Letter pubs.acs.org/macroletters
Boron-Catalyzed C3-Polymerization of ω‑2-Methyl Allylarsonium Ylide and Its C3/C1 Copolymers with Dimethylsulfoxonium Methylide De Wang, Zhen Zhang, and Nikos Hadjichristidis* King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, Thuwal 23955, Kingdom of Saudi Arabia S Supporting Information *
ABSTRACT: A novel arsonium ylide, ω-2-methylallylarsonium ylide, was synthesized and used as monomer for polyhomologation with triethyborane as initiator. It was found that the terminal methyl group leads to C3 polymerization. Furthermore, the copolyhomologation of arsonium ylide with dimethylsulfoxonium methylide is reported for the first time.
B
other polymerization methods leading to: (i) well-defined PMbased di- and triblock co- or terpolymers by reaction of an appropriate living polymer or block copolymer with BF3·Et2O to produce 3-arm-borane stars which served as macroinitiator for the polyhomologation of dimethylsulfoxonium methylide, followed by oxidation/hydrolysis of the 3-arm borane star block co/terpolymers,7 (ii) well-defined polymethylene brushes by ring opening metathesis polymerization (ROMP) of norbornyl PM-macromonomers,8 and (iii) well-defined PM-based miktoarm stars by using the in situ generation of boron-thexylsilaboracyclic, which served as initiator to afford the PM-based miktoarm star.9 In this communication, we report the synthesis of ω-2methylallylarsonium ylide, terminal monosubstituted arsonium ylide, as well as its polymerization with boranes as initiators. It was found that the methyl group leads to C3 polymerization. Furthermore, as part of our ongoing investigation on PM-based polymers, the copolymerization of arsonium ylide with dimethylsulfoxonium methylide is reported for the first time. New copolymers both containing “soft” (C3 polymerization) and “rigid” (C1 polyhomologation) segments were achieved by this strategy. A terminal methyl substituted arsonium ylide salt 1 (Z/E ratio is 1/3.3 based on 1H NMR analysis, Scheme S1, Figures S1−S3, Supporting Information (SI)) was prepared and used as precursor of the monomer. The activated intermediate ylide 2 was generated in situ in tetrehydronfuran (THF) at −78 °C using n-butyllithium as base under argon atomosphere. The mixture (red solution) was allowed to warm to room
oron compounds have been widely used as catalysts in the polymerization of diazomethane and other diazoalkanes for several years.1 Recently, Shea developed a living polymerization procedure leading to linear well-defined hydroxylterminated polymethylene, PM (equivalent to polyethylene, PE) by using boron compounds as initiators.2 The general mechanism involves the formation of a complex between the ylide (monomer) and the organoborane (initiator) followed by migration/insertion of −CH2− into the initiator. As a consequence, the methylene groups are randomly inserted one by one (C1 polymerization) into the three arms of the initiator leading to a 3-arm polymethylene star with boron at the junction point. By oxidizing/hydrolyzing the 3-arm boron star, an OH-terminated polymethylene (polyethylene) is obtained. Based on this seminal discovery of Shea, a lot of excellent work has been developed leading to PM-based polymeric homo- and block copolymers with narrow molecular weight distribution (living) with well-defined topology (linear, star, cyclic, etc.).3,4 Extension of polyhomologation to substituted sulfoxonium methylide monomers, even ethyl-substituted sulfoxonium ylide, faces a big challenge under the current experimental conditions.2 Mioskowski, Gall, and co-workers developed a new class of substituted allylylide monomers, 2-substituted allylic arsonium ylides, which are compatible with borane polymerization (C3 polymerization).5a,b They found that by using allylarsonium ylide (nonsubstituted) as monomer a poly(propenylene-co-propenylidene) copolymer was obtained, while by using a dimethyl substituted allylic arsonium ylide monomer a C1 homopolymer5c,6 was obtained. Consequently, arsonium ylides could be successfully applied in polyhomologation and thus open new horizons in polymer synthesis. Our group has continuously worked on PM-based macromolecular architectures by combining polyhomologation with © 2016 American Chemical Society
Received: January 29, 2016 Accepted: February 25, 2016 Published: March 1, 2016 387
DOI: 10.1021/acsmacrolett.6b00082 ACS Macro Lett. 2016, 5, 387−390
Letter
ACS Macro Letters
Figure 1. (A) General reactions for the polymerization of terminal methyl substituted arsonium ylide and the GPC chromatogram of 3a. [1]/[BEt3] = 160, [nBuLi] = 1.6 M, THF as solvent. (B) 1H NMR spectra (600 MHz, CDCl3) of C3 polymer 3a (DPNMR = 90, determined by 1H NMR, see Figure S7, SI). The asterisks from left to right are deuterated chloroform peak, petroleum peak and silicon grease peak, respectively; the terminal ethyl group a and b are overlapped by n-hexane and water peaks. Mn,NMR = (DPNMR + 1) × MWallyl‑Me + MWEt + MWOH.
