Ultralong-Chain-Spaced Crystalline Poly(H-phosphonate)s and Poly

Article pubs.acs.org/Macromolecules

Ultralong-Chain-Spaced Crystalline Poly(H-phosphonate)s and Poly(phenylphosphonate)s Hanna Busch, Eva Schiebel, Annika Sickinger, and Stefan Mecking* Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78457 Konstanz, Germany S Supporting Information *

ABSTRACT: Aliphatic poly(H-phosphonate)s were obtained by polyesterification of dimethyl H-phosphonate with bio-based long-chain diols. Nonadecane-1,19-diol, tricosane-1,23-diol, and octatetracontane-1,48-diol with dimethyl H-phosphonate yield the corresponding polyphosphoesters (PPE19H, PPE23H, and PPE48H) with molecular weights (Mn) up to 4.3 × 104 g mol−1. Postfunctionalization of these polymers via Hirao cross-coupling yields the selectively functionalized poly(H-phosphonate)s PPE19Ph, PPE23Ph, and PPE48Ph. DSC analysis revealed significantly enhanced crystallinities and melting points (up to Tm = 110 °C) with increasing methylene sequence lengths. Hydrolytic degradation of polymer powder of poly-(Hphosphonate) occurred up to 95% in 2 days. The degradation rates decreased with increasing methylene sequence length. After postfunctionalization, degradation occurred only to a minimal extent over 3 months in basic and in acidic media.

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INTRODUCTION Polycondensates containing long-chain aliphatic building blocks differ from their established shorter chain congeners. The long methylene sequences dominate the polymer properties by van der Waals interactions leading to “polyethylene-like” behavior in some respect.1,2 Notwithstanding, the functional groups along the polymer chain impact polymer properties, in particular the interaction with polar surfaces and degradability of the polymer chain. As an example, polyphosphoesters are potential candidates for hydrolytic or enzymatic degradability.3 In addition, the similarity of polyphosphoesters (PPEs) to nucleic acids allows for biocompatibility.4,5 In particular, poly(H-phosphonate)s are studied as rapidly degrading polymers.6−8 Furthermore, they allow for postpolymerization reactions and modifications of the polymer backbone.8−11 Poly(H-phosphonate)s are accessible via polycondensation reactions or ring-opening polymerization methods.8 The polymers studied to date are restricted to short- and midchain linear aliphatic repeat unit based on the corresponding α,ω-diols (C2−6, C8, C10, and C12) and poly-(ethylene glycol) up to date.6,10−21 Via transesterification, taking place at 160 °C poly(H-phosphonate)s with high molecular weights of up to 3.3 × 104 g mol−1 are accessible by polycondensation methods.22−24 So far, poly(H-phosphonate)s bearing highly hydrophobic long-chain aliphatic repeat units, longer than C12, were not synthesized. New approaches for generation of long-chain building blocks by isomerizing alkoxycarbonylation25−27 or chain multiplication28 of fatty acids open up new possibilities for crystallizable polymer segments. The currently known poly(H-phosphonate)s are generally unstable under aqueous conditions. This limitation can potentially © XXXX American Chemical Society

be overcome by longer chain, more hydrophobic aliphatic repeat units. The P−H function also offers itself for further selective functionalization, generating polymers with phosphorus bound aryl or alkyl side chains. Postpolymerization functionalization of the P−H bond is expected to impact key properties like hydrolytic stability or mechanical properties. A catalytic P−C bond formation between phosphonates and aryl electrophiles was first investigated in1981 by Hirao et al.29 The cross-coupling reaction catalyzed by Pd(0) was further investigated with respect to ligand, base, solvent, nucleophiles, and mechanism.30−36 Only small molecule phosphonates such as diethyl H-phosphonate, diisopropyl H-phosphonate, or nucleosidic compounds were studied as substrates.29,35−40 The utility for polymers as multifunctional substrates remains unexplored to day. This would offer the possibility of a direct and catalytic postfunctionalization strategy of poly(H-phosphonate)s under mild conditions. We now report the synthesis of crystalline poly-(H-phosphonate)-s and their postpolymerization functionalization reactivity as well as key properties of the resulting polymers.

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RESULTS AND DISCUSSION Synthesis of Poly(H-phosphonate)s. Based on the reported polycondensation procedures of dimethyl phosphonate with short- or midchain diols, different polymerization conditions were evaluated. Usually, these polycondensation reactions are conducted in the melt using different temperature Received: June 27, 2017 Revised: August 11, 2017

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DOI: 10.1021/acs.macromol.7b01368 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Long-Chain Poly(H-phosphonate)s.

