of DMSO as a co-solvent to TFP is necessary for high monomer conversion along with narrow molecular weight distribution. However, when the reaction was conducted at room temperature in TFP containing 30% DMSO a low monomer conversion (50%) was obtained after a long reaction time with M_n = 201 400 and M_w/M_n = 1.29 (entry 7, Table 1).

Table 1 SET-LRP of nBA during catalysis with Cu(0)/Me₆-TREN in a binary mixture of TFP or TFE and DMSO

Entry	[M]/[I]/[L]	Solvent	Temp. (°C)	k ^appp (min^−1)	Conv. (%)	M th	M n (GPC)	M w/Mn
1	3110/1/1	DMSO	50	—	98	390 970	460 100	2.15
2	3110/1/1	TFP	50	0.0014	58	231 530	225 100	1.21
3	3110/1/1	TFE	50	—	68	271 390	281 300	1.32
4	3110/1/1	30% DMSO in TFP	50	0.0038	81	323 210	315 200	1.18
5	3110/1/1	50% DMSO in TFP	50	0.0041	86	343 140	335 400	1.17
6	3110/1/1	70% DMSO in TFP	50	0.0061	88	351 110	381 200	1.56
7	3110/1/1	30% DMSO in TFP	25	—	50	199 640	201 400	1.29
8	6220/1/1	30% DMSO in TFP	50	—	33	263 410	249 300	1.28
9	6220/1/2	30% DMSO in TFP	50	0.0027	67	534 470	527 700	1.21
10	3110/1/1	30% DMSO in TFE	50	0.0104	82	327 190	340 900	1.35
11	6220/1/2	30% DMSO in TFE	50	—	52	414 890	453 700	1.52

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Fig. 1 Kinetic plots (left) and M_n, and M_w/M_nvs. theoretical M_th (right) for SET-LRP reactions of n-butyl acrylate (nBA) performed in a binary mixture of DMSO and trifluoroethanol (TFE) or tetrafluoropropanol (TFP). Conditions: (a and b) nBA = 1 mL, TFP containing 30% DMSO = 0.5 mL, and [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1, non-activated 1.0 cm Cu(0) wire, (c and d) nBA = 1 mL, TFP containing 50% DMSO = 0.5 mL, and [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1, non-activated 1.0 cm Cu(0) wire, (e and f) nBA = 1 mL, TFE containing 30% DMSO = 0.5 mL, and [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1, non-activated 1.0 cm Cu(0) wire.

Fig. 2 GPC traces for SET-LRP reactions of nBA performed in a binary mixture of DMSO and TFE or TFP with nonactivated 1.0 cm Cu(0) wire as a catalyst at 50 °C. Conditions: (a) nBA = 1 mL, 30% DMSO in TFP = 0.5 mL, and [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1, (b) nBA = 1 mL, 50% DMSO in TFP = 0.5 mL, and [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1, (c) nBA = 1 mL, 30% DMSO in TFE = 0.5 mL, and [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1, (d) nBA = 1 mL, 30% DMSO in TFP = 0.5 mL, and [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 6220/1/2. The change in the slope of the baseline in the GPC chromatograms provides artificial shoulders in the GPC traces.

When TFE was used as the solvent under similar reaction conditions 68% conversion was obtained after 12 h resulting in poly(nBA) with M_n = 281 300 and M_w/M_n = 1.32 (entry 3, Table 1). Addition of 30% DMSO to TFE gave rise to 82% conversion after 9 h resulting in poly(nBA) with M_n = 340 900 and M_w/M_n = 1.35 (entry 10, Table 1). In this solvent the polymerization takes place at a higher rate (k^app_p = 0.0104 min⁻¹) compared to that in TFP containing 30% DMSO (k^app_p = 0.0038 min⁻¹) and a linear dependence of ln([M]₀/[M]) on time was obtained up to 60% monomer conversion which then drastically deviates from linearity. Due to the faster rate of propagation than initiation, during the early stages of the polymerization^29,49 when using TFE with 30% DMSO as a co-solvent and BPE as the initiator, a very small extent of bimolecular molecular weight distribution can be observed (Fig. 1e, f and 2c). However, the change in the baseline of the GPC traces as indicated in Fig. 2 can provide an artificial bimolecular weight distribution observed as a shoulder on these chromatograms.