temperature (20 °C) slowly and kept at room temperature for half an hour, then triethylborane was added (a rapid discoloration observed when added borane) followed by heating the mixture immediately at 50 °C (Figure S4, SI). After oxidation/hydrolysis with H2O2/NaOH, a poly(3-methyl1-propenylene) homopolymer 3a (Mn,NMR = 4.9 × 103 g/mol, DPNMR = 90, PDIGPC = 1.21) was obtained, in which the chain has been elongated by three carbon atoms at a time (C3 polymerization, Figure 1A). To identify that the monomeric unit of polymer 3 was C3, different analytical methods (1H NMR, 1H−1H COSEY and FT-IR) were used. The peak at 971 cm−1 (FT-IR data, Figure S5, SI) indicates that the double bonds possess the E-configuration (trans). A representative 1H NMR spectrum of polymer 3 is presented in Figure 1B (enlarged spectra are given in Figures S6 and S7, as well as detailed information, SI). By careful analysis of COSEY and 1H NMR methyl group data the C3 structure of polymer 3 was further confirmed (Figures S8 and S9, SI). The polymerization of ylide 1 (100 or 200 equiv) in the presence of 1 equiv triethylborane was also performed (Table S1, entries 1 and 2, SI). Two samples with different molecular weights and polydispersities have been obtained. Using borane tetrahydrofuran complex solution as initiator, the polymerization leads to a slightly higher molecular weight with higher PDI (3d: Mn,NMR = 5.7 × 103 g/mol, PDIGPC = 1.94, Table S1, entry 3, SI); it may be because BH3 has a different reactivity. The GPC results show that an increase in the ratio of monomer to initiator shifts the peak to higher molecular weight (3b: Mn,NMR = 2.1 × 103 g/mol, PDIGPC = 1.28; 3c: Mn,NMR = 5.3 × 103 g/mol, PDIGPC = 1.24, Table S1, entries 1 and 2, Figure S10, SI). The 1H NMR spectra of polymers 3b−3d are given in Figures S11−S13 (SI). Having shown that the terminal methyl substituted allylic arsonium ylide directs the polymerization of C3 mode in the presence of a borane, we further investigated the possibility to gain access to copolymers derived from two different types of ylides-arsonium and sulfoxonium ylide. For the synthesis of block copolymer 7 (the mole ratio of [MAs]/[MS]/[I] = 200/ 600/1, MAs is arsonium ylide 2, MS is sulfoxonium ylide and I is triethylborane, Mn,NMR = 8.9 × 103 g/mol, PDIGPC = 1.22, Scheme 1), triethylborane was added to the solution of 2 to form “macro-boron initiator” at room temperature and then heated at 50 °C immediately, the red solution discolorated rapidly within 5 min and the reaction mixture became neutral
Scheme 1. Synthesis Route to Block Copolymer 7 and Its HT-GPC Trace
(pH = 7.0), indicating that the ylide monomer was completely consumed. Then the solution of dimethylsulfoxonium methylide was added immediately after discoloration and heated to 70 °C until the reaction mixture became neutral. After oxidation/ hydrolysis and purification, the 1H NMR spectrum displayed the expected signals corresponding to the methyl substituted allylic and polymethylene groups arising from the two ylide monomers (Figure S14, SI). Three more samples with different molecular weight were synthesized and their characteristics are summarized in Table 1. Molecular weights were determined by NMR end group analysis (Mn,NMR = 2.64−6.22 × 103 g/mol, Table 1, Figure S15, SI) and polydispersity index (PDI) by HTGPC with polystyrene as standards (Mw/Mn = 1.14−1.27). Increasing the ratio of arsonium ylide and sulfoxonium ylide from 15/50 to 45/150 (DPcal(As/S)), the molecular weight of polymers increased correspondingly. It was noticed that the DPNMR(S) of copolymer is a little higher than calculated, it was probably attributed to part of “macro-boron initiator” had been oxidized or decomposed before adding dimethylsulfoxonium methylide as the second monomer in polymerization reaction. The HT-GPC curves in Figure 2 shows the variation of different molecular weights. A clear shift from low molecular weight to high molecular weight was observed. The in situ prepared ylide 2 was mixed with dimethylsulfoxonium methylide, and then triethylborane was added to the mixture where both ylides consumed in 3 h. After oxidation/ hydrolysis, the random copolymer 8 (the mole ratio of [MAs]/ [MS]/[I] = 160/450/1, Mn,NMR = 5.2 × 103 g/mol, PDIGPC = 388
DOI: 10.1021/acsmacrolett.6b00082 ACS Macro Lett. 2016, 5, 387−390
Letter
ACS Macro Letters Table 1. Preparation of Polymer 7 by Using Ylide 2 and Sulfoxonium Ylide Catalyzed by Triethylborane entrya
polymer
[MAs]/[MS]/[I]
DPcal(As/S)b
DPNMR(As/S)c
Mn,NMRc (×10−3)
Mn,GPCd (×10−3)
PDId
yielde (%)
1 2 3
PAM17-b-PE120 PAM33-b-PE195 PAM51-b-PE244
45/150/1 90/300/1 135/450/1
15/50 30/100 45/150
17/120 33/195 51/244
2.64 4.55 6.22
2.08 4.24 10.1
1.27 1.14 1.20
79 73 85
a
All the reaction carried out as above-described. bDPcal was calculated by [M]/3[I]. cMn,NMR and DPNMR (degree of polymerization) were calculated from 1H NMR spectrum (toluene-d8) using the area ratio of protons in terminal CH2OH at δ = 3.4 ppm to the ones on the backbone. dPDI = Mw/ Mn, determined by HT-GPC (1,2,4-trichlorobenzene, 150 °C, PS standards). eIsolated yield.
