Semicrystalline Long-Chain Polyphosphoesters from Polyesterification

Article pubs.acs.org/Macromolecules

Semicrystalline Long-Chain Polyphosphoesters from Polyesterification Hanna Busch,† Sumit Majumder,‡ Günter Reiter,‡ and Stefan Mecking*,† †

Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78457 Konstanz, Germany Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, 79104 Freiburg, Germany

‡

S Supporting Information *

ABSTRACT: Semicrystalline aliphatic polyphosphoesters can be obtained in a one-step approach by polyesterification of readily available bio-based long-chain diols with dichlorophosphorus compounds. Nonadecane-1,19-diol and tricosane-1,23-diol with respectively methylphosphonic dichloride or phenyl dichlorophosphate yield polymers (PPE19Me, PPE23Me, PPE19(OPh), and PPE23(OPh)) with molecular weights Mn up to 3 × 104 g mol−1. DSC analysis of these polymers and a C12 analogue reveal significantly enhanced crystallinities and melting points (up to Tm = 87 °C) with increasing methylene sequence length. DMA on injection-molded samples shows glass transitions at −19 °C (PPE19Me) and −12 °C (PPE23Me). Single crystals of PPE19Me accommodate a single C19 repeat unit, as concluded from a lamella thickness of only 3 nm thick determined by AFM. Hydrolytic degradation of solid polymer samples under ambient conditions occurred only to a minimal extent over three months by hydrolysis of very small amounts of in-chain anhydride defects.

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INTRODUCTION Long-chain aliphatic polycondensates differ from their established shorter chain congeners in that their solid state structures are not dominated by the functional groups, but rather reflect the van der Waals interactions between the long methylene sequences.1 In this sense, they can be considered “polyethylenelike”. Notwithstanding, the functional groups in the chain will significantly impact properties like e.g. the degradability of the chain and interactions with polar surfaces. A particular group of interest here are phosphoesters. Polyphosphoester (PPE) materials in general have been studied among others due to their flame retardancy, degradability, and biocompatibility.2 The latter can be related to their similarity to nucleic acids in terms of the phosphate motifs.3 Hydrolytic cleavage of the phosphoester moiety can bring about degradation of the polymers.4−11 The trifunctional nature of the P atoms in the chain allows for the attachment of versatile side chains on a linear polymer backbone. These extensive studies have largely addressed shorter aliphatic repeat units or water-soluble polyphosphoesters. Long-chain aliphatic PPEs have been reported recently by Wurm et al. They were prepared by acyclic diene metathesis (ADMET) polymerization of a specific monomer prepared for these purpose, followed by exhaustive postpolymerization hydrogenation.12−15 This differs from the more established convenient preparation of polyphosphoesters by polycondensation of diols with dichlorophosphorus compounds.3,16 Thus, poly(arylene alkyl- or arylphosphonate)s and poly(arylene phosphate)s are com© XXXX American Chemical Society

monly synthesized by polycondensation of the corresponding RP(O)Cl2 dichlorophosphorus compounds with aromatic diols.17−22 These polymerization reactions are conducted either in two-phase systems20−30 or in the bulk.19,31 Polymerizations of dichlorophosphorus compounds with diols in chlorinated organic solvents have also been reported,17,18 but notably, despite their straightforward single step nature, such polyesterifications have not been delineated for long-chain aliphatic α,ω-diols. One possible reason is the limited availability of these monomers, which has been overcome only more recently by novel catalytic conversions of fatty acids. For example, long-chain α,ω-diols are accessible by isomerizing alkoxycarbonylation of fatty acids and subsequent catalytic hydrogenation. Nonadecane-1,19-diol and tricosane-1,23-diol are obtained by this approach from high oleic sunflower oil or erucic acid ester, respectively, in high purity.32,33 Also, compared to the ADMET synthesis demonstrated for poly(icosamethylphosphonate) (PPE20Me),12 poly(icosaphenylphosphate) 13,14 (PPE20(OPh)), and poly(icosachlorophosphonate)15 (PPE20Cl), a polyesterification of long-chain diols will be subject to other restrictions in terms of side reactions and achievable PPE molecular weights. We now report semicrystalline long-chain aliphatic poly(alkylene phenylphosphate)s and poly(alkylene methylReceived: February 2, 2017 Revised: March 8, 2017

