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. 2019 Dec 19;4(27):22363–22372. doi: 10.1021/acsomega.9b02648

Synthesis of a Regioregular Series of Poly(aryl ether carbonates) and Their Glass Transition Temperatures

Mohammad S Alomar †,*, David A Boyles ‡
PMCID: PMC6941183  PMID: 31909319

Abstract

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Bisphenol A polycarbonate (BPA-PC) is a remarkable high-performance engineering polymer, although it is susceptible to photo-Fries and hydrolytic degradation. New poly(aryl ether carbonates) were synthesized to address these limitations by replacing the chain backbone carbonate ester functionality with aryl ether functionality. The monomers for these new polymers were synthesized by a variation of the Ullmann condensation accelerated by 2,2,6,6-tetramethylheptane-3,5-dione and promoted by Cs2CO3 and 1-methyl-2-pyrrolidinone under mild conditions. Four such bisphenol A-based diarylether monomers containing different mass ratios of carbonate ester groups were prepared and polymerized with phosgene gas to give novel poly(aryl ether carbonates). Polymers were named as di-o-BPA-PC 9′, tri-o-BPA-PC 11′, tetra-o-BPA-PC 13′, and penta-o-BPA-PC 15′ where di-, tri-, tetra-, and penta- reflect the number of diphenylisopropylidene units in each of the respective polymers. The molecular weights of the resulting four poly(aryl ether carbonates) were measured by gel permeation chromatography. Differential scanning calorimetry was used to measure glass transition temperature (Tg). The polymers exhibited weight-average molecular weights up to 4.09 × 105 g/mol and Tg in the range of 136 to 149 °C with no melting temperature peak, indicative of their amorphous character. The new polymers formed transparent and flexible films by solution casting from chloroform solution.

1. Introduction

Bisphenol A polycarbonate (BPA-PC, Figure 1) is a widely used engineering thermoplastic with remarkable properties including ductility, toughness, and optical clarity. As a strong, high-performance amorphous engineering thermoplastic, it is well-known in the modern chemical industry for its temperature resistance, optical properties, and impact strength.1

Figure 1.

Figure 1

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Molecular structure of BPA-PC.

BPA-PC is not without its problems, however. Its BPA monomer is associated with early sexual maturation, altered behavior, and effects on prostate and mammary glands as demonstrated in rodent studies. In humans, BPA has been associated with cardiovascular disease, diabetes, and male sexual dysfunction in exposed workers. As a known endocrine disruptor, BPA is included in the EU list of potential endocrine disrupting substances. Although controversy continues over the toxicology of BPA, BPA-PC itself has been banned by the FDA in 2012 in products such as sippy cups and baby bottles which are intended for use by children under 12 years of age, owing to documented hydrolysis and migration of BPA.2−11

The polar carbonate ester groups in the backbone chains of BPA-PC subject the polymer to moisture uptake and have been demonstrated to be the reason for hydrolysis of the polymer,12−17 the kinetics of which are second-order.15 This mechanism is responsible in part for chemical aging of the material which leads to degradation of the physical properties of BPA-PC and which ultimately leads to the release of BPA. The labile carbonate ester functionality in BPA-PC thus has implications for the mechanical strength and ductility of the material, in addition to potential environmental and toxicological concerns.18,19

In addition to hydrolysis, BPA-PC also undergoes photodegradation in sunlight.18,20,21 Photodegradation is known to occur via the photo-Fries rearrangement with concomitant photooxidation and has been studied extensively for several decades.1,20 Photoexcitation of the carbonate ester groups is responsible for this reaction which converts the carbonate esters into various hydroxyketones. As per the First Law of Photochemistry (the Grotthuss–Draper law): “Only light which is absorbed by a molecule is effective in producing a reaction which changes the molecule.”22 The loci responsible for the photo-Fries reaction in BPA-PC are the same as those responsible for chain scission by hydrolysis, viz., the labile carbonate ester linkages in the backbone of the BPA-PC polymer chain. The net effect of both hydrolysis and the photo-Fries reaction is to decrease efficient chain entanglements necessary for the otherwise superior mechanical properties of the bulk material.23−26

Unlike carbonate ester linkages, diarylether linkages are exceptionally difficult to hydrolyze, requiring strong acids or bases,27 conditions which commercial BPA-PC is not subject to in its range of practical applications. This difficulty of hydrolysis is due to the stability of the aryl C–O bond.29,30,32−35 Barnett notes: “Phenolic ethers are stable, inert substances.”28,31 Ghashwall notes that aryl ether “hydrolysis requires concentrated hydrochloric or hydroiodic acid in a sealed tube.”36 More recent cleavage methods involve a catalytic reaction for aryl ether cleavage,37,38 but, again, such conditions are laboratory conditions outside those in which commercial BPA-PC finds application.

