RAFT copolymerization of a phosphorus-containing monomer with α-hydroxy phosphonate and methyl methacrylate

Abstract

Phosphorus-containing polymers have been found wide applications in many fields. In this work, a new kind of phosphorus-containing monomer with α-hydroxy phosphonate 4-((diethoxyphosphoryl)(hydroxy)methyl)phenyl methacrylate (PHMA) was firstly synthesized, and then copolymerization of PHMA and methyl methacrylate (MMA) was carried out via reversible addition–fragmentation chain transfer (RAFT) polymerization. The copolymerization kinetics and successful chain-extension reaction using the resultant PPHMA-co-PMMA confirmed the features of “living”/controlled radical polymerization. According to the thermal gravimetric analyzer (TGA) test, the thermal stability of the resultant phosphorus-containing copolymers increased significantly compared to the conventional methacrylate polymer materials due to the attachment of the phosphonate groups into the side chain of the copolymers. In addition, its superior flame-retardant performance was verified by the micro-scale combustion calorimetry (MCC) test. Meanwhile, the hydrophilicity of the phosphorus-containing copolymers was reflected by water contact angle measurement.

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1. Introduction

Phosphorus-containing polymers functionalized either at the main chain (e.g., polyphosphazene, polyphosphoesters) or at the side chain have found wide applications in many fields,¹ such as dental application,² biomedical applications,³ flame retardancy,⁴ proton conducting phosphonated polymers and membranes for fuel cells.⁵ This triggers more attention for various phosphorus-type monomers and related polymers from researchers. Generally speaking, the syntheses of phosphorus-containing polymers include the following different ways: (i) direct (co)polymerization of phosphorus-based monomers and (ii) post-modification of polymers using phosphorus-based groups.

On the other hand, most of phosphorus-containing monomers reported in previous literatures are common phosphonate monomers⁶ such as 2-methacryloyloxyethyl phosphorylcholine (MPC),⁷ dimethyl(methacryloyloxymethyl) phosphonate (DMM AMP),⁸ 4-(bis(diethoxyphosphoryl)methyl)phenyl methacrylate (BPMA),⁹ 10-(N-methylacrylamido)-decylphosphonic acid and 10-(methacryloyloxy)-decylphosphonic acid.¹⁰ In order to obtain controlled structure or molecular weights for the phosphorus-based polymers, different “living”/controlled radical polymerization (LRP) techniques were introduced to polymerize phosphorus-based monomers, such as nitroxide-mediated polymerization (NMP),¹¹ atom transfer radical polymerization (ATRP),^7,8,12 and reversible addition–fragmentation chain transfer (RAFT) polymerization.^9,13

It is well known that α-hydroxy phosphonates have important physiological activity.¹⁴ Some of the enzymes (α-hydroxy phosphonic acid derivatives) in the human body have the ability to inhibit the activity, also have the ability to anti-virus and anti-cancer.¹⁵ In addition, because it is easy to be biodegradable, it has a great application prospect in the field of medicine. Unfortunately, there have been only few examples of direct polymerization of α-hydroxy phosphonates monomers.¹⁶ For example, Heath et al. reported a kind of allyl monomers with hydroxyl-functionalized phosphonic acids in a patent.^16a Margel et al. reported a kind of novel hydroxy-bisphosphonate vinylic monomers, polymers and particles that have utility as biologically active molecule (such as drugs) carriers, medical device coatings and for imaging/radiology applications, especially in bone and dental applications.^16b,16c

In this work, we firstly synthesized a novel phosphorus-containing monomer with α-hydroxy phosphonate (4-((diethoxyphosphoryl)(hydroxy)methyl)phenyl methacrylate, abbreviated PHMA). Then the RAFT copolymerization of PHMA and methyl methacrylate (MMA) was investigated. The copolymerization kinetics and the performance (e.g., flame-retardant performance and hydrophilicity) of the obtained copolymers were also studied in detail.