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DOI: 10.1021/acs.macromol.7b00243 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Aliphatic Long-Chain Polyphosphonates via Polycondensation

Figure 1. 1H NMR spectrum (400 MHz, CDCl3, 25 °C) of poly(nonadecane methylphosphonate) (PPE19Me) after work-up by precipitation in methanol.

Figure 2. 31P{1H} NMR spectra (161 MHz, CDCl3, 25 °C) of poly(nonadecane methylphosphonate) (PPE19Me).

dures of aromatic diols with dichlorophosphorus compounds in chlorinated organic solvents as a starting point, different polymerization conditions were evaluated (cf. Supporting Information). Utilization of pyridine as a base in dichloromethane at 60 °C resulted in the highest conversions in the polycondensation reaction. These conditions were found to be suitable for the polycondensation reaction of long-chain nonadecane-1,19-diol and tricosane-1,23-diol with methylphosphonic dichloride and phenyl dichlorophosphate (Scheme 1).

phosphonate)s obtained in a single-step polyesterification procedure and their properties.

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RESULTS AND DISCUSSION For comparison of the reactivity and properties of the resulting polymers with material based on the long-chain diols of interest, the mid-chain length analogue dodecane-1,12-diol was also investigated. Methylphosphonic dichloride and phenyl dichlorophosphate were employed as the second monomer component. Based on the reported polycondensation proceB


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Macromolecules Table 1. Polycondensation of Methylphosphonic Dichloride with Different α,ω-Diolsa entry 1 2 3

α,ω-diol

Mn,GPCb [g mol−1]

1.12 1.19 1.23

1.1 × 10 2.1 × 104 3.1 × 104 4

Mw/Mnb

Mn,NMRc [g mol−1]

Tmd [°C]

Tcd [°C]

ΔHmd [J g−1]

crystallinitye [%]

1.7 2.0 1.7

1.0 × 10 2.9 × 104 2.5 × 104

13 73 87

1; 7 62 78

75 100 116

26 34 40

4

Reaction conditions: α,ω-diol (18.1 mmol), methylphosphonic dichloride (18.1 mmol), pyridine (39.8 mmol), 65 mL of CH2Cl2, 60 °C, 24 h. Determined by GPC in THF (50 °C) versus polystyrene standards. cCalculated from 1H NMR spectroscopic analysis of the end groups. d Determined 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 293 J g−1 for 100% crystalline polyethylene. a b

Table 2. Polycondensation of Phenyl Dichlorophosphate with Different α,ω-Diolsa entry 1 2 3

α,ω-diol

Mn,GPCb [g mol−1]

1.12 1.19 1.23

4 × 10 7 × 103 1.5 × 104 3

Mw/Mnb

Mn,NMRc [g mol−1]

Tmd [°C]

Tcd [°C]

ΔHmd [J g−1]

crystallinitye [%]

1.9 1.5 1.8

8 × 10 1.0 × 104 1.4 × 104

−29 46 63

−38 36 55

30 88 90

10 30 31

3

Reaction conditions: α,ω-diol (16.6 mmol), phenyl dichlorophosphate (16.6 mmol), pyridine (36.6 mmol), 64 mL of CH2Cl2, 60 °C, 24 h. Determined by GPC in THF (50 °C) versus polystyrene standards. cCalculated from 1H NMR spectroscopic analysis of the end groups. d Determined 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 293 J g−1 for 100% crystalline polyethylene. a b