Diarylethers are additionally unable to undergo the photo-Fries reaction, although photoexcitation of diphenyl ether has been shown to occur under irradiation, albeit only in the presence of a solvent.39 Substitution of diarylether functionality for carbonate ester functionality would therefore be anticipated to mitigate the photochemical and hydrolytic disadvantages of bulk BPA-PC and could additionally be expected to enhance the chain flexibility40−43 of the polymer, owing to replacement of the trigonal planar carbonate ester groups in BPA-PC which are responsible for restricted rotation of the polymer chains. Replacement of these latter groups by aryl ether groups which are tetrahedral about oxygen would therefore also be anticipated to lead to potential processing advantages: it is well-appreciated by industry that the molecular weight of BPA-PC itself must be kept deliberately low because high molecular weight material leads to increased viscosity which in turn hinders polymer flow during processing, particularly during melt molding.44

Although our synthetic strategy to tailored poly(aryl ether carbonates) having precisely spaced carbonate ester groups in their backbones is novel, the late Allan S. Hay (1929–2017) of the General Electric Research Laboratory in Niskayuna, New York was the first to envisage an all poly(aryl ether) polymer based on diphenylisopropylidene containing no carbonate ester groups whatsoever, as seen in Figure 2.

Figure 2.

Figure 2

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Poly(aryl ether) polymer molecular structure containing repeat diphenylisopropylidene units.

Inspired by Mark’s work that bisphenols in the presence of a large excess of another phenol and methanesulfonic acid as catalyst underwent a transalkylation reaction producing a new bisphenol,45 Hay varied the conditions for spirobiindane bisphenol formation, substituting diphenyl ether for phenol in an attempt to produce the polymer shown in Figure 3.

Figure 3.

Figure 3

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Hay’s envisaged polymer produced by transalkyation of BPA with diphenyl ether.

First published in 1990, Hay’s proposed reaction for the synthesis of the polymer in Figure 3 was only moderately successful: an acid-catalyzed mixture of acetone, phenol, 3,5-dimethylphenol, and diphenylether gave the trimer of the polymer shown in Figure 3 and in 10% yield only.45,46 Nonetheless, a patent quickly followed in 1993 for polymeric species production.47 Although Hay claimed the polymer of Figure 3 which had a degree of polymerization anywhere from 20 to 100, the polymers in the embodiments of the patent had relatively low glass transition temperatures, ranging from 70 °C seen in his example 9 to a maximum of 93 °C seen in example 10. The highest degree of polymerization attained was cited to be 34 in example 10, which correlated with a molecular weight of approximately 7149.52, and no embodiments in the patent admit to a high molecular weight polymer. Hay, in fact, makes specific reference to oligomeric species: “The oligomers of the invention can be used in polymerization reactions with various monomers and reagents to produce a wide variety of polymers including polyethers, polysulfides, polyesters, polycarbonates, and polyformals. Employing oligomers of the invention, polymers suitable as engineering or industrial plastics, having high Tg in the range 100–200 °C may be produced.” No follow up work on this patent was reported in the literature.

Thus, the envisaged polymer in Figure 2 has never been synthesized as a high molecular weight polymer. Our current work represents the development of a synthetic protocol by which carbonate ester groups are successively able to be replaced by diarylether linkages, converging on the structure of the polymer in Figure 2, yet containing some carbonate ester linkages in the polymer backbone, unlike Hay’s projected polymer.

The objective of this work is to synthesize a series of new bisphenol monomers which incorporate diphenylisopropylidene units characteristic of BPA, yet in which the diphenylisopropylidene units—unlike in BPA-PC—are connected by diarylether linkages, as shown in Figure 2. These resultant monomers will then be polymerized using phosgene gas to link the bisphenol units by carbonate ester functionality. Although not identical to Hay’s work, the monomeric unit in this work are aryl ethers of diphenylisopropylidene units seen in Hay’s oligomer. In the current work, the smaller molecular weight monomers would result in poly(aryl ether carbonates) having a greater number of carbonate ester linkages per average polymer chain compared with the larger molecular weight monomers, which would instead result in fewer carbonate ester linkages per average polymer chain. The Tgs of these materials will be compared to those of both Hay’s oligomeric polyether and to that of BPA-PC.

2. Results and Discussion

In the current work, many variants of the classic Ullmann48−51 reaction were attempted for the synthesis of new diarylether monomers, but only one method was successful. Failed methods included utilization of copper(II)-promoted coupling of arylboronic acids and phenols,52 room-temperature copper(II) acetate-mediated coupling of substituted phenols with arylboronic acid from BPA and pyridine,53 ligandless Raney Ni–Al alloy/copper cross-coupling reaction from aryl iodide with BPA phenols,54 1,1,1-tris(hydroxymethyl)ethane as a tripod ligand for copper-catalyzed cross-coupling reaction of aryl iodide from BPA and phenols from BPA,55 and a palladium-catalyzed Buchwald–Hartwig reaction.56 Buck’s modified Ullmann copper-catalyzed coupling reaction using copper(I) chloride and 2,2,6,6-tetramethylheptane-3,5-dione (TMHD) as the ligand57 was the only successful method found in the current work for the synthesis of the new diarylether, diphenylisopropylidene-containing monomers.

Using the above method, four new bisphenolic monomers (9, 11, 13, and 15) have been successfully synthesized utilizing a known starting material, the monoamino analogue, 2, of BPA. Diazotiation of the amino functionality of this latter compound with subsequent iodine replacement proceeded smoothly to afford 4, the phenolic hydroxyl, which was then protected by benzylation to afford 7. This latter compound was elaborated using the method of Buck57 into the new bisphenolic monomers, 9, 11, 13, and 15. These four new monomers contain 2, 3, 4, and 5 diphenylisopropylidene units, respectively, which are linked by diarylether functionality. Although the synthetic strategy employed could be further extended to synthesize additional monomers with even greater numbers of diphenylisopropylidene units, this was not done.

