Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Fluorinated Poly(substituted methylene)s Prepared by Pd-Initiated Polymerization of Fluorine-Containing Alkyl and Phenyl Diazoacetates: Their Unique Solubility and Postpolymerization Modification Hiroaki Shimomoto, Tomohiko Kudo, Shogo Tsunematsu, Tomomichi Itoh, and Eiji Ihara* Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan S Supporting Information *
ABSTRACT: Pd-initiated polymerization of fluorine (F)containing alkyl and phenyl diazoacetates is described. Polymerization of 2,2,2-trifluoroethyl diazoacetate [Et(CF3)DA] with π-allylPdCl afforded a C−C main chain polymer bearing a 2,2,2-trifluoroethoxycarbonyl group on each main chain carbon atom. The polymer showed upper critical solution temperature (UCST)-type phase separation in multiple common organic solvents with differing polarities. Although homopolymerization of 3,3,4,4,5,5,6,6,6-nonafluorohexyl diazoacetate [Hex(C4F9)DA] with a higher fluorine content yielded an insoluble product, copolymerization of Hex(C4F9)DA with non-fluorinated ethyl diazoacetate (EDA) proceeded homogeneously to give a soluble F-containing copolymer. Polymerization of a series of F-containing phenyl diazoacetates was also conducted with the same initiator, giving poly[(F-containing aryloxycarbonyl)methylene]s, which showed significant solubility differences depending on the substitution pattern of F atoms on the phenyl ring. Efficient postpolymerization modification of poly[(F-containing aryloxycarbonyl)methylene]s was achieved with a primary amine, affording a polymer with both a five-membered cyclic imide structure and an N-alkylcarbamoyl group in its side chains.
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include oligo(ethylene glycol)s,5 phosphonic acid,6 long alkyl chains,7 mesogens,8 polycyclic aromatic hydrocarbons,9 and substituted cyclophosphazenes.10 As a new functional group to be explored as an ester substituent of diazoacetates, fluorine (F)-containing alkyl and phenyl groups could be highly intriguing because we can possibly realize accumulation of these groups along the polymer main chain following the synthetic strategies outlined in Scheme 1, which we will indeed employ in this study. In Scheme 1A, perfluoromethyl (CF3) and perfluorobutyl (nC4F9) groups are introduced into ester groups of diazoacetates with methylene and ethylene spacers, respectively, enabling us to achieve accumulation of these perfluoroalkyl (Rf) groups along the resulting polymer main chains. The products could be highly promising as unique polymeric materials, considering that a variety of polymers derived from Rf-containing monomers have been developed and utilized as functional polymeric materials.11,12 While, in these reported examples, the Rf-containing materials exhibit unique properties such as a very
INTRODUCTION Transition-metal-catalyzed polymerization of alkyl or aryl diazoacetates is a useful method to prepare sp3−sp3 C−C main chain polymers [poly(substituted methylene)s].1−3 Unlike polymerization of vinyl compounds, which utilize the addition to CC double bond (two carbon units) for the polymer synthesis to give poly(substituted ethylene)s, polymerization of diazoacetates constructs the C−C main chain from one carbon unit with N2 elimination in each chaingrowth step, yielding polymers bearing an ester substituent on all main chain carbon atoms. The resulting polymers have been shown to possess unique properties compared to the corresponding vinyl polymer counterparts, i.e., poly(meth)acrylates, due to densely accumulated functional groups around the polymer backbone. For example, we have recently synthesized hydroxy-containing poly(substituted methylene)s by Pd-initiated polymerization of silyl-protected alkyl diazoacetates and subsequent deprotection or, interestingly, direct polymerization of hydroxy-containing diazoacetates.4 The resulting polymers bearing a hydroxy-containing ester substituent on each carbon of the backbone showed higher hydrophilicity than those of the corresponding vinyl polymers. Other functional groups introduced into the polymers so far © XXXX American Chemical Society
Received: September 11, 2017 Revised: November 24, 2017
A
DOI: 10.1021/acs.macromol.7b01964 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