Molecular weights of up to 3.1 × 104 g mol−1 were obtained in these polycondensation reactions according to GPC vs polystyrene standards. These apparent molecular weights agree reasonably with molecular weights determined by 1H NMR spectroscopic end-group analysis (Table 1). From the polycondensations with phenyl dichlorophosphate (Table 2), poly(dodecane phenylphosphate) (PPE12(OPh)) was obtained as a colorless oil. By contrast, poly(phenylphosphate)s PPE19(OPh) and PPE23(OPh) from the long-chain diols nonadecane-1,19-diol and tricosane-1,23diol are semicrystalline solids. Compared to the analogous poly(methyl phosphonates), the effect of methylene chain lengths on the melting temperature is even more dramatic, with a Tm of −29 °C for the C12 polyphosphoester PPE12(OPh) vs 63 °C for the C23 analogue PPE23(OPh). In contrast to the corresponding poly(methylphosphonate)s (PPEXMes), the crystallinities as well as the peak melting temperatures are lower. The bulkier phenylphosphate moiety affects the crystallization of the PE like long-chain alkylene moieties between the functional groups more than the methyl phosphonate functionality (Table 2), as also discussed by Wurm and co-workers for the example of PPE20Me and PPE20(OPh)39 Notably, the peak melting temperatures of PPE19Me (Tm = 73 °C) and PPE19(OPh) (Tm = 46 °C) exceed those reported for PPE20Me (Tm = 64 °C12) and PPE20(OPh) (Tm = 44 °C13) from ADMET polymerization and hydrogenation. While it can be noted that odd−even effects on the melting points are frequently observed for polycondensates with various different types of interchain interaction,1 the explicit origin of these different melting points at this point remains unclear. Nothwithstanding, this data shows that the different types of in-chain defects and also end groups resulting from the different polycondensation mechanisms, polyesterification vs previously studied ADMET, are not reflected in an inferior crystalline order or thermal properties of the long-chain polyphosphoesters. Molecular weights of (Mn) of 7.4 × 103 and 1.5 × 104 g mol−1 (GPC vs PS) were determined for PPE19(OPh) and PPE23(OPh), respectively (Table 2). In comparison to the poly(methylphosphonate)s (PPEXMes), the degrees of polymerization of PPEX(OPh)s are lower. This can be related to the higher reactivity of methylphosphonic vs phenyl dichlor-

Upon work-up of the crude polymer mixtures in dry methanol, the phosphorus chloride end groups (P−Cl) are converted to the corresponding methoxy end groups (P-OMe), observable by 1H NMR spectroscopy at δ = 3.72 ppm for poly(methylphosphonate)s (PPEXMe) (d, 2JPH = 11.0 Hz) (Figure 1) and at δ = 3.84 ppm for poly(phenylphosphate)s (PPEX(OPh)) (d, 2JPH = 11.0 Hz). The hydroxy alkyl end group signal (HO−CH2−) arising from the long-chain diol appear at δ = 3.62 ppm. As a side reaction occurring to a small extent, alkyl chloride is also formed34 which results in chloro alkyl end groups (Cl−CH2−) that give rise to a triplet at δ = 3.52 ppm in the 1H NMR spectra of PPEXMe and PPEX(OPh). The phosphorus hydroxide species (P−OH) nominally formed along with alkyl chloride further reacts with a phosphorus chloride species to the P−O−P anhydride group, which occurs to a small extent in the polymer backbone.35 This results in the formation of three different stereoisomers: the meso-anhydride as well as the two racemic anhydrides. These species are observable at δ = 22.6 and δ = 22.3 ppm for PPEXMe36,37 and at δ = −19.1 ppm and δ = −19.3 ppm for PPEX(OPh)38 by 31P{1H} NMR spectroscopy (Figure 2). The virtually identical integrals of these two signals agree with a 1:1 ratio of the rac- and meso-anhydride formed. The singlet nature of the two resonances was also confirmed by acquisition of spectra on instruments with different fields (31P resonance frequencies of 161 and 242 MHz). The connectivity of these anhydride species to the polymer backbone was confirmed by 1H 31P-HMBC spectroscopy (cf. Supporting Information). For PPEX(OPh), the P−OH end groups are also observed at −0.3 ppm in 31P NMR spectroscopy. The P−OMe end groups are observed at δ = −5.1 ppm for PPEX(OPh) and at δ = 31.6 ppm only for low molecular weight PPEXMe’s. The poly(dodecane methylphosphonate) (PPE12Me) obtained is a colorless soft wax, whereas polycondensation of the long-chain aliphatic α,ω-diols resulted in solids of poly(nonadecane methylphosphonate) (PPE19Me) and poly(tricosane methylphosphonate) (PPE23Me). 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 1). C