Polymerization of the four new bisphenols using phosgene gas gave the new poly(aryl ether carbonates), 9′, 11′, 13′, and 15′, which were characterized using gel permeation chromatography (GPC), 1H NMR, and differential scanning calorimetry (DSC). Although the degree of polymerization of the polymers was lower with increasing molecular weights of the respective monomers (88, 47, 30, and 25 for 9′, 11′, 13′, and 15′, respectively) as seen in Table 2, the average molecular weights of the polymers were comparable with that of commercial BPA-PC, with the exception of 9′ which had a molecular weight 1.5× higher than that of commercial BPA-PC.

Table 2. Poly(aryl ether carbonate) Data.

polymer molecular weight (g/mol) glass transition temperature (°C) repeat unit formula and Mw (g/mol) degree of polymerization number of carbonate ester groups per average polymer chain % reduction of carbonate ester groups per average polymer chain compared to BPA-PC
BPA-PCa 26 700 150.0 C16H14O3 = 254 105 105  
Hay polymer, example 10b 7158 93.0 C15H14O = 210 34 0  
9′ 40 860 149.0 C31H28O4 = 464 88 88 16
11′ 31 450 144.3 C46H42O5 = 674 47 47 55
13′ 26 230 136.1 C61H56O6 = 884 30 30 71
15′ 26 900 136.0 C76H70O7 = 1094 25 25 76
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a

Commercial Lexan pellets were dissolved in methylene chloride and reprecipitated in methanol. This was repeated twice before molecular weight measurements were made.

b

Reference (47).

As seen in Table 2, there are numerically 88, 47, 30, and 25 carbonate ester groups per average polymer chain in polymers 9′, 11′, 13′, and 15′, respectively. With regard to the current objective of reducing the average numbers of carbonate ester groups per polymer chain compared to the number found in commercial BPA-PC (which has 105 carbonate ester groups per chain), polymers 9′, 11′, 13′, and 15′ therefore display a 16, 55, 71, and 76% reduction, respectively, in the number of their carbonate ester groups compared to commercial BPA-PC.

The low Tgs noted by Hay in his patent were likely due to low molecular weights obtained in the polymerization process. There is no reason that the polymer in Figure 3 should cease to undergo electrophilic aromatic substitution with acetone as long as acetone was in constant supply, which was the case under Hay’s conditions. This suggests that chain growth may have been arrested as a result of premature end-capping of oligomeric species, possibly by the formation of spirobiindane moieties, the latter a well-known product of acid-catalyzed BPA decomposition and recombination as previously mentioned above.58−66 In his 1993 article, Hay, in fact, makes specific reference to the limitations inherent in his polymerization method, which results in premature termination of polymerization preventing the obtention of high molecular weight material.47 If spirobiindane end-capping occurred, the large free volume induced by any spirobiindane endcapping would be reasonably expected to additionally lower the Tg of the final materials.

2.1. Synthesis of Diarylether Diphenylisopropylidene-Containing Monomers Having Different Diphenylisopropylidene–Ether Ratios

The synthesis of the desired diphenylisopropylidene monomers was achieved in multiple steps from commercially available 4,4′-isopropylidenediphenol. Important starting materials were necessary to begin the synthesis of the new diarylether diphenylisopropylidene-containing monomers.

Mono-iodination of arenediazonium tetrafluoroborate salt of 2,2′-(4-hydroxy-4′-aminodiphenyl)propane, 3, with NaI/I2 in acetonitrile gave I-BPA–OH, 4, in good yield (90%). 2,2′-(4-Benzyloxy-4′-iododiphenyl)propane, 7, reacted with compound 4, in the presence of benzyl chloride in ethyl acetate and gave compound 7 in good yield (90%). The synthesis of compound 7 is shown in Scheme 1.

Scheme 1. Synthesis Route of 2,2-(4-Benzyloxy-4′-iododiphenyl)propane (7).

Scheme 1

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Compound 7 was necessary to synthesize di-o-BPA, 9, as shown in Scheme 2—route 1.

Scheme 2. Synthesis of Diarylether Monomers Including Varied Number of Ether Linkages.

Scheme 2

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Key: (a) benzyl chloride, NaOH/H2O, 100 °C, 24 h, toluene, 89%; (b) compound 7, Cs2CO3, CuCl, TMHD, NMP, 72 h, 100 °C, 56%; (c) H2/Pd, ethyl acetate, 4 h, RT, 98%; (d) compound 7, Cs2CO3, CuCl, TMHD, NMP, 72 h, 100 °C, 77%; (e) H2/Pd, ethyl acetate, 8 h, RT, 71%; (f) compound 7, Cs2CO3, CuCl, TMHD, NMP, 72 h, 100 °C, 73%; (g) H2/Pd, ethyl acetate, 7 h, RT, 63%; (h) compound 7, Cs2CO3, CuCl, TMHD, NMP, 72 h, 100 °C, 81%; (i) H2/Pd, ethyl acetate, 8 h, RT, 75%.