dimethylcarbamoylmethylene) by heating poly(di-tert-butyl fumarate) prepared by radical polymerization of di-tert-butyl fumarate followed by treatment with hexamethylphosphoramide (HMPA), although the substituent is inevitably limited to N,N-dimethylcarbamoyl unit in this strategy. More recently, Theato and co-workers19 have reported postpolymerization modification of poly(benzyloxycarbonylmethylene), which was prepared by Rh-mediated polymerization20−25 of benzyl diazoacetate, with amines (Scheme 2B), where the formation of five-membered cyclic imide was observed when reacted with primary amines. Although a relatively high conversion of a benzyloxy group was realized under an appropriate condition, quantitative transformation has not been achieved in this study. In contrast, we can expect more efficient transformation could be possible because of the advantage of using a highly reactive F-containing phenoxy group instead of a benzyloxy group, with the use of PhFcontaining poly(substituted methylene) as a precursor (Scheme 2C). Although the above-described fluorinated poly(substituted methylene)s might be alternatively prepared by radical polymerization of F-containing alkyl or phenyl fumarate, successful homopolymerization of such monomers has not been reported so far, to the best of our knowledge. Accordingly, in this paper, we will report Pd-initiated polymerization of a variety of F-containing diazoacetates for the first time and investigation of the resulting polymers with respect to solubility toward various solvents and postpolymerization modification attempts with a primary amine.
Scheme 1. Polymerization of F-Containing Alkyl and Phenyl Diazoacetates
low critical surface tension, good transparency, low refractive index, and low moisture absorption, which derive from the arraignment of the Rf groups along the polymer main chains, we can expect positively enhanced effects because of the denser packing of Rf groups in the poly(substituted methylene)s than the reported Rf-containing polymers.11,12 Meanwhile, F-containing phenyl (PhF) groups are incorporated into diazocetates in Scheme 1B to afford polymers in which PhF groups are accumulated along the main chain. In this class of PhF-containing polymers, we can reasonably expect that postpolymerization modification could be feasible to obtain other poly(substituted methylene)s because PhFcontaining ester groups are highly susceptible to nucleophilic substitution with a variety of nucleophiles such as amine and alcohol.13−16 In particular, the amine substitution is quite significant here because the resulting product, poly(Nalkylcarbamoylmethylene), cannot be obtained efficiently by direct polymerization of N-alkyldiazoacetamide so far,17 while the products could be expected to exhibit unique properties with respect to comparison with their vinyl polymer counterpart, poly(N-alkylacrylamide). Indeed, postpolymerization modification to poly(alkoxycarbonylmethylene)s has been examined to prepare poly(N-alkylcarbamoylmethylene) as illustrated in Schemes 2A and 2B. In Scheme 2A, Chujo and co-workers 18 have successfully synthesized poly(N,N-
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RESULTS AND DISCUSSION Synthesis and Polymerization of F-Containing Alkyl Diazoacetates. As F-containing alkyl diazoacetates, 2,2,2trifluoroethyl diazoacetate [Et(CF3)DA] and 3,3,4,4,5,5,6,6,6nonafluorohexyl diazoacetate [Hex(C4F9)DA] were employed in this study. The former monomer was synthesized according to the literature (Scheme 3A)26 and used as a CH2Cl2 solution for polymerization. The latter one was synthesized by a general procedure for the synthesis of alkyl diazoacetate reported by Fukuyama and co-workers,27 where bromoacetates prepared from the corresponding alcohols are treated with N,N′-
Scheme 2. Postpolymerization Modification Utilizing Poly(substituted methylene)s as Reactive Precursor Polymers
Scheme 3. Synthesis of Et(CF3)DA (A) and Hex(C4F9)DA (B)
B
DOI: 10.1021/acs.macromol.7b01964 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 1. Homopolymerization and Copolymerization of F-Containing Alkyl Diazoacetatesa run
monomer (M1)
comonomer (M2)
[M1]/[M2]/[Pd]b
boratec
yield (%)d
Mne
Mw/Mne
1 2 3 4 5
Et(CF3)DA Et(CF3)DA Et(CF3)DA Hex(C4F9)DA Hex(C4F9)DA
none none none none EDA
100/0/1 200/0/1 100/0/1 100/0/1 25/75/1
none none NaBPh4 none none
54 65 54 n.d.f 51
12800 18800 12500 n.d.f 8600
1.23 1.29 1.10 n.d.f 1.64