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Figure 3. Dynamic mechanical analysis (frequency of 1 Hz) of poly(nonadecane methylphosphonate) (PPE19Me) (left) and poly(tricosane methylphosphonate) (PPE23Me) (right).

maximum of −71 °C from the Tan Delta and −82 °C from the loss modulus.

ophosphate, leading to a higher conversion and therefore higher molecular weights.

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ORDERING IN SINGLE CRYSTALS First insights into the solid-state structure of PPE19Me from polyesterification were obtained on a crystallized thin film of PPE19Me. A solution-cast thin film of polymer was molten and then crystallized isothermally slowly. The formed crystals were analyzed by atomic force microscopy (AFM) (Figure 5). Remarkably, these crystals feature distinct terraces of 3 nm height. This equals the expected length of the stretched methylene segments between the functional phosphoester groups. The number of the lamellar crystals stacking on top of each other was dependent on the time of crystallization, the film thickness, and crystallization temperature. The thickness of these single crystals corresponds to lamella dimensions in bulk samples of PPE20Me from ADMET/hydrogenation observed by Lieberwirth et al. via TEM with staining.39 The narrow melt transitions (cf. Supporting Information) of the long-chain polyphosphoesters PPE19 and PPE23 in the second heating cycle also agree with a uniform ordered state of these polymers in the bulk.

DYNAMIC MECHANICAL ANALYSIS (DMA)

No glass transitions were observable reliably in DSC (vide supra) even upon rapid cooling and heating. Dynamic mechanical analysis (DMA at a frequency of 1 Hz) was more conclusive to this end (Figure 3). For PPE19Me, a glass transition temperature (Tg) of −19 °C was observed from the maximum of the Tan Delta. In the loss modulus, the glass transition is not observable. A maximum at −82 °C of the loss modulus indicates another transition (Tγ). The latter transition can be assigned to the motion of the long methylene sequences of the polyphosphoester main chain.40,41 In comparison to PPE19Me, the longer chain PPE23Me exhibits a slightly higher glass-transition temperature of −12 °C (PPE19Me: −19 °C) as observed from the maximum of the Tan Delta. This difference can be attributed to the longer methylene sequences and the resulting stronger restriction in mobility of PPE23Me (Figure 3).42 The low molecular weight and brittle nature of PPE19(OPh) hampered DMA analysis of this polymer, but DMA data could be acquired for PPE23(OPh) (Figure 4). A glass transition (Tg) was observed from the maximum of the Tan Delta trace (−12 °C) as well as in the loss modulus (−20 °C). Both traces also show a second transition (Tγ) at the

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DEGRADATION OF PPE19Me and PPE19(OPh) Polyphosphoesters can degrade in aqueous media by hydrolysis of P−O−C bonds.9−11 In particular, hydrolysis of poly(phosphate)s occurs at the end group, the side chain, and the backbone phosphate functional groups.6 To evaluate the hydrolytic degradability of the materials prepared, compact pellets (0.1 cm × 0.4 cm × 1.0 cm, m ≈ 40 mg) of the C19 spaced polyphosphoesters PPE19Me and PPE19(OPh) were placed in water over several months at constant stirring and temperature in order to obtain more insights into the degradation rates as well as degradation mechanism of the new PPEs. 1 H NMR analysis of the specimens (PPE19Me and PPE19(OPh)) after this aqueous treatment in comparison to the initial specimens revealed no changes of the intensities of the P−OMe end groups at δ = 3.72 ppm relative to the polymer backbone, indicating that this end group is not degraded preferentially (Figure 6). In addition, no degradation of the alkyl chloride end group (Cl−alkyl) at δ = 3.51 ppm is evident. In contrast, the hydroxy alkyl end group (HO−alkyl) at δ = 3.62 ppm increases in intensity, suggesting a hydrolytic cleavage in the chain.