Monomer 9 was successfully prepared by hydrogen deprotection of compound 8, which was prepared by reacting compound 6 with compound 7 in the presence of Cs2CO3, CuCl, and the TMHD ligand in 1-methyl-2-pyrrolidinone (NMP).

Monomer 9 was prepared from commercially available bisphenol A. The method of Buck57 was used to effect the condensation reaction of 1.00 equiv compound 6 with 1.10 equiv compound 7. Anhydrous Cs2CO3 (1.50 equiv) as base and CuCl (0.500 equiv) as the catalyst were added to the anhydrous NMP system. The reaction proceeded smoothly under reflux in a nitrogen atmosphere for 72 h to afford the desired Ullmann condensation product 8 at 56% yield. The dihydroxy condensation product 9 in Scheme 2—route 1 was obtained by dissolving compound 8 in 10 mL of ethyl acetate, 10 mol % of the Pd catalyst was added, and the mixture was degassed twice under vacuum (using a water pump), replacing each time the vacuum by hydrogen. The reaction mixture was left at room temperature for 4 h connected to a double-layer balloon of hydrogen. The catalyst was filtered off and washed with ethyl acetate. The filtrate was concentrated to give compound 9. Flash chromatography was used for purification. Yield: 98%. This compound has previously been reported, but only as a product of γ-irradiated BPA-PC.67

2.2. General Procedure for Polymerization

After successful preparation of the di-o-BPA dihydroxy monomer, 9, polymerization was performed by solution polymerization using phosgene gas as described above and in a previous work.68

Tri-o-BPA monomer 11 was successfully prepared by dehydrogenation of compound 10, the latter of which was prepared by reacting compound 1 with compound 7 in the presence of Cs2CO3, CuCl, and the TMHD ligand in NMP. Buck’s methodology57 was used to effect the condensation reaction of 2.00 equiv compound 7 with 1.0 equiv compound 1. Anhydrous Cs2CO3 (2.5 equiv) as the base and CuCl as the catalyst were added to the anhydrous NMP system. The reaction proceeded smoothly under reflux in a nitrogen atmosphere for 72 h to afford the desired Ullmann condensation product 10 in 72% yield. This material was purified by flash chromatography. The dihydroxy condensation product, 11, in Scheme 2–route 2, was obtained in good yield (63%) in 7 h using the H2/Pd balloon method at room temperature in the ethyl acetate solvent (10.0 mL). Monomer 13 was prepared successfully as shown in Scheme 2—route 3. This polymer has three diarylether linkages and four diphenylisopropylidene units.

Tetra-o-BPA monomer 13 was successfully prepared by hydrogen deprotection of compound 12, which was prepared by reacting compound 7 with monomer 9 in the presence of Cs2CO3, CuCl, and the TMHD ligand in NMP. Again, Buck’s methodology57 was used to effect the condensation reaction of 2.00 equiv compound 7 with 1.0 equiv monomer 9. Anhydrous Cs2CO3 (2.5 equiv) as the base and CuCl as the catalyst were added to the anhydrous NMP system. The reaction proceeded smoothly under reflux in a nitrogen atmosphere for 72 h to afford the desired Ullmann condensation product 12 in 77% yield. The dihydroxy condensation product, 13, in Scheme 2—route 3 was obtained in good yield in 3 h under H2/Pd at room temperature using ethyl acetate as the solvent (10.0 mL). Yield: 71%. Monomer 15 was prepared successfully as shown in Scheme 2—route 4. This polymer has four diarylether linkages and five diphenylisopropylidene units.

The penta-o-BPA monomer, 15 was successfully prepared by hydrogen deprotection of compound 14, which was prepared by reacting compound 7 with monomer 11 in the presence of Cs2CO3, CuCl, and the TMHD ligand in NMP. The method of Buck57 was used to effect the condensation reaction of 2.00 equiv compound 7 with 1.0 equiv monomer 11. Anhydrous Cs2CO3 (2.5 equiv) as the base and CuCl as the catalyst were added to the anhydrous NMP system. The reaction proceeded smoothly under reflux in a nitrogen atmosphere for 72 h to afford the desired Ullmann condensation product 14 in 81% yield. The dihydroxy condensation product, 15, in Scheme 2—route 4, was obtained in good yield in 3 h using the H2/Pd method at room temperature using ethyl acetate as the solvent (10.0 mL). Yield: 75%.

All poly(aryl ether carbonates) produced are labeled as di-o-BPA-PC 9′, tri-o-BPA-PC 11′, tetra-o-BPA-PC 13′, and penta-o-BPA-PC 15′. All monomers and their corresponding polymers are summarized in Table 1.

Table 1. Monomer Structures and Their Corresponding Polymers.

2.2.

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All monomers and their corresponding polymers were characterized by proton nuclear magnetic resonance (1H NMR). All spectra are shown in the Supporting Information file.

Average molecular weights for all polymers as well as their corresponding glass transition temperatures were measured with GPC and DSC, respectively. Weight average molecular weights along with the glass transition temperatures, repeat unit molecular formulae and weights, degree of polymerization, and number of carbonate ester groups per average polymer chain are summarized in Table 2.