In THF/CH2Cl2 [3.6 mL (v/v = 1/0.8) for runs 1−3 and 7.3 mL (v/v = 1/0.2) for run 5] or in THF (3.0 mL) for run 4, at −20 °C for 13 h; (πallylPdCl)2 = 2 mg; Et(CF3)DA was used as a 0.64−0.66 M solution in CH2Cl2; EDA was used as a 2.37 M solution in CH2Cl2. b[Pd] = 2[(πallylPdCl)2]. c[NaBPh4]/[Pd] = 1.8. dDetermined by gravimetry after purification by reprecipitation into toluene (runs 1−3) or with preparative SEC (run 5). eDetermined by SEC (PMMA standards). fNot determined. a
ditosylhydrazine in the presence of 1,8-diazabicyclo[5.4.0]-7undecene (DBU) (Scheme 3B). Polymerization of these monomers was investigated using πallylPdCl-based initiating systems (π-allylPdCl/NaBPh4 or πallylPdCl alone),28 which have been reported to be effective for polymerization of alkyl and aryl diazoacetates. Polymerization of Et(CF3)DA was carried out using π-allylPdCl as a catalyst with a feed ratio of [monomer]/[Pd] = 100 in tetrahydrofuran (THF) at −20 °C, giving a polymer (P[Et(CF3)DA′]) with Mn = 12 800 in 54% yield after purification by reprecipitation into toluene (run 1 in Table 1).29 With the increase of the [Et(CF3)DA]/[Pd] feed ratio to 200, we can observe increase of Mn to 18 800 (run 2). The addition of borate as a cocatalyst, which was effective for improving polymer yield in the case of some diazoacetates including oligo(ethylene glycol)- and cyclophosphazene-containing diazoacetates,5,10 did not improve the polymerization behavior in terms of Mn and yield (run 3). Figure 1A shows the 1H NMR spectrum of P[Et(CF3)DA′] recorded in acetone-d6. The signals for main chain methine
When Hex(C4F9)DA with a longer F-containing alkyl group was used as a monomer, the reaction mixture turned heterogeneous shortly after the polymerization was initiated, and the product was insoluble in any common organic solvents (run 4). The low solubility of the product (P[Hex(C4F9)DA′]) assumed to be the lipophobicity of relatively long Rf pendants (n-C4F9). Thus, the polymerizability of Hex(C4F9)DA was confirmed by copolymerization with a non-fluorinated comonomer to yield a product with improved solubility; a 1:3 random copolymerization of Hex(C4F9)DA with ethyl diazoacetate (EDA) proceeded homogeneously in THF to give a copolymer with high solubility toward common organic solvents (run 5). The copolymer structure was confirmed by 1 H NMR analysis (Figure 1B). Based on the integral ratio of signals for methylene protons (c) derived from Hex(C4F9)DA and methyl protons ( f) derived from EDA appearing independently, the comonomer composition of the copolymer was calculated to be [Hex(C4F9)DA′]/[EDA′] = 1:2.7, which nearly corresponds to the feed ratio. These polymerization behavior clearly demonstrated that Fcontaining alkyl diazoacetates have similar reactivities toward Pd-initiated polymerization as other alkyl diazoacetates to afford homo- and copolymers with Mn = ca. 10 000, and thus, a variety of Rf groups can be incorporated into poly(substituted methylene) framework, possibly imparting unique properties to the resulting polymers. In addition to the unique solubility of P[Et(CF3)DA′] described in the later section, poly(substituted methylene)s with Rf groups are expected to show some characteristic properties leading to creation of novel polymeric materials. Synthesis and Polymerization of F-Containing Phenyl Diazoacetates. At first, we tried to synthesize pentafluorophenyl diazoacetate (N2CHCO2C6F5) as a F-containing phenyl diazoacetate because pentafluorophenyl ester is wellknown as a representative activated ester for amidation,13−16 used for transformation in various research areas including peptide synthesis and, thus, regarded as the most suitable candidate for our purpose of postpolymerization modification here. However, pentafluorophenyl diazoacetate could not be obtained in our attempts using some reported synthetic methods for diazoacetates, probably because the extremely strong electron-withdrawing pentafluorophenyl group renders the target compound highly unstable. Accordingly, we switched the targets to diazoacetates with less electronwithdrawing PhF groups listed in Chart 1, which were successfully synthesized in a similar procedure27 as that used for Hex(C4F9)DA. Polymerization of these F-containing phenyl diazoacetates was investigated using the π-allylPdCl-based initiating systems (Table 2). The polymerization of 3-fluorophenyl diazoacetate
Figure 1. 1H NMR spectra of (A) P[Et(CF3)DA′] (run 1 in Table 1) recorded in acetone-d6 and (B) the copolymer prepared by copolymerization of Hex(C4F9)DA and EDA (run 5 in Table 1) recorded in CDCl3.