2. Experimental section

2.1. Materials

Methacryloylchloride (95%) was purchased from Aladdin. Diethyl phosphate (99%) was purchased from Energy Chemical (Shanghai, China). Azobisisobutyronitrile (AIBN) was purchased from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China) and was purified by recrystallization. Methyl methacrylate (MMA, +99%) was purchased from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China) and the inhibitor was removed by passing through a neutral alumina column. Potassium phosphate tribasic trihydrate (analytical reagent), p-hydroxybenzaldehyde (CP), tetrahydrofuran (analytical reagent), anisole (chemically pure), and calcium chloride anhydrous (analytical reagent) were obtained from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China) and used. The RAFT agent 2-cyanoprop-2-yl 1-dithionaphthalate (CPDN) was prepared according to the method reported by the literature,¹⁷ and the detailed procedure showed in ESI.† Triethylamine (analytical reagent), methylene chloride (analytical reagent), petroleum ether (analytical reagent), ethyl acetate (analytical reagent), N,N-dimethylformamide (DMF, analytical reagent), ethanol absolute (analytical reagent), and toluene (analytical reagent), sodium sulfate (analytical reagent) were purchased from Chinasun Specialty Products Co. Ltd. And used as received unless mentioned.

2.2. Synthesis of phosphorus-containing monomer 4-((diethoxyphosphoryl)(hydroxy)methyl)-phenyl methacrylate (PHMA)

The synthetic route of PHMA is shown in Scheme 1. Details are as follows: methacryloyl chloride (8.2 g, 78.0 mmol) in dichloromethane (15.0 mL) was added within 0.5 h to a solution of p-hydroxybenzaldehyde (9.5 g, 80.0 mmol) and triethylamine (20.2 g, 200.0 mmol) in methylene chloride (60.0 mL). After the reaction mixture had been stirred at room temperature for 24 h, the crude product solution was first filtered to remove ammonium salt, diluted with 50 mL methylene chloride, then washed with water (2 × 100 mL) and extracted with methylene chloride (2 × 100 mL), collection of preserving organic layer. The organic layer was dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure and the residue was purified by flash silica gel chromatography (petroleum ether : ethyl acetate = 5 : 1, v/v) to give benzaldehyde methacrylate (7.9 g, 52.9%) as a white solid. ¹H NMR (CDCl₃, 300 MHz), δ ppm: 2.08 (s, 3H), 5.82 (s, 1H), 6.39 (s, 1H), 7.30–7.34 (d, 2H), 7.92–7.96 (d, 2H), 10.01 (s, 1H).

Scheme 1 Synthetic route of phosphorus-containing monomer PHMA.

Next, to a stirred mixture of an aromatic aldehyde (4-formylphenyl methacrylate) and diethyl phosphonate (the molar ratio of the two chemicals was 1 : 1.2) was added potassium phosphate tribasic trihydrate (5 mol%) and stirring continued. Upon completion of the reaction (TLC), the solution has become sticky or frozen, then methylene chloride was added. After stirring for 5 min to obtain a crude product solution, firstly filtered to remove insoluble impurities, then 50 mL of methylene chloride was added to the resultant solution, washed with water (2 × 100 mL) and extracted with methylene chloride (2 × 100 mL), collection of the preserving organic layer. The organic layer was dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure and the residue was purified by flash silica gel chromatography (petroleum ether : ethyl acetate = 2 : 1, v/v) to give the final product PHMA (6.6 g, 45.1%) as a white solid. ¹H NMR (CDCl₃, 300 MHz), δ ppm: 1.14–1.23 (t, 6H), 2.01 (s, 3H), 1.99 (s, 3H), 3.89–4.13 (m, 4H), 4.93–4.97 (d, 1H), 5.69 (s, 1H), 6.27 (s, 1H), 6.43 (s, 1H), 7.05–7.08 (d, 2H), 7.43–7.47 (dd, 2H).

