Article pubs.acs.org/Macromolecules
Soluble, Allyl-Functionalized Deoxybenzoin Polymers Umesh Choudhary, Aabid A. Mir, and Todd Emrick* Polymer Science and Engineering Department, Conte National Center for Polymer Research, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: We describe the synthesis of allyl-substituted deoxybenzoin-based aromatic polyesters that combine the advantageous thermal properties of deoxybenzoin with the processability and reactivity of pendent unsaturated groups. A thermally induced Claisen rearrangement of bis-allyl ether-substituted deoxybenzoin 2 produced 3,3′-bis-allyl-4,4′-bishydroxydeoxybenzoin (BA-BHDB, compound 1) as a novel A2 monomer. BA-BHDB 1 polymerized readily with isophthaloyl chloride to produce a novel set of functional aromatic polyesters. These allyl-substituted polymers exhibited marked solubility advantages over deoxybenzoin-based polymers lacking such pendent groups, affording homogeneous solutions in numerous organic solvents. The processability contributed by the allyl groups afforded access to relatively high molecular weight aromatic polyesters, with estimated number-average molecular weight (Mn) values exceeding 20 kDa. Interestingly, evaluation of the thermal properties of these polymers revealed that the pendent allyl groups led to small increases in heat release relative to the unsubstituted deoxybenzoin polymers. Thus, this work represents an advantageous design of nonflammable polymer materials, offering benefits with respect to both processability and heat release properties. Moreover, the utility of these functional deoxybenzoins in post-polymerization cross-linking was demonstrated using multifunctional thiols and thiol−ene reactions.
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INTRODUCTION The inherent flammability characteristics of synthetic polymers represent a significant societal threat, especially when present in enclosed or isolated spaces; as such, flame-retardant additives are required to mitigate the danger associated with polymer flammability. Halogenated molecules, such as brominated aromatics, function as flame retardants by inhibiting gas phase combustion.1 Unfortunately, such halogen-rich structures have been identified as environmentally accumulative and harmful to human health,2−4 and thus some compounds are under scrutiny or prohibited from use in Europe, Japan, and the United States.3,5 These additives would be eliminated from finished polymer products if suitable replacements were available. Nonhalogenated flame retardants, such as inorganic metal hydrates6−8 and phosphorus-containing organic molecules,9−11 inhibit flammability through solid phase mechanisms such as glass formation.12 These flame-retardant materials are favorable for their halogen-free compositions, but the highweight-percent loadings typically required may adversely impact the physical/mechanical properties of the filled polymers.13 Therefore, new materials discoveries must emerge in order to realize hydrocarbon-based polymers that combine the desirable properties of excellent processability and low flammability. We previously reported the synthesis of deoxybenzoin-based polyarylates and studied their decomposition by thermogravimetric analysis (TGA) and pyrolysis combustion flow calorimetry (PCFC) to reveal high char yields (>40%) and remarkably low heat release capacity values (20 kDa. However, as would be expected, TGA characterization revealed these alkylated polyarylates to possess lower char yield than the unsubstituted versions (27% vs 43%) and higher HRC values (285 J/(g K) vs 62 J/(g K)) due to the fuel provided by the pendent hydrocarbon group.14 Generally speaking, discovering facile methods to solubilize polymers without significantly increasing their hydrocarbon Received: March 7, 2017 Revised: April 25, 2017
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DOI: 10.1021/acs.macromol.7b00460 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules content, and thus flammability, is challenging and requires a balance of chemical/structural properties in order to simultaneously achieve low flammability along with any of a wide range of physical/mechanical properties desired for a particular application. We address this challenge by preparing the novel bis-allylsubstituted bis-hydroxydeoxybenzoin (BA-BHDB) monomer shown as compound 1 in Figure 1 and using it to afford low-
Figure 1. Structure of the novel monomer BA-BHDB 1.
flammability aromatic polyesters. Specifically, BA-BHDB was utilized in esterification polymerizations with isophthaloyl or terephthaloyl chloride under interfacial polycondensation conditions. Notably, the thermal properties of the resulting polyarylates indicated that despite the presence of the allyl substituents, high char yields were maintained and polymer solubility improved significantly over the unsubstituted versions. The pendent allyl groups proved additionally useful in postpolymerization reactions, such as thiol−ene cross-linking that converted the polymers to robust, insoluble cross-linked films or gels.