Table 1. Polycondensation of Dimethyl H-phosphonate with Different α,ω-Diolsa entry 1 2 3 4

α,ω-diol

Mn,GPCb [g mol−1]

1.12 1.19 1.23 1.48

8 × 10 4.3 × 104 2.3 × 104 3

Mw/Mnb

Mn,NMRc [g mol−1]

Tmd [°C]

Tcd [°C]

ΔHmd [J g−1]

crystallinitye [%]

2.1 2.0 2.0

2.5 × 10 1.5 × 104 1.1 × 104 2.4 × 104

38 76 90 98; 110

27 65 82 94; 102

87 109 111 149

30 37 38 51

4

Reaction conditions: α,ω-diol (13.3 mmol), dimethyl phosphonate (1.8 g, 16.0 mmol), sodium (0.9 mmol), 120 °C for 4 h, 160 °C for 20 h and 0.1 mbar. bDetermined by GPC in THF (50 °C) versus polystyrene standards. For PPE48H, no GPC analysis was possible due to the insolubility of the polymer in hot THF. cCalculated from 1H NMR spectroscopic analysis of the end groups. dDetermined by DSC with a heating/cooling rate of 10 K min−1. Peak Tm determined from the second heating cycle. eFrom DSC measurements, calculated vs 100% crystalline polyethylene (293 J g−1).

a

programs starting from 80 °C and reaching up to 200 °C.20−22 Base-catalyzed reaction conditions using sodium accelerated the reaction.6 Compared to dodecane-1,12-diol (Tm = 79−81 °C), the plant oil based long-chain diols have higher melting points (C19: Tm1 = 90 °C, Tm2 = 104 °C; C23: Tm1 = 98 °C, Tm2 = 110 °C; C48: Tm = 125 °C). This required sufficiently high initial temperatures. Furthermore, temperatures above 160 °C did not show any improvement regarding the degrees of polymerization. The different long-chain diols were stirred with an excess of dimethyl H-phosphonate at 120 °C in the presence of sodium for 4 h before the temperature was raised to 160 °C. The reaction mixture was stirred for 24 h and 0.1 mbar at 160 °C to obtain the semicrystalline long-chain PPEs (Scheme 1). Polycondensation of the long-chain aliphatic α,ω-diols resulted in solids of poly(dodecane H-phosphonate) (PPE12H), poly(nonadecane H-phosphonate) (PPE19H), poly-(tricosane Hphosphonate) (PPE23H), and poly-(octatetracontane H-phosphonate) (PPE48H). An increased crystalline order with increasing methylene chain length is suggested by the heats of fusion ΔHm and the melting points as observed by DSC (Table 1). Molecular weights of up to 4.3 × 104 g mol−1 were obtained in these polycondensation reactions according to GPC vs PS standards. NMR spectroscopic analysis of the end groups agrees reasonably well with GPC data for PPE19H and PPE23H, considering that GPC vs PS standards generally overestimates molecular weights for aliphatic polymers. DSC measurements clearly show that the melting temperatures increase with increasing alkylene chain lengths between the functional group. While PPE12H exhibits a peak melting temperature Tm of 38 °C, the Tm for PPE19H is already remarkably higher (76 °C). The highest peak melting temperature was observed for PPE48H with 98 °C/110 °C, where two melting peaks are observed. The 1H NMR spectrum for PPE12H displays the highest signal intensity for the functional group relative to the polymer backbone and was therefore taken as a reference for assignment. The shifts of the discussed signals are comparable for all the polymers independent of the length of the alkylene spacer. Other PPEXH polymers (with X naming the number of the carbon atoms of the diol monomer) only differ by the integrals of the polymer backbone. The structure of the polymers was confirmed by 2D 1H31P-HMBC spectroscopy (Figure 1). The most specific signal for the poly(H-phosphonate)s is the P−H resonance, observable at 6.74 ppm in the 1H NMR spectrum as a doublet with a coupling constant of 1JPH = ca. 690 Hz.

Figure 1. 1H31P NMR HMBC spectrum of poly(dodecane H-phosphonate) (PPE12H).