Given the success of SET-LRP for nBA at [nBA]₀/[BPE]₀ = 3110/1, SET-LRP of nBA targeting a higher degree of polymerization by using [nBA]₀/[BPE]₀ = 6220/1 was carried out in binary mixtures of TFP and TFE containing 30% DMSO. It was observed that when 30% DMSO in TFP was employed as a solvent at 50 °C, addition of 2 equivalents of the ligand gave rise to 67% monomer conversion resulting in poly(nBA) with M_n = 527 700, M_w/M_n = 1.21 (entry 9, Table 1 and Fig. 2d) which is almost two times the monomer conversion in the presence of 1 equivalent of the ligand (33%, entry 8, Table 1). However, when TFE was employed as a solvent for [nBA]₀/[BPE]₀ = 6220/1 a broader molecular weight distribution was obtained (entry 11, Table 1) with M_n = 453 700, M_w/M_n = 1.52, similar to the case of [nBA]₀/[BPE]₀ = 3110/1.

Synthesis of high molar mass poly(EHA)

The synthesis of high molar mass poly(EHA) was carried out by SET-LRP in the same binary solvent mixtures targeting [EHA]₀/[BPE]₀ = 5000 and 10 000. It is noteworthy that, although SET-LRP of EHA in both TFE and TFP continues in a single phase, unlike nBA the addition of 30% DMSO results in phase separation after ∼60% monomer conversion (Table 2 and Fig. 3). It was observed that for TFE containing 30% of DMSO at [EHA]₀/[BPE]₀ = 5000/1 addition of 2 equivalents of the ligand exhibited higher conversion compared to when 1 equivalent of the ligand was used (entry 1, 2, Table 2). Kinetic plots for [EHA]₀/[BPE]₀/[Me₆-TREN]₀ = 5000/1/2 and [EHA]₀/[BPE]₀/[Me₆-TREN]₀ = 10 000/1/2 in 30% DMSO in TFE are shown in Fig. 3. In both cases ln([M]₀/[M]) showed near-linear increment with time until ∼60% monomer conversion. At higher conversion a deviation from linearity was observed. However, the number average molecular weight (M_n) of poly(EHA) follows a linear dependence on monomer conversion with narrow molecular weight distribution (M_w/M_n ∼1.2). For [EHA]₀/[BPE]₀/[Me₆-TREN]₀ = 5000/1/2 in TFE containing 30% DMSO poly(EHA) was obtained with M_n = 539 500, M_w/M_n = 1.20, entry 4, Table 2 and for [EHA]₀/[BPE]₀/[Me₆-TREN]₀ = 10 000/1/2 poly(EHA) with M_n = 913 100 and M_w/M_n = 1.21 (entry 3, Table 2) was obtained. When TFP containing 30% DMSO was employed for [EHA]₀/[BPE]₀/[Me₆-TREN]₀ = 5000/1/2 a lower monomer conversion (68%) was obtained compared to the TFE–DMSO solvent mixture resulting in poly(EHA) with M_n = 435 100 and M_w/M_n = 1.35 (entry 4, Table 2).

Table 2 SET-LRP of EHA during catalysis with Cu(0)/Me₆-TREN in binary mixtures of TFP or TFE with DMSO

Entry	[M]/[I]/[L]	Solvent	Temp. (°C)	k ^appp (min^−1)	Conv. (%)	M th	M n (GPC)	M w/Mn
1	5000/1/1	30% DMSO in TFE	50	—	66	617 670	352 900	1.23
2	5000/1/2	30% DMSO in TFE	50	0.0024	76	700 600	539 500	1.20
3	10 000/1/2	30% DMSO in TFE	50	0.0021	67	1 235 010	913 100	1.21
4	5000/1/2	30% DMSO in TFP	50	—	68	626 880	435 100	1.35
5	10 000/1/2	30% DMSO in TFP	50	—	20	368 560	210 600	1.25

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Fig. 3 Kinetic plots (left) and M_n and M_w/M_nvs. theoretical M_th (right) for SET-LRP reactions of 2-ethylhexyl acrylate (EHA) performed in a binary mixture of DMSO and trifluoroethanol (TFE). Conditions: (a and b) EHA = 1 mL, TFE containing 30% DMSO = 0.5 mL, and [EHA]₀/[BPE]₀/[Me₆-TREN]₀ = 5000/1/2, non-activated 1.0 cm Cu(0) wire, (c and d) EHA = 1 mL, TFE containing 30% DMSO = 0.5 mL, and [EHA]₀/[BPE]₀/[Me₆-TREN]₀ = 10 000/1/2, non-activated 1.0 cm Cu(0) wire.