2.3. Typical procedures for RAFT polymerization

The RAFT copolymerization of MMA and phosphorus-containing monomer with α-hydroxy phosphonate (PHMA) was conducted using CPDN as the RAFT agent and AIBN as the thermal initiator. A typical polymerization procedure for the molar ratio of [PHMA]₀/[MMA]₀/[AIBN]₀/[CPDN]₀ = 200/600/1/3 is as follows: a mixture was obtained by adding PHMA (0.31 mg), MMA (0.3 mL), AIBN (0.77 mg), CPDN (3.80 mg), and solvent (ethanol, 2.0 mL) to a clean ampoule with a stir bar. The mixture was thoroughly bubbled with argon for 20 min to eliminate the dissolved oxygen, and then flame-sealed and transferred into an oil bath held by a thermostat at the desired temperature (70 °C) to polymerize under stirring. After the desired polymerization time, the ampoule was cooled by immersing it into iced water. Afterwards, open it and take out mixture diluted by tetrahydrofuran. After precipitation with plenty of petroleum ether in a glass, and standing overnight, it was filtered and dried in a vacuum oven at 40 °C until constant weight. The monomer conversion was determined by gravimetrically.

2.4. Characterization

The number-average molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) of the resultant polymers were determined by using a TOSOH HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive-index detector (TOSOH), using TSKgel guardcolumn SuperMP-N (4.6 × 20 mm) and two TSKgel SupermultiporeHZ-N (4.6 × 150 mm) with measurable molecular weights ranging from 5 × 10² to 5 × 10⁵ g mol⁻¹. DMF (+LiBr 0.1% weight) was used as the eluent at a flow rate of 0.6 mL min⁻¹ and 40 °C. GPC samples were injected using a TOSOH plus autosampler and calibrated with polystyrene (PS) standards purchased from TOSOH. ¹H NMR spectra were recorded on Bruker 300 MHz nuclear magnetic resonance (NMR) instrument using CDCl₃ as the solvent and tetramethylsilane (TMS) as the internal standard at ambient temperature. Thermogravimetric analysis (TGA) was carried out on a 2960 SDT TA Instruments with a heating rate of 5 °C min⁻¹ from the room temperature to 700 °C under nitrogen atmosphere. The flammability tests of samples were characterized with an MCC-2 type microscale combustion calorimeter (MCC, Govmark Organization, Inc., USA) according to the standard ASTM D 7309. In the MCC instrument, the sample (∼5 mg) was heated under a nitrogen stream flow (80 mL min⁻¹) in a pyrolyzer (the maximum pyrolysis temperature was 750 °C) at a certain heating rate (typically 1 °C s⁻¹) and suffered thermal decomposition. The water contact angle was measured using a SL200C contact angle meter (USA KINO INDUSTRY Co., Ltd).

3. Results and discussion

3.1. Effect of solvent and molar ratio of two monomers on polymerization

In order to investigate the effect of solvent on the RAFT copolymerization of PHMA and MMA, various solvents including toluene, anisole, ethyl acetate, tetrahydrofuran, and ethanol were selected. The results are shown in Table 1.

Table 1 Effect of solvent on the copolymerization^a

Entry	R	Solvent	Time (h)	Conv. (%)	Mn,th^b (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: R = [PHMA]0 : [MMA]0 : [AIBN]0 : [CPDN]0; VMMA = 0.3 mL, Vsolvent = 2.0 mL, temperature = 70 °C.b Mn,th = ([PHMA]0/[CPDN]0 × Mw,PHMA + [MMA]0/[CPDN]0 × Mw,MMA) × conv.% + Mw,CPDN. Conv. (%) = mcopolymer/(mPHMA + mMMA) × 100.
1	200 : 600 : 1 : 3	Toluene	11.5	85.1	36 000	18 900	2.20
2	200 : 600 : 1 : 3	Anisole	11.5	88.8	37 500	16 000	1.28
3	200 : 600 : 1 : 3	EtOAc	11.5	79.0	33 400	14 300	1.36
4	200 : 600 : 1 : 3	THF	11.5	90.5	38 200	10 400	1.50
5	200 : 600 : 1 : 3	Ethanol	9	61.4	26 000	13 400	1.30

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In view of the resultant molecular weight distributions, the copolymerization in the case of toluene (entry 1 in Table 1) was out of control. Taking into account the molecular weight and molecular weight distribution of the resultant copolymers, ethanol (entry 5 in Table 1) as the solvent additionally preferred to used due to its almost harmlessness to humans and the environment.