analysis of block and random copolymers are shown in Figure S18 (SI). The mechanism of ylide 2 initiated by borane could be explained according to Scheme 3. An ate complex 9, initially Scheme 3. Proposed Mechanism for the Polymerization of Ylide 2
Figure 2. HT-GPC chromatography of PAM17-b-PE120, PAM33-bPE195, and PAM51-b-PE244.
1.13, Scheme 2) was obtained, and confirmed by GPC and NMR (Figure S16, SI) results. Using dimethylsulfoxonium methylide as monomer initiated by triethylborane to form PEbased macroinitiator at 80 °C first, then lower the temperature to 50 °C and react with the second monomer 2 to generate copolymer 9 (the mole ratio of [MAs]/[MS]/[I] = 45/120/1, Mn,GPC = 0.98 × 103 g/mol, PDIGPC = 1.19, Scheme 2, Figure S17, SI) with very low ratio of allylic segment, and copolymer 9 is mixed with PE−OH from unreacted macroinitiator. This is because the activated ylide 2 will be decomposed at high temperature whereas PE-based macroinitiator have poor solubility at a relative low temperature (Scheme 2). The DSC
produced from ylide 2 and triethylborane, is rearranged by losing triphenylarsine to an allylic borane intermediate 10. The terminal methyl substituted intermediate 10 undergoes a [1,3]σ rearrangement (or boratropic rearrangement),5,10 leading to isomeric allylic borane 12, which is involved in several cycles of
Scheme 2. Synthesis of Copolymers 8 and 9 and Their HT-GPC Trace
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DOI: 10.1021/acsmacrolett.6b00082 ACS Macro Lett. 2016, 5, 387−390
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ACS Macro Letters
Cao, S.; Ma, Z. Polym. Chem. 2013, 4, 307−312. (f) Chen, J.; Cui, K.; Zhang, S.; Xie, P.; Zhao, Q.; Huang, J.; Shi, L.; Li, G.; Ma, Z. Macromol. Rapid Commun. 2009, 30, 532−538. (g) Li, J.; Zhao, Q.-L.; Chen, J.-Z.; Li, L.; Huang, J.; Ma, Z.; Zhong, Y.-W. Polym. Chem. 2010, 1, 164−167. (5) (a) Goddard, J.-P.; Lixon, P.; Gall, T. L.; Mioskowski, C. J. Am. Chem. Soc. 2003, 125, 9242−9243. (b) Mondiere, R.; Goddard, J.-P.; Carrot, G.; Gall, T. L.; Mioskowski, C. Macromolecules 2005, 38, 663− 668. (c) Mondiere, R.; Goddard, J.-P.; Huiban, M.; Carrot, G.; Gall, T. L.; Mioskowski, C. Chem. Commun. 2006, 42, 723−725. (6) For other C1 type polymerizations, see: (a) Jellema, E.; Jongerius, A. L.; Reek, J. N. H.; de Bruin, B. Chem. Soc. Rev. 2010, 39, 1706− 1723. (b) Franssen, N. M. G.; Remerie, K.; Macko, T.; Reek, J. N. H.; de Bruin, B. Macromolecules 2012, 45, 3711−3721. (7) (a) Zhang, H.; Alkayal, N.; Gnanou, Y.; Hadjichristidis, N. Chem. Commun. 2013, 49, 8952−8954. (b) Zhang, H.; Alkayal, N.; Gnanou, Y.; Hadjichristidis, N. Macromol. Rapid Commun. 2014, 35, 378−390. (8) (a) Zhang, H.; Gnanou, Y.; Hadjichristidis, N. Polym. Chem. 2014, 5, 6431−6434. (b) Zhang, H.; Gnanou, Y.; Hadjichristidis, N. Macromolecules 2015, 48, 3556−3562. (9) Zhang, Z.; Zhang, H.; Gnanou, Y.; Hadjichristidis, N. Chem. Commun. 2015, 51, 9936−9938. (10) (a) Aggarwal, V. K.; Fang, G. Y.; Schmidt, A. T. J. Am. Chem. Soc. 2005, 127, 1642−1643. (b) Fang, G. Y.; Aggarwal, V. K. Angew. Chem., Int. Ed. 2007, 46, 359−362.