The phosphorus atom of the polymer backbone (signal at 7.72 ppm in 31P NMR spectrum) couples with this hydrogen atom as well as with the protons of the alkylene backbone (4.02 ppm; m) (Figure 1). The signal for the end group (P-OMe) species of low molecular weight poly-(H-phosphonate) appears at 9.11 ppm in the 31P NMR spectrum. 1H31PHMBC NMR analysis revealed the connectivity of this signal with the protons of the phosphonate end group (P-OMe) at 3.67 ppm (d, 3JPH = 11.96 Hz) (cf. Supporting Information). 31 P NMR spectroscopy of high molecular weight poly-(Hphosphonate) does not show these end group signals as they are too low in intensity. In 1H NMR spectroscopy, the hydroxy end group arising from the long chain diols appears at 3.59 ppm. The same type of end group has been described for literature known short chain PPEs. Therefore, we assume that the polycondensation follows literature known mechanism.23 A third end group is detectable at 3.28 ppm arising from a methyl ether end group. The methyl ether end group is the result of a side reaction of an alcoholate anion with a phosphorus methoxy end group (P-OMe) (Scheme 2a).22 Moreover, the alcoholate anion can also react with any phosphorus bound longchain diol (Scheme 2b). The ether groups were further investigated by 1H NMR spectroscopy. The ether signals are observable at 3.27−3.37 ppm (Figure 2). B


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starting material in order to obtain functionalized poly(phenylphosphonate)-s (Scheme 3). To find a suitable solvent and reaction temperature, PPE19H was cross-coupled with bromobenzene, and the conversion was monitored by 31P NMR spectroscopy (Table 2).

Scheme 2. Suggested Ether Formation Reactions during Polycondensation Reaction

Table 2. Hirao Cross-Coupling Reaction of PPE19H with Bromobenzene in Different Solvents at Different Temperaturesa entry

solvent

temperature [°C]

conversionb [%]

1 2 3 4 5 6 7 8 9

CH2Cl2 C2H4Cl2 C2H4Cl2 CH3CN CH3CN THF THF DMAc DMAc

40 68 90 68 90 40 68 68 120

4 0 0 5 25 7 100 60 100

a

Reaction conditions: PPE19H (200 mg, 0.58 mmol), bromobenzene (111 mg, 0.71 mmol), Pd(OAc)2 (3 mg, 0.01 mmol), dppf (16 mg, 0.03 mmol), KOAc (5 mg, 0.06 mmol), 10 mL solvent, Et3N (78 mg, 0.06 mmol), 24 h. bValues are determined by the integrals of the 31 1 P{ H} NMR (161 MHz, CDCl3, 25 °C) spectra of the crude reaction mixture.

In dichloromethane, only 4% of the P−H functional groups were converted to the desired P−Ph groups within 24 h (Table 2, entry 1). In 1,2-dichloroethane, no conversion was found at all (Table 2, entries 2, 3). Higher conversions were observed in acetonitrile, THF, and N,N-dimethylacetamide (DMAc) and in particular at elevated temperatures. For those reaction mixtures a less turbid suspension was observed, indicating a better solubility of all components in the corresponding solvent. Notwithstanding, in CH3CN only a maximum conversion of 25% at 90 °C was achieved. By contrast, full conversion of PPE19H to PPE19Ph within 24 h was obtained in THF at 68 °C and DMAc at 120 °C (Table 2, entries 7, 9). The optimized reaction conditions were applied in the functionalization of various crystalline poly(H-phosphonate)s with aryl bromides via Hirao cross-coupling. In all cases, full conversion to the corresponding poly(aryl-phosphonate)s was achieved. For the more soluble polymers PPE19H and PPE23H, the reaction was performed in THF at 68 °C. Because of the decreased solubility of PPE48H, the reaction was performed in DMAc at 120 °C in order to obtain full conversion. The progress of the reaction, namely the conversion of the H-phosphonate moieties toward the phenylphosphonate moieties, was monitored via 31P NMR spectroscopy (Figure 3). During the reaction, the signal of the H-phosphonate moiety in the backbone of PPE19H at 7.8 ppm decreases while the signal for the phenylphosphonate moiety observable at 18.7 ppm increases. The integrals of these signals directly yield the conversion of the phosphonate units in the backbone.

Figure 2. Detail of the 1H NMR spectrum (400 MHz, CDCl3, 25 °C) of PPE12H.