As polymerizations of each new monomer in this article were run under identical conditions, no attempt was made to optimize molecular weights. Thus, detailed comparisons dependent on factors including molecular weights and degree of polymerization, all of which varied from polymer to polymer, are not possible. Nonetheless, several general trends may be observed from Table 2. First, other than Hay’s reported polymer 10, the molecular weights of all four new polymers were comparable with or above that of commercial BPA-PC, even though the degree of polymerization varied. Second, the size of the monomer was clearly influential on the degree of polymerization for the resulting polymers: the series of polymers decreased in their degree of polymerization as the size of the monomer increased. Third, the four new polymers had Tgs and molecular weights significantly higher than the polymer 10 documented in Hay’s work.48 Polymer 9′, in fact, had a Tg nearly that of BPA-PC itself.

It appears that, despite 9′ having fewer carbonate ester groups per average chain than BPA-PC to stiffen it, the higher molecular weight of 9′ compared with BPA-PC (1.5× greater) may alone have been influential in raising the Tg. Polymers 13′ and 15′ had nearly equivalent Tgs as well as nearly equivalent molecular weights. Whether a lower degree of polymerization was influential in a lower Tg among the four polymers is difficult to assess: it is well appreciated by the literature that a lower degree of polymerization leads to a lower entanglement density as well as a greater number of chain ends in the bulk, imparting greater free volume and thereby lowering Tg.69 Restricted rotational degrees of freedom particularly around the carbonate ester functionality in BPA-PC’s backbone chain create a more rigid structure,1 and this may partly account for the increasingly higher Tg with the increasing number of carbonate ester groups per average polymer chain in the respective polymers. Finally, in spite of their differences, the Tgs of polymers 13′ and 15′ appear to be converging on a Tg that could be close to that of the ideal poly(aryl ether) polymer of Figure 2 which is yet to be synthesized. This Tg (136 °C) is 43 °C higher than that of Hay’s highest Tg polymer (93 °C), indeed suggesting the oligomeric nature of the polymers obtained by Hay, if not large spirobiindane end-caps on his materials.

3. Conclusions

Synthetic routes for four new bisphenol monomers were successfully designed, followed by successful synthesis of the monomers. The monomers were polymerized to their corresponding poly(aryl ether carbonates), reestablishing historical continuity with the objective of Hay’s work some 25 years ago, namely, addressing the issue of BPA-PC chain scission. The synthesis of the monomers for these polymers relied on an iterative method to elaborate monomers using the same starting materials and synthetic processes for each, a method which in principle could conceivably be extended to even further reduce the number of carbonate ester groups in the final polymers.

The synthesis of the four new monomers relied on Buck’s57 modified Ullmann condensation reaction using TMHD as the reaction-rate accelerating material, cesium carbonate as the base, and CuCl as the catalyst. This was found to be the only method among many tried which was able to successfully effect the aryl coupling reaction for the desired monomers.

Solution polymerization of the four new bisphenols using phosgene gas in methylene chloride solvent resulted in four new poly(aryl ether carbonates). These polymers were characterized by 1H NMR spectroscopy, GPC, and DSC. They exhibited weight-average molecular weights up to 4.09 × 105 g/mol and Tgs in the range from 136 to 149 °C with no melting temperature peak, indicative of their amorphous character. As the Tg of commercial BPA-PC is 150 °C, this was surprising in that diarylethers are known to be more flexible about the C–O bond than their counterparts, the carbonate esters which tend to rigidify the polymer chain owing to resonance effects into the carbonate ester group. The new polymers formed transparent and flexible films by solution casting from chloroform solution.

Although the polymer of Figure 2 is yet to be synthesized, the new synthesized polymers converge on the structure of Hayes’ ideal of a no-carbonate ester-containing polymer. In one case, the Tg approaches that of BPA-PC, giving an indication of the maximum Tg Hay’s no-carbonate ester-containing polymer could conceivably approach, had it been of higher molecular weight.

Experimental work on the stability properties of these new polymers relative to BPA-PC is yet to be undertaken. However, as these four new polymers have greatly reduced numbers of carbonate ester groups, the polymers would therefore be anticipated to have a reduced number of chain scission “events” such as hydrolysis and the photo-Fries reaction relative to BPA-PC itself. Furthermore, as these new polymers contain no BPA monomer when hydrolyzed, they would not lead to environmental and toxicological effects characteristic of BPA in BPA-PC.

Additional work to optimize the polymerization conditions is in progress which will better allow us to understand the effect of the degree of polymerization on the melt viscosity and glass transition properties, particularly as compared with that of commercial BPA-PC, as well as on the environmental stability of these new materials.

4. Experimental Section

Starting materials and reagents necessary to synthesize the corresponding monomers follow.

4.1. Materials

All reagents were used without further purification unless otherwise mentioned. Bisphenol A (99+%), boron trifluoride etherate, methyl iodide, anhydrous tetrahydrofuran (THF), tert-butyl nitrite, sodium chloride, copper(I) chloride, potassium carbonate (K2CO3, 99+%), benzyl chloride, TMHD, sodium iodide, cesium carbonate, and anhydrous NMP 99% were purchased from Sigma-Aldrich Chemical Co. and used as received. Hexane, anhydrous diethyl ether, anhydrous sodium thiosulfate, methylene chloride stabilized, sodium hydroxide (97%), toluene, methanol, ethyl acetate, and 37% hydrochloric acid were purchased from Fisher Chemicals and used without further modification. Acetonitrile was purchased from Spectrum and used without further modification. Sodium sulfate was purchased from Alfa Aesar and used without further modification. All solvents were dried and purified by using the standard procedures.