protons (a) and side chain methylene protons adjacent to CF3 unit (b) appear at 3.2−4.2 and 4.5−5.0 ppm, respectively. Judging from the split and broad appearance of the main chain methine signals (a), the products obtained here are likely atactic polymers,30 as is the case with other polymers prepared by polymerization of non-fluorinated diazoacetates with the πallylPdCl-based initiating systems.4−6,9,10 C
DOI: 10.1021/acs.macromol.7b01964 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
exemplified in the polymerization of Ph(4-CF3)DA (runs 7 vs 9). The polymer structures were confirmed by NMR measurements, and their elemental analyses gave satisfactory values for repeating unit structures derived from each monomer after N2 elimination (Supporting Information). On the other hand, a precipitate was formed in the course of polymerization of Ph(2-F)DA and Ph(2,6-F2)DA (runs 1 and 4). Although Mn and Mw/Mn values of the products from these monomers could not be determined due to their low solubility in solvents for our size-exclusion chromatography (SEC) measurements (THF or CHCl3), NMR analyses recorded in dimethyl sulfoxide (DMSO)-d6 suggest that they were transformed into polymers with expected structures (Supporting Information). In addition, the data of elemental analysis for the product from Ph(2-F)DA also support the formation of the expected polymer structure [Calcd for (C8H5O2F)n: C, 63.16; H, 3.31. Found: C, 62.58, H, 3.43]. The origin of the low solubility of the products with ortho-F-substituted phenyl group will be discussed later. In our previous publication describing polymerization of non-fluorinated aryl diazoacetates using the same initiating systems,9 the polymerization of 4-methylphenyl diazoacetate using π-allylPdCl/NaBPh4, for example, gave a polymer with Mn = 7200 in 73% yield under a similar condition. The comparable polymerizability of F-containing phenyl diazoacetates described above demonstrates that these monomers can be generally utilized to incorporate a variety of PhF groups into poly(substituted methylene) framework. Solubility of F-Containing Poly(substituted methylene)s. We examined the solubility characteristics of polymers prepared from a series of F-containing alkyl and phenyl diazoacetates; Table 3 summarizes the solubility
Chart 1. Structures of F-Containing Phenyl Diazoacetates Used in This Study
Table 2. Polymerization of F-Containing Phenyl Diazoacetatesa run 1 2 3 4 5 6 7 8 9
monomer Ph(2-F)DA Ph(3-F)DA Ph(4-F)DA Ph(2,6-F2) DA Ph(3,5-F2) DA Ph(3,5-F2) DA Ph(4-CF3) DA Ph(4-CF3) DA Ph(4-CF3) DA
solvent
borateb
yield (%)c
Mnd
e
e
Mw/Mnd
THF THF THF THF
none none none none
n.d. 85 87 n.d.e
n.d. 7000 4800 n.d.e
n.d.e 1.47 1.50 n.d.e
THF
none