Conclusions

Excellent reaction media for SET-LRP of hydrophobic acrylates nBA and EHA were obtained when a combination of TFP or TFE with DMSO was used as a solvent. Very high monomer conversion and narrow molecular weight distribution with a relatively fast rate of reaction compared to pure TFP or TFE were obtained. Using these solvent mixtures for the first time, high molar mass poly(nBA) (M_n = 527 700, M_w/M_n = 1.21, 12 h) and poly(EHA) (M_n = 913 100, M_w/M_n = 1.20, 15 h) were produced. In both cases narrow molecular weight distribution of the polymers demonstrated that the polymerization reactions are living in nature and there is no significant impact of side reactions. Previously we reported that SET-LRP provided access for the first time to ultrahigh molar mass poly(MA) in DMSO (M_n = 1 400 000, M_w/M_n = 1.15, 10 h),³ in MeOH (M_n = 800 000, M_w/M_n = 1.15, <3 h),⁴¹ poly(2-hydroxyethyl methacrylate) in DMSO (PHEMA, M_n = 1 017 900, M_w/M_n = 1.49, 38 h),¹⁷ and high molar mass poly(tBA) (M_n = 505 120 and M_w/M_n = 1.30, 15 h) in TFE.⁴⁵ In addition, high molecular weight linear polymers were synthesized by Single-Electron-Transfer Degenerative-chain-Transfer mediated Living Radical Polymerization (SET-DTLRP) initiated with iodoform, and catalyzed by sodium dithionite producing poly(nBA) with M_n = 85 000, M_w/M_n = 1.9 in 5 h,⁵⁵ and poly(tBA) with M_n = 823 000, M_w/M_n = 1.15 in 71 h.⁵⁶

Experimental

Materials

2-Ethylhexyl acrylate (99%, Acros) and n-butyl acrylate (99%, Sigma Aldrich) were passed through a short column of basic Al₂O₃ prior to use in order to remove the radical inhibitor. Dimethyl sulfoxide (DMSO) (Fisher, Certified ACS, 99.9) was used as received, TFP (SynQuest Laboratories) and TFE (AlfaAesar) were used as received. Hexamethylated tris(2-aminoethyl)amine (Me₆-TREN) was synthesized as described in the literature.⁵⁷ The bifunctional initiator BPE was synthesized by esterification of ethylene glycol with 2-bromopropionyl bromide in the presence of pyridine.⁴²

Techniques

Gel Permeation Chromatography (GPC) analysis was performed on a Perkin-Elmer Series 10 high-performance liquid chromatograph, equipped with an LC-100 column oven at 30 °C, a Nelson Analytical 900 Series integration data station, a Perkin-Elmer 785 UV-vis detector (254 nm), a Varian star 4090 refractive index (RI) detector, and three AM gel columns (5 μm particle size and 500 Å, 1000 Å, and 10 000 Å pore size). HPLC grade THF (Fisher) was used as an eluent at a flow rate of 1 mL min⁻¹. The number-average molecular weight (M_n) and molecular weight distribution (M_w/M_n) were determined with poly(methyl methacrylate) (PMMA) standards purchased from American Polymer Standards.

Typical procedure for SET-LRP of nBA at [nBA]₀/[BPE]₀/[Me₆-TREN]₀ = 3110/1/1

A stock solution containing 1 mL nBA (1 mL, 6.89 mmol), initiator BPE (2.21 μmol, 0.7385 mg), ligand Me₆-TREN (2.21 μmol, 0.510 mg), 0.5 mL of solvent and a stirring bar was charged to a Schlenk tube. The reaction mixture was then deoxygenated by six freeze-pump thaw cycles using liquid nitrogen as a freezing mixture. After these cycles the Schlenk tube was opened under a positive argon pressure to add the Cu(0) wire. It was further degassed and refilled by argon three times and transferred to a thermostatted oil bath operating at the desired temperature (50 °C). The introduction of the Cu(0) wire defines t = 0. Approximately one or two drops of samples were periodically removed from the reaction mixture by purging the side arm with argon for more than 2 min using a deoxygenated syringe and needle. The collected samples were then analyzed by ¹H NMR in CDCl₃ and subsequently by GPC to obtain the molecular weight and M_w/M_n data respectively.

Acknowledgements

Financial support by the National Science Foundation (DMR-1120901, DMR-1066116) and the P. Roy Vagelos Chair at Penn are gratefully acknowledged.

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