4.2. Characterization

1H NMR was recorded on a 300 MHz/52 MM Bruker Cryomadnet NMR (USA, coil# 244 30 67G) spectrometer with tetramethylsilane as an internal reference. 1H NMR chemical shifts were reported as δ values (ppm) relative to CDCl3 (7.26). Reactions were monitored by thin-layer chromatography performed on Kieselgel 60 F254, 0.2 mm plates (Merck) with visualization under UV light (254 or 366 nm). DSC measurements were conducted with a DSC Q100 instrument. All samples (250–350 mg) were subjected to two heating–cooling cycles at the ramp rate of 10 °C/min under a flow of nitrogen in aluminum pans. GPC/static light scattering: The GPC/SEC system consists of Shimadzu’s solvent delivery unit; the column heater includes PL gel 5 μm mixed-D columns from Polymer Laboratories Ltd. for size-exclusive separation, connected also to a miniDAWN Tristar light scattering detector (Wyatt). An Optilab DSP interferometric refractometer (Wyatt) was used to determine the average molecular weight of the polymer with a range from 200 to over 1 000 000 g/mole. The miniDAWN Tristar is a multiangle laser light scattering photometer containing two photodiodes to monitor the laser beam intensity. The light source was a 30 mW semiconductor diode laser with a wavelength of 690 nm. Measurements of molecular weight (Mw): polymers were prepared in a 100.00 mL volumetric flask (grade A, 100.00 mL ± 0.08) by weighing approximately 0.10000 g and then dissolving in THF. A 1.0000 mg/mL stock solution of the polymer was prepared. Then, samples were filtered before injection using Iso-Disc filters (Iso-Disk Filters, PTFE-25-2, 25 mm × 0.2 μm, Supelco). An autosampler was used to perform the analysis in duplicates with a total run time of 25 min. The analysis was performed using a miniDAWN Tristar (Wyatt) connected to a GPC (Shimadzu).

4.3. Precursor Synthesis

Synthesis of 2,2-(4-hydroxy-4′-aminodiphenyl)propane (2) was carried out according to the literature.18,70

4.3.1. Synthesis of Arenediazonium Tetrafluoroborate Salt of 2,2-(4-Hydroxy-4′-aminodiphenyl)propane (3)

Boron trifluoride etherate (17.73 g, 0.1250 mol) was added to compound 2 (18.87 g, 0.0830 mol) in an anhydrous THF (120 mL). Prior to addition of the amine, the boron trifluoride etherate was cooled at −15 °C in an ice-acetone bath. tert-Butyl nitrite (10.21 g, 0.0991 mol) was added dropwise to the rapidly stirred reaction solution over a 10 min period. Following complete addition, the temperature of the reaction solution was maintained at −15 °C for 10 min and then allowed to warm to 5 °C in an ice-water bath over a 20 min period. A crystalline precipitate usually formed during the addition of tert-butyl nitrite and following the 20 min period at 5 °C, precipitation was complete. Hexane (20 mL) was then added to the reaction solution, and the solid was suction filtered, washed with cold ether, and dried in a desiccator. Yield: 20.69 g (78%). 1H NMR (300 MHz, DMSO-d6): δ 9.38 (br s. 1H, exchangeable with D2O, OH), 8.50–8.53 (d, 2H, ArH), 7.74–7.77 (d, 2H, ArH), 6.97–7.00 (d, 2H, ArH), 6.66–6.68 (d, 2H, ArH), and 1.62 (s, 6H, CH3).

4.3.2. Synthesis of 2,2-(4-Hydroxy-4′-iododiphenyl)propane (4)

Arenediazonium tetrafluoroborate salt (3) (55.61 g, 0.1705 mol) was dissolved in a solution of NaI (37.82 g, 0.2523 mol)/I2 (32.03 g, 0.1262 mol) in 667 mL of acetonitrile. After complete addition of the salt, the mixture was stirred for 15 min. After 15 min, a saturated solution of sodium thiosulfate (800 mL) was followed by the addition of 800 mL CH2Cl2. The two layers were separated, and the organic layer was washed with H2O (three times) and brine (once). The washed organic layer was dried over sodium sulfate, Na2SO4, and then the solvent was evaporated under reduced pressure.71 Yield: 49.66 g (86%). 1H NMR (300 MHz, CDCl3): δ 7.05–7.07 (d, 2H, ArH), 6.74–6.77 (d, 2H, ArH), 6.97–7.00 (d, 2H, ArH), 7.55–7.58 (d, 2H, ArH), 4.68 (s, 1H, OH), and 1.62 (s, 6H, CH3).