In addition, we also investigate the molar ratio of the two monomers on the copolymerization. As shown in Table 2, it can be found that the copolymerization of PHMA and MMA could be conducted smoothly with a wide range of molar ratio of PHMA to MMA ([PHMA]₀ : [MMA]₀ = 1 : 1–10) while keeping relatively narrow molecular weight distributions (M_w/M_n ≤ 1.49). In Table 2, it can be seen that there is large difference of molecular weights from theoretical and GPC results. It may be contributed to the following issues: (1) the hydrodynamic volumes of the PHMA copolymers probably differ substantially from those of the linear PMMA standards; and (2) the calculation of the theoretical value does not consider the effect of monomer reactivity ratio.^10,18

Table 2 The impact on the ratio of the two monomers^a

Entry	R	Time (h)	Conv. (%)	Mn,th^e (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: R = [PHMA]0 : [MMA]0 : [AIBN]0 : [CPDN]0, Vethanol = 2.0 mL, temperature = 70 °C.b mPHMA = 0.309 g.c VMMA = 0.3 mL.d VMMA = 1.0 mL.e Mn,th = ([PHMA]0/[CPDN]0 × Mw,PHMA + [MMA]0/[CPDN]0 × Mw,MMA) × conv.% + Mw,CPDN. Conv. (%) = mcopolymer/(mPHMA + mMMA).
1^b	200 : 200 : 1 : 3	9	72.3	20 900	6600	1.23
2^b	200 : 600 : 1 : 3	9	61.4	26 000	13 400	1.30
3^b	200 : 800 : 1 : 3	9	74.4	36 400	29 400	1.29
4^b	200 : 1000 : 1 : 3	9	61.3	34 100	52 700	1.46
5^b	200 : 2000 : 1 : 3	9	27.8	24 900	76 800	1.49
6^c	100 : 1500 : 1 : 3	9	76.9	47 200	57 200	1.23
7^c	200 : 1500 : 1 : 3	9	79.5	57 500	55 800	1.32
8^c	300 : 1500 : 1 : 3	9	82.6	68 700	58 700	1.43
9^c	500 : 1500 : 1 : 3	9	86.0	90 400	40 800	1.31
10^c	600 : 1500 : 1 : 3	9	84.8	98 400	40 100	1.17
11^c	0 : 1500 : 1 : 3	9	87.5	44 100	52 300	1.11
12^d	600 : 1800 : 1 : 3	11.5	74.4	93 800	20 300	1.38
13^d	800 : 2400 : 1 : 3	11.5	69.4	116 600	25 100	1.27

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3.2. Kinetics of RAFT copolymerization

In order to further investigate the copolymerization behaviors, we conducted the RAFT copolymerization of PHMA and MMA with the molar ratios of [PHMA]₀ : [MMA]₀ : [AIBN]₀ : [CPDN]₀ = 200 : 1500 : 1 : 3 and 100 : 1500 : 1 : 3 at 70 °C and 80 °C, respectively. It can be seen from Fig. 1(A) that first-order copolymerization kinetics was observed in all cases, which means a constant concentration of propagating radicals during the copolymerization processes. It should be noted that there is an obvious positive intercept in the pseudo-first order kinetic plot, especially temperature is 80 °C. This may result from the thermal initiation of the co-monomers at higher temperature.¹⁹ Fig. 1(B) shows the evolution of M_n,GPC and M_w/M_n versus monomer conversion. The molecular weight of the resultant polymers increased linearly with monomer conversion while keeping relatively narrow molecular weight distribution (M_w/M_n < 1.33). All these indicated the “living” feature of this copolymerization system. In addition, we can also calculate the molecular weights by ¹H NMR spectrum (as shown in Fig. 2, M_n,NMR = (328.1 × I_d/2 + 100.1 × I_h/3)/(I_b/2) + 271.4, where 328.1, 100.1 and 271.5 are the molecular weights of PHMA, MMA and CPDN, respectively). It can be seen that most of the molecular weights from ¹H NMR results and theoretical ones are closer, which indicating a high degree of chain end retention and the controlled character of the copolymerization reactions.²⁰