The methoxy end group (alkyl-OMe) is observable at 3.29 ppm. The CH2 groups adjacent to the oxygen of the two ether species appear as two overlapping triplets. The triplet at 3.32 ppm is assigned to the CH2 of the alkyl-OMe end group based on the signal intensity in comparison to the methoxy end group signal. Hirao Cross-Coupling. The trifunctional nature of the phosphorus moiety allows for the introduction of functional side chains. Such side chains can have a strong impact on the mechanical behavior as well as on the thermal properties.41 In order to investigate the influence of an aromatic moiety directly attached at the phosphoester functional group, the Hirao cross-coupling reaction was applied. To the best of our knowledge, this is the first postfunctionalization resulting in a fully functionalized poly(phenyl phosphonate). This provides access to semicrystalline poly(alkyl phenylphosphonate)s for the first time. In comparison to small molecule modifications, postpolymerization reactions have to deal with high viscosities as well as lower solubility of the polymer. In addition, full conversion is required in order to obtain a well-defined polymer with a homogeneous composition of the entire chain. Reaction conditions for the Hirao cross-coupling with bromobenzene35 were adapted to the low solubility of the polymeric

Scheme 3. Hirao Cross-Coupling of Poly(H-phosphonate)s with Aryl Bromides

C


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Figure 3. 31P{1H} NMR spectra (CDCl3, 25 °C, 161 MHz) of the crude reaction mixture during the conversion of PPE19H (top) to PPE19Ph (bottom) via Hirao cross-coupling after different time intervals.

Figure 4. 1H NMR spectra (C2D2Cl4, 120 °C, 400 MHz) of poly-(nonadecane H-phosphonate) (PPE19H) (top) and the resulting poly-(nonadecane phenylphosphonate) (PPE19Ph) after Hirao cross-coupling (bottom).

The characteristic P−H signal in 1H NMR disappeared whereas the signals of the polymer backbone do not shift. In addition, new aromatic signals appeared, which differ from signals of bromobenzene. Notably, during the experiment no decrease of molecular weight (Mn) was observed by NMR spectroscopy.

The connection of the phosphorus to the polymer backbone as well as to the new aromatic functionality were confirmed by 1 31 H P-HMBC spectroscopy (cf. Supporting Information). Complete conversion was also confirmed by 1H NMR spectroscopy (Figure 4). D


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Macromolecules Table 3. Molecular Weights and Thermal Properties of Different Poly(phenylphosphonate)sa entry 1 2 3 4

substrate PPE12H PPE19H PPE23H PPE48H

Mn,GPCb [g mol−1]

Mw/Mnb

Mn,NMRc [g mol−1]

Tmd [°C]

Tcd [°C]

ΔHmd [J g−1]

crystallinitye [%]

2.3 2.1

1.0 × 10 1.8 × 104 1.1 × 104

−40 41 62 103

−45 32 54 89

12 47 72 155

4 16 25 53

4

2.0 × 104 8 × 103

a

Reaction conditions: poly(H-phosphonate) (0.58 mmol), bromobenzene (111 mg, 0.71 mmol), Pd(OAc)2 (3 mg, 0.01 mmol), dppf (16 mg, 0.03 mmol), KOAc (5 mg, 0.06 mmol), 10 mL THF, Et3N (78 mg, 0.06 mmol), 68 °C, 24 h. bDetermined by GPC in THF (50 °C) versus polystyrene standards. cCalculated from 1H NMR spectroscopic analysis of the end groups. dDetermined by DSC with a heating/cooling rate of 10 K min−1. Peak Tm determined from the second heating cycle. eFrom DSC measurements, calculated vs 100% crystalline polyethylene (293 J g−1).

Table 4. Molecular Weights and Thermal Properties of Hirao Cross-Coupling Functionalized Polyphosphoesters (PPE19X)a

a Reaction conditions: PPE19H (200 mg), aryl halide (0.71 mmol), Pd(OAc)2 (3 mg, 0.01 mmol), dppf (16 mg, 0.03 mmol), KOAc (5 mg, 0.06 mmol), 10 mL THF, Et3N (78 mg, 0.06 mmol), 68 °C, 48 h. bDetermined by GPC in THF (50 °C) versus polystyrene standards. cCalculated from 1H NMR spectroscopic analysis of the end groups. dDetermined by DSC with a heating/cooling rate of 10 K min−1. Peak Tm determined from the second heating cycle. eFrom DSC measurements, calculated vs 100% crystalline polyethylene.