Figure 3. 1H NMR spectra of deoxybenzoins 1 and 2 as solutions in DMSO-d6.
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RESULTS AND DISCUSSION As shown in Figure 2, BHDB (synthesized by demethylation of desoxyanisoin14) was converted to the corresponding bis-allyl ether 2 in high yield by refluxing with allyl bromide in anhydrous acetone and potassium carbonate. Compound 2, when heated neat to 185 °C for 12−15 h under inert (N2(g)) atmosphere, underwent a Claisen rearrangement to afford BABHDB 1. The rearrangement proceeded in high yield and 1 was obtained as a white solid in ∼80% yield following purification by column chromatography on silica gel. High-resolution mass spectroscopy−electrospray ionization (HRMS-ESI) analysis of 1 and 2 revealed each to possess the expected molecular weight of 309.1490 g/mol, though with markedly different fragmentation patterns as shown in Figures S14 and S15. Nuclear magnetic resonance (NMR) spectroscopy of 1 and 2, shown in Figure 3, distinguished their chemical structures. The 1H NMR spectrum of 2 showed characteristic allyl ether resonances centered at 5.96 (Ha), 5.36 (Hb), and 5.25 ppm (Hc) as well as signals from 6.7 to 8.0 ppm anticipated for the aromatic protons. The 1H NMR spectrum of Claisen product 1 showed an upfield shift in the methylene protons (Hc), from 4.6 ppm in 2 to 3.2 ppm in 1, owing to the transformation from the allylsubstituted ether to the allyl-substituted aryl group. 13C NMR
spectroscopy in DMSO-d6 showed a similarly significant upfield shift for the saturated methylene group in going from diallyl ether 2 (O−CH2CHCH2, 69 ppm) to the aromatic bis-allyl monomer 1 (Ar−CH2CHCH2, 34 ppm). The deoxybenzoin ketone signal moved little following the structural rearrangement, shifting very slightly from 196.7 ppm (for 2) to 196.9 ppm (for 1). BA-BHDB 1 was then utilized as an A2 monomer in step growth polymerization with aromatic diacid chlorides. By interfacial polycondensation, aromatic polyesters 3 and 4 (Figure 4) were synthesized from 1 and isophthaloyl and terephthaloyl chloride, respectively, under phase-transfer conditions employing dichloromethane, Na(OH)(aq), and benzyltriethylammonium chloride as the phase transfer catalyst (PTC). As a precursor to these polymerizations, the disodium salt of 1 was generated in NaOH(aq), followed by addition of the PTC, CH2Cl2, and the aromatic diacid chloride. This reaction mixture was stirred vigorously for 30 min, and the resulting viscous liquid was poured into excess methanol to precipitate the polymer. The isolated polymer was purified further by Soxhlet extraction using acetone, then filtered, and dried under vacuum to give a white fibrous polymer material. As shown in Table 1, gel permeation chromatography (GPC) in DMF (for
Figure 2. Synthesis of BA-BHDB 1 by a thermally induced Claisen rearrangement of bis-allyl ether 2. B
DOI: 10.1021/acs.macromol.7b00460 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 4. Synthesis of aromatic polyesters 3 and 4 by polymerization of 1 with isophthaloyl and terephthaloyl chloride, respectively.