Fig. 1 ln([M]₀/[M]) as a function of time (A) and number-average molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) versus conversion (B) for RAFT copolymerization of PHMA and MMA with different feed ratios. Polymerization conditions: [PHMA]₀ : [MMA]₀ : [AIBN]₀ : [CPDN]₀ = 100 : 1500 : 1 : 3 or 200 : 1500 : 1 : 3; temperature = 70 °C or 80 °C, V_MMA = 1.0 mL, V_ethanol = 2.0 mL. In (B), solid icon represents M_n,GPC; semi-solid icon represents M_n,NMR; hollow icon represents M_w/M_n and the linear line means M_n,th.

Fig. 2 ¹H NMR spectrum of copolymer PPHMA-co-PMMA (M_n,GPC = 12 400 g mol⁻¹, M_w/M_n = 1.27) with CDCl₃ as solvent.

3.3. End analysis and chain extension

The chain end of the copolymer prepared by RAFT copolymerization with CPDN as the chain transfer agent in this system was analyzed by ¹H NMR spectroscopy, as shown in Fig. 2. The signals at δ = 7.4–8.1 ppm are corresponded to the aromatic protons of the naphthalene units in CPDN, which reveal that the dithiocarbonate moieties of CPDN are attached to the copolymer chain ends (ω chain end). The signal at δ = 3.6 ppm is assigned to the protons of methoxy group in PMMA repeat units. Therefore, these results show that the CPDN moieties were successfully attached onto the chain end of the obtained copolymer and that the random copolymerization of MMA and PHMA by RAFT could be carried out successfully in ethanol. In order to further confirm the “living” feature of the resultant copolymer PPHMA-co-PMMA, a chain extension experiment was conducted with fresh MMA monomer using PPHMA-co-PMMA as the macro-RAFT agent. From Fig. 3, there is a peak shift from the compounds would be generated. Consequently, it macro-RAFT agent (M_n,GPC = 19 100 g mol⁻¹ and M_w/M_n = 1.19) to the chain-extended PPHMA-co-PMMA-b-PMMA (M_n,GPC = 28 000 g mol⁻¹ and M_w/M_n = 1.28). Therefore, the successful chain-extension actually confirmed the features of “living”/controlled radical copolymerization of PHMA and MMA.

Fig. 3 GPC traces of before and after chain extension using random copolymer PPHMA-co-PMMA as the macro-RAFT agent (M_n,GPC = 19 100 g mol⁻¹ and M_w/M_n = 1.19). Polymerization conditions: [MMA]₀ : [AIBN]₀ : [PPHMA-co-PMMA]₀ = 2100 : 1 : 3, V_ethanol = 2.0 mL, V_MMA = 1.0 mL, temperature = 70 °C, t = 5 h.