The molecular weights before and after the Hirao cross-coupling do not show any changes as analyzed by NMR spectroscopy. No degradation is observable. Nevertheless, molecular weights, measured by GPC vs polystyrene, show lower molecular weights after the functionalization indicating a modified interaction with the GPC column. The scope of the Hirao cross-coupling on PPE19H was further expanded, and additional side-chain functionalities were introduced by the application of different functionalized aryl halides (Table 4). Full conversion of the Hirao cross-coupling of all applied arylhalides with PPE19H toward the functionalized PPEs was achieved within 2 days. The functional group in para-position to the phosphonate only had a limited influence on the thermal properties of the polymers. Compared to PPE19Ph, which has a peak melting temperature at 41 °C, the new polymers have comparable melting points (35−42 °C) (Table 4). Also, the crystallinities are only little affected by the new functionalities, except for PPE19(PhCOOH) where the crystallinity decreased from 16% to 4% in comparison to PPE19Ph or PE19(PhCOOMe) (Table 4, entry 3 vs 4). Dynamic Mechanical Analysis. No glass transitions (Tg) were observable reliably in DSC even upon rapid cooling and heating. Dynamic mechanical analysis (DMA at a frequency of 1 Hz) was more conclusive to this end (Figure 5). PPE19Ph and PPE23Ph show a similar behavior in dynamic mechanical analysis (cf. Supporting Information), which is discussed here in detail for PPE23Ph. From the Tan Delta, a first transition (Tγ) is observable at −133 °C, which is attributed to the motion of the methylene sequences of the polyester main chain.42,43 The second transition at −76 °C is assumed to be a glass transition (Tg). A third transition Tα at −12 °C is observable, which is suspected to originate from

The poly(dodecane phenylphosphonate) (PPE12Ph) obtained is a colorless soft wax, whereas postfunctionalization of the long-chain spaced polymers PPE19H, PPE23H, and PPE48H resulted in solids of poly(nonadecane phenylphosphonate) (PPE19Ph), poly(tricosane phenylphosphonate) (PPE23Ph), and poly(octatetracontane phenylphosphonate) (PPE48Ph). An increased crystalline order with increasing methylene chain length is suggested by the heats of fusion ΔHm and the melting points observed by DSC (Table 3). Compared to the parent poly(H-phosphonate)s, the crystallinities as well as the peak melting temperatures are lower, except for PPE48Ph. For example, the peak melting temperatures decreases from 38 °C (PPE12H) to −40 °C for PPE12Ph after functionalization. A longer alkylene segment reflects in smaller differences in peak melting temperatures Tm of the poly(Hphosphonate)-s and the corresponding poly(phenylphosphonate)s. For the PPE19 polymer, the peak melting point decreases from 76 to 41 °C after functionalization, for PPE23 polymers from 90 to 62 °C. In both cases, also the melting enthalpies and therefore the crystallinities (calculated vs 100% crystalline PE) are decreasing from 109 J/g for PPE19H to 47 J/g for PPE19Ph and from 111 J/g for PPE23H to 72 J/g for PPE23Ph. These changes show the strong influence of the substituents at the phosphonate group on the thermal properties. The increase in steric demand causes the decrease in crystallinity most likely. The crystalline long chain polyphosphoesters with methylene sequences longer than C12 exhibit smaller changes due to the higher linear polyethylene like content. For the ultralong-chainspaced PPE48 polymers PPE48H and PPE48Ph the peak melting temperatures, as well as the crystallinities, are similar. Because of the relatively low functional group content, the hydrocarbon segments of the polymer are mainly responsible for the thermal behavior. E