Figure 4. Dynamic mechanical analysis (frequency of 1 Hz) of poly(tricosane phenylphosphate) (PPE23(OPh)). D


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Figure 5. AFM image of a PPE19Me crystal stack (left) and height profile thereof (right), formed by isothermal crystallization at 68 °C from a polymer thin film (15 nm) for 14 h.

Figure 6. 1H NMR spectra (400 MHz, CDCl3, 25 °C) of poly(nonadecane methylphosphonate) (PPE19Me) before (top) and after (bottom) three months in water.

Figure 7. 31P{1H} NMR spectra (161 MHz, CDCl3, 25 °C) of poly(nonadecane phenylphosphate) (PPE19(OPh)) before (top) and after (bottom) degradation.

E


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Macromolecules Scheme 2. Proposed Degradation Mechanism of PPE19Me and PPE19(OPh) in Water

In the corresponding 31P NMR spectra for PPE19Me and PPE19(OPh), the signals for the P−O−P anhydride bonds disappear, indicating the cleavage of these functionalities. Since in the 31P NMR spectra no new P−OH end group signals are observed for PPE19Me, hydrolysis presumably proceeds to free acid and an HO−alkyl end group. This is also underlined by the disappearance of a minor signal observed at δ = −0.29 ppm in the initial 31P NMR spectra of PPE19(OPh) (Figure 7), assumed to originate from a P−OH end group. No new signals appear in the 31P NMR spectra of PPE19(OPh) and PPE19Me; thus, it can be further concluded that no side-chain degradation to new in-chain polyphosphoester functionalities occurs. Overall, this data suggests that chain degradation under the conditions studied occurred exclusively via the hydrolytic cleavage of anhydride groups, present in small amounts in the chains. This results in shorter HO−alkyl-terminated chains, along with free low-molecular-weight acids (Scheme 2). Indeed, a slight decrease in pH of the aqueous phase was observed over the course of these experiments (Figure S1 in the Supporting Information; pKa = 2.19 and 7.5443 for methylphosphonic acid and pKa = 1.42 and 5.8544 for phenylphosphate). This degradation results in roughly one cleavage event per polymer chain on average. An increasingly brittle nature of the specimens was observed over the time of the degradation experiments. This hampered a meaningful monitoring of their weight due to partial disintegration upon handling.

melting points are much higher. This effect of chain length is particularly pronounced compared to polycondensates with other, more compact functional groups like esters or acetals. Exposure of these semicrystalline rather hydrophobic solids to water over extended periods of time resulted in a degradation to a limited extent via chain cleavage at the small number of anhydride defects.

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MATERIALS AND METHODS

Materials. All manipulations were carried out under an inert gas atmosphere using standard Schlenk or glovebox techniques, unless stated otherwise. Dichloromethane was distilled over sodium. Methanol was distilled from magnesium turnings. Phenyl dichlorophosphate supplied by Alfa Aesar was distilled in vacuo before use. Dimethyl methylphosphonate was supplied by ABCR and used as received. Dodecane-1,12-diol was supplied by ACROS. High oleic sunflower oil methyl ester (92.5% of methyl oleate) was kindly donated by Dako AG, and methyl erucate (>90%) was purchased from TCI. Long-chain aliphatic diols were prepared according to ref 32. Methylphosphonyl dichloride was prepared according to ref 45. Deuterated solvents were supplied by Eurisotop. NMR spectra were recorded on a Bruker Avance 400/600 or on a Varian Inova 400 spectrometer. 1H and 13C 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 second heating cycles. GPC analyses were performed on a Polymer Laboratories GPC50 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 specimens (length × width × thickness = 25 × 6 × 2 mm3) using a Triton Technology TTDMA instrument equipped with single cantilever geometry. Measurements were performed from −100 to 60 and 40 °C (depending on nominal melting point) at a heating rate of 3 K min−1 and a frequency of 1 Hz. The Triton Technology DMA software was used to acquire and process the data. Glass transition temperatures (Tg) were determined from the temperature position of the maximum in Tan Delta. pH values were measured with a 691 pH meter device (Metrohm), equipped with a flat-membrane pH electrode (Metrohm, 6.0256.100).