4.3.3. Synthesis of 2,2-(4-Iodo-4′-methoxydiphenyl)propane (5)

To a mixture of K2CO3 (8.130 g, 0.0588 mol) in 100 mL of acetonitrile, the weight of compound 4 was added (5.000 g, 0.0148 mol) and refluxed. After complete dissolution, methyl iodide CH3I (8.351 g, 0.0588 mol) was added dropwise. The mixture was stirred for 12 h. After 12 h, the mixture was cooled to room temperature and washed with H2O and brine in a separatory funnel. The resultant organic layer was dried over Na2SO4. The dried organic layer was concentrated under reduced pressure. The concentrated product was recrystallized from ethanol to yield yellow crystals. Yield: 4.45 g (86%). 1H NMR (300 MHz, CDCl3): δ 6.83–6.85 (d, 2H, ArH), 7.19–7.21 (d, 2H, ArH), 6.97–7.00 (d, 2H, ArH), 7.55–7.58 (d, 2H, ArH), 3.78 (s, 3H, OCH3), and 1.62 (s, 6H, CH3).

4.3.4. Synthesis of 2,2-(4-Benzyloxy-4′-hydroxydiphenyl)propane (6)

Compound 1 (50.00 g, 0.2190 mol) was dissolved in NaOH/100 mL of H2O solution (8.852 g, 0.2212 mol) under reflux. A reddish solution was obtained; then 200 mL of toluene was added followed by the addition of benzyl chloride (40.20 g, 0.3176) dropwise. After complete addition of benzyl chloride, 60 mL of methanol was added. The reaction was allowed to reflux with stirring for 24 h. After 24 h, the reaction mixture was concentrated under reduced pressure, and washed with H2O and brine. The resultant yellowish oil was recrystallized from toluene to afford 59.79 g (86%). 1H NMR (300 MHz, CDCl3): δ 7.30–7.50 (m, 5H, ArH), 6.73–6.76 (d, 2H, ArH), 7.08–7.11 (d, 2H, ArH), 7.15–7.18 (d, 2H, ArH), 7.89–7.91 (d, 2H, ArH), and 1.62 (s, 6H, CH3).

4.3.5. Synthesis of 2,2-(4-Benzyloxy-4′-iododiphenyl)propane (7)

Compound 4 (20.00 g, 0.0591 mol) was dissolved in NaOH/(H2O: ethanol) (1:1) solution (3.552 g, 0.0887 mol). After complete dissolution of compound 4 under reflux, 100 mL of ethyl acetate and 50 mL of methanol were added. Benzyl chloride (11.23 g, 0.0887 mol) was subsequently added and the solution was stirred for 24 h under reflux. The reaction mixture was worked up by first concentrating the solution under reduced pressure. The resultant light-brown oil product was next dissolved in CH2Cl2 and washed with H2O and brine using a separatory funnel. The solution was dried with sodium sulfate prior to concentration under reduced pressure. A light yellow oil was produced which was recrystallized from ethanol to yield white crystals. Yield: 28.32 g (89%). 1H NMR (300 MHz, CDCl3): δ 7.30–7.50 (m, 5H, ArH), 6.73–6.76 (d, 4H, ArH), 7.08–7.11 (d, 4H, ArH), 6.97–7.01 (d, 4H, ArH), 7.55–7.58 (d, 4H, ArH), and 1.62 (s, 6H, CH3).

4.4. Monomers and Their Corresponding Polymers Synthesis

4.4.1. Synthesis of 4-[2-(4-{4-[2-(4-Hydroxyphenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenol, Di-o-BPA (9)

A four-step synthetic strategy for the monomer possessing one diarylether linkage is described. Compound 7 (10.00 g, 0.0233 mol), compound 6 (14.87 g, 0.0467 mol), TMHD (0.6501 g, 0.0035 mol), and cesium carbonate (15.21 g, 0.0467 mol) were added to 40 mL of anhydrous NMP. The mixture was degassed and filled with nitrogen. To this mixture was added CuCl (0.0600 g, 0.0006 mol), and the mixture was degassed and filled with nitrogen three times. This mixture was then heated to 120 °C under nitrogen and kept stirring for 72 h. The reaction was diluted with ether and filtered. The filtrate was washed with 2 M HCl and then 0.6 M HCl, 2 M NaOH, and 10% NaCl. The desired product was isolated by flash column chromatography to give compound 8. Yield 8.20 g (57%). Monomer 9 was obtained by deprotection of compound 8 by the H2/Pd method at room temperature using ethyl acetate as solvent (10.0 mL). Yield: 5.52 g (95%).

4.4.2. Synthesis of 4-[2-(4-{4-[2-(4-{4-[2-(4-Hydroxyphenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenol, Tri-o-BPA (11)

A six-step synthetic strategy for monomer 11 possessing two ether linkages is outlined in Scheme 2—route 2. Compound 7 (7.001 g, 0.0156 mol), compound 1 (4.971 g, 0.0156 mol), TMHD (1.150 g, 0.0062 mol), and cesium carbonate (5.081 g, 0.0344 mol) were added to 28 mL of anhydrous NMP. The mixture was degassed and filled with nitrogen. To this mixture was added CuCl (1.110 g, 0.0011 mol), and the mixture was degassed and filled with nitrogen three times. This mixture was then heated to 120 °C under nitrogen and kept stirring for 72 h. The reaction was diluted with ether and filtered. The filtrate was washed with 2 M HCl and then 0.6 M HCl, 2 M NaOH, and 10% NaCl. The desired product was isolated by flash column chromatography to give compound 10. Yield: 9.50 g (73%). Monomer 11 was obtained by deprotection of compound 10 (8.101 g, 0.0098 mol) under H2/Pd at room temperature using ethyl acetate as the solvent (10.0 mL). Yield: 6.11 g (63%).