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observable at 3.64 ppm in the 1H NMR spectra. This corresponds to a substantial degradation (for example, starting from a molecular weight of 1.5 × 104 g mol−1 (DPn = 42) a degradation to a nominal DPn = 2 occurred in 2 days). The degradation of PPE19H pellets in water could not be calculated by weight loss due to the complete decomposition of the polymer. After 22 h, the pellets had disintegrated, and after 2 days, no pellet parts were observed in the turbid suspension. An alternative for monitoring the degradation is provided by following the pH value. The hydrolytic cleavage of the phosphonate group forms a diol as well as phosphonic acid. Pellets of PPE19H and PPE48H were exposed to deionized water. The mass of the initial pellet provides the amount of functional phosphoester units per polymer pellet before degradation. Furthermore, from the decreasing pH value the amount of released phosphonic acid and therefore the amount of cleaved functional groups can be estimated. This data confirms that degradation occurs to a large extent (Figure 8). Full hydrolysis to phosphonic acid (pKa 1.5)45 should result in a pH of 2.3, a value which is observed after 2 days. PPE48H, featuring a more hydrophobic long-chain building block degraded slower than the shorter chain spaced PPE19H pellets (Figure 8), as expected.46 After 1 day, only small cracks were observable in the polymer specimen, and the polymer pellets did not disintegrate during the experiment. After 2 days, substantial degradation has occurred also (full conversion to phosphonic acid would result in pH 2.6; a pH of 2.9 is observed). Contrary to the relatively unstable poly(H-phosphonate)s the phenyl-functionalized polyphosphonates like PPE19Ph did not show any degradation under these conditions according to monitoring the pH value over 63 days. NMR spectroscopy of the pellets also confirmed that no degradation of the functional groups occurred. Analysis of the degradation by weight of PPE19Ph specimen in basic (2 M NaOHaq) and acidic media (2 M HClaq) revealed a slow degradation over long time periods (Figure 9). PPE19Ph seems to degrade slightly faster in basic than in acidic media. However, a degradation of about 0.5% is also observable in water and 2 M HClaq (this is on the limit of experimental accuracy). In summary, the degradation under basic, neutral, and acidic conditions is very slow. Comparative NMR analysis of the specimen before and after basic and acidic treatment did not show any degradation or changes of the polymer and their functional groups. Molecular weight estimation by GPC also did not reveal any decreasing molecular weights after treatment. Nevertheless, SEM images of the specimen surfaces before and after treatment indicate changes in the specimen surfaces (Figure 10). In an initial PPE19Ph specimen, the surfaces were rather smooth (Figure 10a). After acidic treatment, the specimen surface developed humps as well as cracks, observable by SEM (Figure 10b). Deionized water also had a similar effect on the polymer specimen surfaces (Figure 10c). In contrast, the polymer specimen show smaller and less cracks after objection to basic conditions, but the surface eroded to a higher roughness (Figure 10d). Conclusive Summary. Poly(H-phosphonate)s with (ultra)long-chain methylene repeating units are accessible conveniently via polycondensation of long-chain diols with dimethyl H-phosphonate. Molecular weights up to 4.3 × 104 g mol−1 were obtained. Other than short- and midchain congeners, these polymers are semicrystalline as a result of the “polyethylene-like” sequences. Postfunctionalization of these polymers via Hirao

Figure 5. Dynamic mechanical analysis (frequency of 1 Hz) of PPE23Ph.

molecular motions in the crystalline region.43,44 All three transitions are also observable in the loss modulus. Because of its brittle nature, DMA measurements of PPE48Ph were not conclusive. DMA measurements of PPE48H polymer revealed a Tγ transition at −124 °C. A broad transition at 0−20 °C is supposed to correspond to the Tα transition (cf. Supporting Information). Mechanical Characteristics. First tensile tests were performed on dogbone-shaped test specimen of PPE23Ph and PPE48H, prepared by piston injection molding. According to the stress−strain curve (cf. Supporting Information), the polyphosphoesters possess low elongations at break of 15% for PPE23Ph and 8% for PPE48H. PPE23Ph possess a lower E-modulus of 83 MPa in comparison to PPE48H with 157 MPa, indicating the higher rigidity of the more crystalline polymer. The stress at break amounted to 2.8 MPa for PPE23Ph and 3.0 MPa for PPE48H. Deformations of the PPE23Ph and PPE48H specimen (Figure 6) were reversible, and no breaking of the samples occurred.

Figure 6. Deformations of PPE48H (left) and PPE23Ph (right).