respectively. 13C NMR spectra revealed a signal at 196 ppm corresponding to the deoxybenzoin carbonyls as well as signals surrounding 164 ppm reflecting the carbonyl carbons of the ester linkages. Unlike the aromatic polyesters prepared from BHDB and terephthaloyl chloride reported previously,14 allylsubstituted polymer 4 exhibited appreciable room temperature solubility in chloroform (1−2 mg/mL) and DMF (4−5 mg/ mL). Moreover, the copolymerizations of 1 with BHDB and isophthaloyl chloride were performed under similar conditions as for polymer 3 to afford polymers 5−8, which were also soluble in common organic solvents including dichloromethane, chloroform, and DMF (>2 mg/mL). The relative ratios of the two bisphenols, 1 and BHDB, incorporated into the aromatic polyester backbone were calculated for copolymers 5−8 from their 1H NMR spectra. For example, as shown in Figure S11, in polymer 7 a nearly 1:1 incorporation of the two bisphenols was found by integrating the signal for the aromatic protons meta to the carbonyl group of BA-BHDB, at 8.00 ppm, against those of the unsubstituted deoxybenzoin subunits, appearing at 8.13 ppm. The resonances corresponding to the α-carbonyl methylene protons of the two incorporated monomers, at 4.30 and 4.33 ppm, were also distinguishable, though not quite baseline separated to allow reliable integration. Importantly, BA-BHDB 1 was generally found to react in similar fashion to BHDB itself, with no substantive steric impedance to its reactivity resulting from the presence of the allyl groups. Thermal Properties and Heat Release Measurements. Table 2 summarizes PCFC, TGA, and differential scanning calorimetry (DSC) data of BA-BHDB polyarylates 3−8. TGA revealed 5% weight loss temperatures to be in the 330−375 °C range. As shown in Figure 6, the TGA curves for polymers 3−8 show a single stage of degradation, with char yield values ranging from 36 to 44%. Previously, we copolymerized BPA with BHDB to enhance polymer solubility in common organic solvents but found a significant reduction in char yield.14 In
Table 1. Polymer Compositions, Molecular Weights, Yields, and Solubility composition (BABHDB:BHDB)
GPCb Mn (g/mol)
Mw (g/mol)
yield (%)
100
28000
42000
80
100:0
100:0
21000
34000
85
5
70:30
74:26
23000
42000
77
6
60:40
61:39
25000
48000
70
7
50:50
49:51
24500
43000
75
8
40:60
39:61
25500
37000
71
polymer
feed
3
100:0
4c
incorporateda
solubility CHCl3, THF, DMF CHCl3, DMF CHCl3, THF, DMF CHCl3, DMF CHCl3, DMF CHCl3 DMF
a Incorporated ratios of BA-BHDB and BHDB calculated by 1H NMR spectroscopy. bGPC analysis performed eluting THF or DMF and estimated against polystyrene standards. cTerephthaloyl chloride employed as monomer.
polymers 4 and 6−8) and THF (for polymers 3 and 5) (calibrated with polystyrene standards) provided estimates of the number-average molecular weight (Mn) values, in the range of 21−28 kDa, for these allyl-substituted deoxybenzoin polyesters, and polydispersity indices (PDIs, or Mw/Mn) of 1.5−1.9. The polymerizations were generally conducted on a 0.5−2.0 g scale and gave isolated yields after purification of greater than 70% in all cases. The 1H NMR spectrum of polymer 3 in CDCl3 showed the expected resonances at 4.2 ppm and from 7.5 to 9.0 ppm, corresponding to methylene protons of deoxybenzoin and the aromatic protons of isophthaloyl or terephthaloyl moieties,
Figure 5. Interfacial polycondensation of BA-BHDB 1 with isophthaloyl chloride and BHDB to give aromatic polyesters 5−8 (compositions shown in Table 1). C
DOI: 10.1021/acs.macromol.7b00460 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
BA-BHDB, has a Tg of 102 °C, while polymer 7, with 51 mol % BA-BHDB, has a Tg of 128 °C. PCFC analysis of these novel diallyl-substituted deoxybenzoin polymers, and their copolymers with BHDB, revealed exceptionally low HRC values ranging from 75 to 147 J/(g K). Such low HRC values are comparable to those of commercial poly(ether ether ketone) (PEK) (124 J/(g K)), polytetrafluoroethylene (35 J/(g K)), and poly(vinyl chloride) (PVC) (138 J/(g K)).15 Figure 7 shows PCFC plots of HRR (y-axis) vs temperature (x-axis) for polyesters 3−8. A maximum peak HRR was observed at 481 °C for polymer 3 with a corresponding HRC of 147 J/(g K), while the lowest peak HRR was observed at 454 °C for polymer 7, with a HRC of 75 J/(g K). HRC values became progressively lower in BHDB/ BA-BHDB copolymers 5−8, and all were