4.4.3. Synthesis of 4-[2-(4-{4-[2-(4-{4-[2-(4-{4-[2-(4-Hydroxyphenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenol, Tetra-o-BPA (13)

A six-step synthetic strategy for monomer 13 possessing three ether linkages is outlined in Scheme 2—route 3. Compound 7 (3.001 g, 0.0070 mol), compound 9 (3.091 g, 0.0070 mol), TMHD (0.26 g, 0.0014 mol), and cesium carbonate (4560 g, 0.0140 mol) were added to 13 mL of anhydrous NMP. The mixture was degassed and filled with nitrogen. To this mixture was added CuCl (0.6900 g, 0.0070 mol), and the mixture was degassed and filled with nitrogen three times. This mixture was then heated to 120 °C under nitrogen and kept stirring for 72 h. The reaction was diluted with ether and filtered. The filtrate was washed with 2 M HCl and then 0.6 M HCl, 2 M NaOH, and 10% NaCl. The desired product was isolated by flash column chromatography to give compound 12. Yield: 5.60 g (77%). Monomer 13 was obtained by deprotection of compound 12 (5.001 g, 0.0048 mol) under H2/Pd at room temperature using ethyl acetate as the solvent (10.0 mL). Yield: 3.0 g (71%).

4.4.4. Synthesis of 4-[2-(4-{4-[2-(4-{4-[2-(4-{4-[2-(4-{4-[2-(4-Hydroxyphenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenoxy}phenyl)propane-2-yl]phenol, Penta-o-BPA (15)

Compound 7 (2.7601 g, 0.0064 mol), compound 11 (4.090 g, 0.0064 mol), TMHD (0.2402 g, 0.0013 mol), and cesium carbonate (4.170 g, 0.0128 mol) were added to 12 mL of anhydrous NMP. The mixture was degassed and filled with nitrogen. To this mixture was added CuCl (0.6400 g, 0.0064 mol), and the mixture was degassed and filled with nitrogen three times. This mixture was then heated to 120 °C under nitrogen and kept stirring for 72 h. The reaction was diluted with ether and filtered. The filtrate was washed with 2 M HCl and then 0.6 M HCl, 2 M NaOH, and 10% NaCl. The desired product was isolated by flash column chromatography to give compound 14. Yield: 6.30 g (81%). Monomer 15 (Scheme 2—route 4) was obtained by deprotection of compound 14 (5.001 g, 0.0041 mol) under H2/Pd at room temperature using ethyl acetate as the solvent (10.0 mL). Yield: 3.21 g (75%).

4.4.5. Synthesis of 2,2-Bis(4-iodophenyl)propane, I-BPA, 16

A suspension of 2,2′-diphenylpropane (29.40 g, 0.1498 mol), iodine (38.10 g, 0.1501 mol), and PhI(OCOCF3)2 (68.80 g, 0.1600 mol) in tetrachloromethane (600 mL) was stirred at 50–55 °C for 1 h. After cooling to room temperature, the suspension was concentrated under reduced pressure. The cloudy white residue was recrystallized from ethanol (53.70 g, 80%) giving white needles. 1H NMR (300 MHz, CDCl3): δ 7.56–7.59 (d, 4H, ArH), 6.92–6.96 (d, 4H, ArH), and 1.61 (s, 6H, CH3).

4.4.6. General Polymerization Procedure

As a general procedure for solution polymerization, a mixture of monomer (0.0052 mol), water (80 mL), methylene chloride (100 mL), 1.2 mL of 5% w/v triethylamine solution in methylene chloride, and the desired amount of a 5% w/v solution of p-t-butylphenol in methylene chloride was prepared. The pH was adjusted to 12 by the addition of 10% aqueous sodium hydroxide solution, stirred for 5 min, then cooled to 0–5 °C. Phosgene was bubbled in over 8 min, and the pH was maintained between 8 and 9 by the addition of sodium hydroxide solution. The methylene chloride layer was washed with 5% aq HCl solution (2 × 200 mL), then with distilled water (6 × 400 mL), and then concentrated and poured into methanol. The resulting polymer was dried in vacuum producing a powdery material.

Acknowledgments

Department of Defense—Army Research Office (grant #W911NF-08-1-0324). State of South Dakota, Nanoscience & Engineering PhD Program, and South Dakota School of Mines & Technology. Dr. Tsvetanka Filipova, Research Scientist II, Department of Chemistry, SDSMT, for all monomer polymerizations by phosgenation. Shady Awwad, graduate research assistant, Nanoscience and Nanoengineering Ph.D. program, SDSMT, for molecular weight determinations for all polymers. Christopher Fitzpatrick, English editor, for English proofreading.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02648.

  • 1H NMR data of all new monomers and their corresponding polymers (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b02648_si_001.pdf (299KB, pdf)

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