PPE48H is the first crystalline poly(H-phosphonate) showing a ductile behavior. It can be handled in air for a short time. After 24 h in air the specimen turned brittle due to hydrolytic degradation. Degradation Studies. Because of the hydrolytically unstable P−O−C bond, poly(H-phosphonate)s are rapidly degrading materials under humid conditions. The general acid or base catalyzed degradation mechanisms for poly(H-phosphonate)s are well studied.6−8 The degradation of PPE19H occurs already upon exposure of the polymer powder to air, as observed by NMR spectroscopy of the polymer (Figure 7). The degradation is reflected in the 1H NMR spectra by a decrease in intensity of the P−H signal at 6.80 ppm (d, 1JP−H = 693 Hz). In addition, the signal of the CH2 group adjacent to the phosphonate group (−P−O−CH2−) at 4.02 ppm, representative for the backbone, also decreases in intensity. At the same time, the signals of the CH2 group adjacent to the hydroxy end group (HO-alkyl) formed upon degradation, increase, F


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Figure 7. 1H NMR spectra (CDCl3, 25 °C, 400 MHz) of poly(nonadecane H-phosphonate) (PPE19H) before (top) and after (bottom) storage in air for 2 days.

Figure 8. pH values over time of PPE19H and PPE48H exposed to water (size of specimen: ≈0.1 cm × 0.4 cm × 1.0 cm; stirred in 25 mL of H2O at 35 °C with 80 rpm).

is possible. Exposure of the poly(H-phosphonate)s to water revealed rapid degradation, which is slowed significantly with an increasing hydrocarbon chain length. By comparison, the poly(phenylphosphonate)s from postpolymerization functionalization are essentially stable to hydrolysis.

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EXPERIMENTAL SECTION

Materials. Unless stated otherwise, all manipulations were carried out under an inert gas atmosphere using standard Schlenk or glovebox techniques. Dichloromethane was distilled over sodium. Dimethyl phosphonate was supplied from Aldrich and distilled in vacuo before use. Deuterated solvents were supplied by Eurisotop. High oleic sunflower oil methyl ester (92.5% of methyl oleate) supplied by Dako AG and methyl erucate (>90%) from TCI were degassed prior to use. Dodecane-1,12-diol was supplied by ACROS. Long-chain aliphatic diols were prepared according to refs 25 and 28. Pd(OAc)2 was supplied by mcat, and dppf was purchased from ABCR.

Figure 9. Weight loss over time of PPE19Ph in water (2 M HClaq and 2 M NaOHaq stirred in 20 mL of media at 35 °C with 80 rpm).

cross-coupling yields the corresponding poly(phenylphosphonate)s. Thereby, the introduction of various functional groups into the polymer backbone like −PhBr, −Ph−COOH, and −PhCOOMe G


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Macromolecules

Figure 10. SEM images of PPE19Ph specimen surfaces before (a) and after treatment with 2 M HClaq (b), water (c), and 2 M NaOHaq (d). NMR spectra were recorded on a Bruker Avance 400, a Bruker Avance 600, and a Bruker Avance 400 solid-state spectrometer. 1H and 13 C chemical shifts were referenced to the solvent signals. DSC analyses were performed on a Netzsch Phoenix 204 F1 instrument with a heating and cooling rate of 10 K min−1. Data reported are from the second heating cycles. GPC analyses were performed on a Polymer Laboratories GPC050 instrument with refractive index detection, equipped with two Mixed C columns in THF at 50 °C against polystyrene standards. Dynamic mechanical analyses (DMA) were recorded on melt compounded rectangular specimen (length × width × thickness = 25 × 6 × 2 mm3) using a Triton Technology TTDMA instrument equipped with single cantilever geometry. Measurements were performed from −100 °C or −150 °C to 60 °C or up to 120 °C, depending on the melting points of the samples, at a heating rate of 3 °C min−1 and a frequency of 1 Hz. The Triton Technology DMA software was used to acquire and process the data. Transition temperatures (Tγ, Tg, Tα) were determined from the temperature position of the maximum Tan Delta. The rectangular specimen was prepared by piston injection molding using a HAAKE Minijet II (Thermo Scientific) device. The cylinder temperature and the mold temperature were set to 160 °C and 30 °C for PPE19Ph, to 180 °C and 40 °C for PPE23Ph and PPE48Ph, and to 170 °C and 50 °C for PPE48H, respectively. Samples were injected with an injection pressure of 500 bar for 5 s and a postpressure of 200 bar for 5 s. Tensile testing was carried out with dogbone-shaped sample bars (75 × 12.5 × 2 mm3, ISO 527-2, type 5A) prepared by piston injection molding using a HAAKE Minijet II (Thermo Scientific) device. The cylinder temperature, mold temperature, and pressures were set similar to rectangular specimen preparation. In order to prevent oxidative degradation, the polymers were stabilized with 0.5 wt % of Irgafos 168 and 0.5 wt % of Irganox 1076. After preconditioning of the samples overnight at room temperature, tensile tests were performed on a Zwick 1446 Retroline tC II instrument according to ISO 527 (crosshead speed 5 mm min−1). The Zwick test Xpert software version 11.0 was used to collect and analyze the results. Young’s modulus, tensile stress at break, and elongation at break were obtained by averaging the data from two test specimens.