3.4. Thermogravimetric analysis and flame retardant analysis of the polymers with different molecular weights

The thermal stabilities of the obtained copolymers were investigated by thermogravimetric analysis (TGA) and showed weight losses mainly due to ester thermolysis (Fig. 4). Firstly, all phosphorus-containing copolymers began to dehydration with heated, and decompose when the temperature reached to 200 °C due to ester thermolysis.²¹ From the results that the thermal stabilities of the copolymers has increased significantly compared to the conventional methacrylate polymer materials. However, the copolymer has two esters groups (phosphonate and carbonate in MMA units) resulting in two decomposition. From 195 °C to 340 °C, the acyl ester played an important role for the thermal stability. In first stage (195 to 340 °C), the mass decreased at a relatively mild rate, this is due to the difference of molecular weights. In the second period of temperature from 350 °C to 410 °C, phosphate thermo-oxidative degradation occurs after heating to produce phosphoric acid. The next stage (410 to 600 °C) comprises random chain scission of the polymer backbone, because a protective layer of char and poly(phosphoric acid), inhibit heat and oxygen transfer into the polymer bulk and decrease the diffusion of combustible gases into the zone of pyrolysis. It can be noted that the same weight loss, but the corresponding temperature moving to higher temperature (increase from 400 °C to 500 °C) with the increase of the content of PHMA at this stage. When the temperature reached to 600 °C, the degradation process has basically completed.

Fig. 4 TGA curves phosphorus-containing copolymers with different molecular weights which obtained by same polymerization conditions in different time.

Heat release rate (HRR) or the peak of heat release rate (pkHRR) is an important parameter determining the fire hazard of materials, and MCC provides more information related to heat release properties of polymer combustion than DSC, such as DSC cannot be used to evaluate performance of flame retardant samples.²² The higher the HRR, the faster the heat decomposition rate of materials, which also meant more volatile organic will speed up the flame propagation and bring higher fire risk. It shows that HRR of the resultant copolymers with different molecular weights by MCC measurement in Fig. 5. The temperature of the appearance of heat release of all the copolymers starts at about 150 °C along with pkHRR, which has been far more postponed comparing with other conventional flame-retardant materials (<150 °C).²³ Furthermore, the appearance of the second peak means that the materials emerged char layer because of the chemical reaction during combustion. In turn, the formation of the char layer inhibits the combustion of fire and decreases the value of HRR. With the accumulation of heat beneath the char layer, the flame will break through the char layer to some extent and generate the second combustion. Comparing with the first heat peak, the second one is relatively lower when the value of M_n,GPC is low, while the value of M_n,GPC increased the second one same as the first, this phenomenon due to the different number of two monomer units in structure of the copolymers. These all results indicated that such materials have good flame retardant performance.

Fig. 5 The heat release rate (HRR) versus temperature curves of phosphorus-containing copolymers with different molecular weights which obtained by same polymerization conditions in different polymerization times.

3.5. Contact angle measurement

The ability of a surface to be wetted with water is an important parameter that has to be taken into account for biological applications, in general, the hydrophilic material has a better biocompatibility than hydrophobic materials.²⁴ The contact angle that is formed between water and the substrate depends on the mobility of the molecules on the surface, for the measurement of static contact angle of patterned surfaces, the droplet size should be small but larger than the dimension of the structures present on the surfaces. The results demonstrate that the water contact angle of the copolymers around 50° (Fig. 6), which indicated that the copolymers has good hydrophilicity performance²⁵ due to introducing α-hydroxy phosphonates. These results suggest that the obtained α-hydroxy phosphonite materials may have potential biomaterials applications.

Fig. 6 Data of water contact angle on the copolymer PPHMA-co-PMMA surface.

4. Conclusions

RAFT copolymerization of a phosphorus-containing monomer with α-hydroxy phosphonate PHMA and MMA was successfully conducted. The obtained copolymer materials shows good superior flame-retardant properties and good hydrophilicity, indicating that it has the potential serving as not only superior flame-retardant materials but also good biological materials such as dental-based materials.

Acknowledgements

The financial support from the National Natural Science Foundation of China (No. 21274100), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20123201130001), the Project of Science and Technology Development Planning of Suzhou (No. ZXG201413, SYG201430), the Project of Science and Technology Development Planning of Jiangsu Province (No. BK20141192) and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is gratefully acknowledged.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02809h

‡ Chun Tian and Tianchi Xu contributed equally to this work.

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