reaction with ylide 2, eventually affording a C3 polymer 3 after oxidation/hydrolysis. The double bond of polymer 3 is Econfiguration (trans) can be explained by the dynamic process of intermediate 10 (E and Z-configuration), which Econfiguration is less sterically hindered during [1,3]-σ rearrangement. In summary, a novel terminal substituted arsonium ylide, ω2-methylallylarsonium ylide, was synthesized and used as monomer for polyhomologation with triethyborane as initiator. It was found that the methyl group leads to C3 polymerization. Other terminal substituted allylic arsonium ylides are under investigation. Furthermore, block and random copolyhomologation of this arsonium ylide with dimethylsulfoxonium methylide was successfully performed for the first time. This general strategy opens new synthetic horizons since different polyhomologation modes can be achieved by controlling the structure of the ylide monomer, leading to unprecedented complex macromolecular architectures. Our group is working toward this pathway.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00082. Additional kinetic data, SEC traces, and proposed mechanism (PDF).
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AUTHOR INFORMATION
Corresponding Author
*Tel.: + 966-(0)12-8080789. E-mail: nikolaos.hadjichristidis@ kaust.edu.sa. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Research reported in this publication was supported by the King Abdullah University of Science and Technology. REFERENCES
(1) (a) Tufariello, J. J.; Lee, L. T. C. J. Am. Chem. Soc. 1966, 88, 4757−4759. (b) Tufariello, J. J.; Wojtkowski, P.; Lee, L. T. C. Chem. Commun. 1967, 505−506. (2) (a) Luo, J.; Shea, K. J. Acc. Chem. Res. 2010, 43, 1420−1433. (b) Zhao, R.; Shea, K. J. ACS Macro Lett. 2015, 4, 584−587. (3) (a) Shea, K. J.; Walker, J. W.; Zhu, H.; Paz, M. M.; Greaves, J. J. Am. Chem. Soc. 1997, 119, 9049−9050. (b) Shea, K. J.; Lee, S. Y.; Busch, B. B. J. Org. Chem. 1998, 63, 5746−5747. (c) Shea, K. J.; Busch, B. B.; Paz, M. M. Angew. Chem., Int. Ed. 1998, 37, 1391−1393. (d) Shea, K. J. Chem. - Eur. J. 2000, 6, 1113−1119. (e) Wagner, C. E.; Shea, K. J. Org. Lett. 2001, 3, 3063−3066. (f) Wagner, C. E.; Kim, J.-S.; Shea, K. J. J. Am. Chem. Soc. 2003, 125, 12179−12195. (g) Zhou, X.Z.; Shea, K. J. Macromolecules 2001, 34, 3111−3114. (h) Shea, K. J.; Staiger, C. L.; Lee, S. Y. Macromolecules 1999, 32, 3157−3158. (i) Sulc, R.; Zhou, X.-Z.; Shea, K. J. Macromolecules 2006, 39, 4948−4952. (4) (a) Yuan, C.; Lu, H.; Li, Q.; Yang, S.; Zhao, Q.; Huang, J.; Wei, L.; Ma, Z. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 2398−2405. (b) Li, Q.-Z.; Zhang, G.-Y.; Chen, J.-Z.; Zhao, Q.-L.; Lu, H.-C.; Huang, J.; Wei, L.-H.; D’Agosto, F.; Boission, C.; Ma, Z. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 511−517. (c) Wang, X.; Gao, J.; Zhao, Q.; Huang, J.; Mao, G.; Wu, W.; Ning, Y.; Ma, Z. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2892−2899. (d) Lu, H.; Xue, Y.; Zhao, Q.; Huang, J.; Xu, S.; Cao, S.; Ma, Z. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 3641−3647. (e) Xue, Y.; Lu, H.; Zhao, Q.; Huang, J.; Xu, S.; 390
DOI: 10.1021/acsmacrolett.6b00082 ACS Macro Lett. 2016, 5, 387−390