pH values were measured with a 691 pH meter device (Metrohm), equipped with a flat-membrane pH electrode (Metrohm, 6.0256.100). The electrode was calibrated regularly using three pH buffers (MettlerToledo). General Procedures. Poly-(H-phosphonate). The polymerization was carried out in a 100 mL two-necked Schlenk tube equipped with an overhead stirrer. Efficient mixing of the highly viscous polymer melt is achieved by using an agitator with a spiral mixing paddle. Dimethyl H-phosphonate (1.76 g, 15.99 mmol, 1.2 equiv) was added to the corresponding diol (13.33 mmol, 1.0 equiv) and sodium (0.02 g, 0.87 mmol, 0.07 equiv). The reaction mixture was stirred at 120 °C for 4 h and for 24 h at 160 °C and 0.1 bar. After cooling to 40 °C, the polymer was dissolved in dry DCM and precipitated in dry MeOH. The solvent was decanted, and the pure white solid polymer was dried in vacuo. Hirao Cross-Coupling of Poly(H-phosphonate)s. Pd(OAc)2 (3.5 mg, 15 μmol, 0.03 equiv), dppf (17 mg, 30 μmol, 0.06 equiv), and KOAc (6 mg, 60 μmol, 0.13 equiv) were dissolved in 4 mL of THF. After 5 min of stirring, triethylamine (110 μL, 770 μmol, 1.50 equiv) was added to the catalyst solution. Poly-(H-phosphonate) (577 μmol, 1.12 equiv) dissolved in 4 mL of THF at 68 °C and the aromatic compound (711 μmol, 1.38 equiv) dissolved in 2 mL of THF were added to the catalyst solution after an additional 10 min. The reaction mixture was stirred for 1 day at 68 °C before the crude reaction mixture was poured in MeOH. After centrifugation, the polymer was obtained as a white solid. PPE48Ph was synthesized analogous to this procedure in DMAc at 120 °C. Large scale synthesis of PPEXPh. Pd-(OAc)2 (101 mg, 0.5 mmol, 0.03 equiv), dppf (499 mg, 0.9 mmol, 0.06 equiv), and KOAc (175 mg, 1.8 mmol, 0.13 equiv) were dissolved in 120 mL of THF. After 5 min of stirring, Et3N (3.1 mL, 22.5 mmol, 1.50 equiv) was added to the catalyst solution. Poly-(H-phosphonate) (17.3 mmol, 1.12 equiv) dissolved in 60 mL of THF at 68 °C and bromobenzene (21.3 mmol, 1.38 equiv) were added to the catalyst solution after an additional 10 min. The reaction mixture was stirred for 2 days at 68 °C. The crude reaction mixture was poured in MeOH. After filtration, the polymer was obtained as white solid. H


Article

Macromolecules Degradation Experiments. Poly(H-phosphonate)s. Pellets (0.1 cm × 0.4 cm × 1.0 cm, m ≈ 40 mg) of long-chain aliphatic polyphosphoesters were prepared (injection molding using a minicompounder) and exposed to 25 mL of deionized water (pH ≈ 6) at 35 °C (stirring with 80 rpm). pH values were measured after the aforementioned time intervals. Poly(phenylphosphonate)s. Pellets (0.1 cm × 0.4 cm × 1.0 cm, m ≈ 40 mg) of long-chain aliphatic polyphosphoesters were prepared (injection molding using a mini-compounder) and exposed to 20 mL of acidic, basic, or aqueous media at 35 °C (stirring with 80 rpm). Prior to weighing, the specimens were washed briefly with water and acetone and dried in vacuo.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01368. Characterization data of polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.M.). ORCID

Hanna Busch: 0000-0002-4684-727X Stefan Mecking: 0000-0002-6618-6659 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Stiftung Baden-Württemberg is gratefully acknowledged. DSC and GPC measurements were kindly performed by Lars Bolk. Access to pH-titration equipment was provided by the Cölfen group.

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REFERENCES

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Ultralong-Chain-Spaced Crystalline Poly(H-phosphonate)s and Poly

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