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CONCLUSIVE SUMMARY Polyphosphoesters with long-chain methylene repeat units are accessible conveniently via polycondensation of long-chain diols with dichlorophosphorus compounds RP(O)Cl2 (R = Me or OPh). Molecular weights of up to Mn 3 × 104 g mol−1 are obtained, and the chains contain a very small amount of anhydride defects. Single crystals of these novel polycondensates comprise a single long-chain hydrocarbon repeat unit, as concluded from their thickness of only 3 nm. Compared to a shorter chain C12 analogue, the degree of crystalline order and F


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The electrode was calibrated regularly using three pH buffers (MettlerToledo). Methods. General Procedure for the Synthesis of Polyphosphonates (PPEXMe). α,ω-Diol (18.1 mmol) was suspended in 60 mL of CH2Cl2 containing 3.22 mL of pyridine (39.8 mmol) at 60 °C. Methylphosphonyl dichloride (2.40 g, 18.1 mmol) dissolved in 4 mL of CH2Cl2 was added, and the reaction was stirred at 60 °C for 22 h. For PPE19Me and PPE23Me, the reaction mixture was poured in 800 mL of dry methanol, upon which the product precipitated. The polymer was filtrated off and washed with 800 mL of methanol to yield the pure polymers as white solids. In the case of PPE12Me, the reaction mixture was stirred with 20 mL of dry MeOH for 15 min at 60 °C and extracted with water. The phases were separated, and the organic phase was dried over MgSO4. After evaporation of the solvents, PPE12Me was obtained as a white soft wax. General Procedure for the Synthesis of Polyphosphates (PPEX(OPh)). α,ω-Diol (16.6 mmol) was suspended in 60 mL of CH2Cl2 containing 2.96 mL of pyridine (36.6 mmol) at 60 °C. Phenyl dichlorophosphate (3.51 g, 16.6 mmol) dissolved in 4 mL of CH2Cl2 was added, and the reaction was stirred at 60 °C for 22 h. For PPE19(OPh) and PPE23(OPh), the reaction mixture was poured in 800 mL of dry methanol upon which the product precipitated. The polymer was filtrated off and washed with 800 mL of methanol to yield the pure polymers as white solids. In the case of PPE12(OPh) the reaction mixture was stirred with 20 mL of dry MeOH for 15 min at 60 °C and extracted with water. The phases were separated, and the organic phase was dried over MgSO4. After evaporation of the solvents, PPE12(OPh) was obtained as a milky white oil. Crystallization Experiments. Spin-coating of a 0.1−1 wt % solution of PPE19Me in THF resulted in polymer thin films with film thickness of 15−35 nm. These polymer films were heated to 120 °C for 20 min, cooled to 72.7 °C with a cooling rate of 1 K min−1, and crystallized at a constant temperature of 68−72.7 °C for 4−24 h. Degradation Experiments. 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 25 mL of deionized water (pH ≈ 6) at 35 °C (stirring with a magnetic stir bar with 80 rpm) for a given period of time.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00243. Polymer degradation and characterization data of polymers (PDF)

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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REFERENCES

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ACKNOWLEDGMENTS

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


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Semicrystalline Long-Chain Polyphosphoesters from Polyesterification

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