Tailor-made polymethacrylate bearing bicyclo-alkenyl functionality via selective ATRP at ambient temperature and its post-polymerization modification by ‘thiol–ene’ reaction

Abstract

This investigation reports a facile synthetic route for the preparation of tailor-made polymers bearing reactive pendant bicyclo-alkenyl functionality via selective atom transfer radical polymerization at ambient temperature (AT-ATRP). In this case dicyclopentenyloxyethyl methacrylate (DCPMA) was polymerized at ambient temperature (30 °C) using CuBr or CuCl as the catalyst in combination with different ligands, such as N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) and 4,4′-di(5-nonyl)-2,2′-bipyridine (dNbpy). The polymerization was very fast and very high conversion (∼90%) was achieved in 2 min. ¹H NMR and MALDI-TOF-MS analysis confirmed the presence of a bicyclic alkenyl pendant group in the polymer prepared by ATRP. This alkenyl functionality was successfully modified by the ‘thiol–ene’ reaction, as evidenced by ¹H NMR and FT-IR analysis.

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Introduction

The synthesis of tailor-made polymers with well defined multiple functionalities is a theme of contemporary research in polymer chemistry.^1–5 Polymers with a pendant alkenyl functionality group can be used in several post polymerization modifications like the Diels–Alder reaction,^6–8 thiol–ene reaction,^9–13 metathesis reaction,^14–16 chemical and photochemical crosslinking^17–20 leading to interesting polymeric materials having different applications.

It is difficult to prepare tailor-made polymers having pendant reactive alkenyl or cycloalkenyl functionality via conventional free radical polymerization (FRP), because those reactive functionalities lead to several side reactions, leading to uncontrolled molecular weights and in many cases it leads to gel formation. In recent years, controlled radical polymerizations (CRPs) have been important synthetic tools to prepare polymers with a controlled molecular weight and well-defined architecture.^21–25 Among the different types of CRP, nitroxide-mediated radical polymerization (NMP),²² reversible addition–fragmentation chain transfer (RAFT) polymerization,²³ atom transfer radical polymerization (ATRP)²⁴ and single electron-transfer living radical polymerization (SET-LRP)²⁵ are widely used to prepare different tailor-made polymers. The last two CRP methods are catalyzed by transition metals and have several advantages, like they need mild reaction conditions and they have good tolerance towards different speciality functional groups.^24,25 Polymers with alkenyl functionality can be prepared either by direct polymerization of the monomer bearing a pendant alkenyl functional group or by incorporating the same via post-polymerization chemical modification. The latter one is usually used in polymer chemistry,^26,27 as the reactive alkenyl moiety can be modified by different methods. For example, poly(pentafluorophenyl methacrylate) was synthesized via different CRPs^26,27 and then modified with allylamine to incorporate the alkenyl functionality.²⁶ There are only a few reports on the incorporation of alkenyl functionality via CRP directly.^16,28–30 Wooley et al. reported the preparation of alkene-functionalized polymers via RAFT as well as ATRP of nonsymmetrical divinyl monomers.^28–30 To the best of our knowledge there is no report of selective ATRP at ambient temperature of any acrylate bearing bicyclo-alkenyl functionality.

Dicyclopentenyloxyethyl methacrylate (DCPMA) is an interesting monomer which has a long chain pendant group having a bicyclo-alkenyl functional moiety. Unlike conventional methacrylate, DCPMA as well as its polymer has a reactive bicyclo-alkenyl functional group. This alkenyl functionality can be post-modified via several reactions, like the thiol–ene reaction, photo-crosslinking, metathesis reaction etc. leading to various applications. This investigation delineates the first report on the preparation of polymers bearing a pendant bicyclic alkenyl functionality via selective ATRP of dicyclopentenyloxyethyl methacrylate (DCPMA) at ambient temperature (30 °C) (AT-ATRP). It shows the distinct advantages of the preparation of this polymer over FRP. During AT-ATRP of DCPMA the pendant bicyclic alkenyl functionality was not affected during polymerization. The polymers were characterized by ¹H NMR, MALDI-TOF-MS, DSC and TGA analysis. The pendant cyclo-alkenyl functionality was modified via the ‘thiol–ene’ reaction.

Experimental

Materials

The monomer dicyclopentenyloxyethyl methacrylate (DCPMA) (Aldrich) was purified by passing through a basic alumina column followed by vacuum distillation and then was stored in a refrigerator. CuBr and CuCl (98%, Aldrich) were purified by washing with glacial acetic acid, followed by ethyl alcohol and diethyl ether, and then were dried under vacuum. Ethyl 2-bromoisobutyrate (EBiB) (98%, Aldrich), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) (99%, Aldrich), 4,4′-di(5-nonyl)-2,2′-bipyridine (dNbpy) (97%, Aldrich), thioglycolic acid (TGA) (Merck), benzyl mercaptan (97%, Aldrich) and 2,2′-azobisisobutyronitrile (AIBN) (98%, ACROS Chemical) were used as received. Solvents were distilled prior to use.

Characterization

Molecular weights and molecular weight distributions of the polymers were measured at ambient temperature using a Viscotek GPC instrument equipped with a refractive index detector (model VE 3580). THF was used as the eluent at a flow rate of 1.0 mL min⁻¹ and poly(methyl methacrylate) (PMMA) of narrow polydispersity index was used as a calibration standard. GPC data were collected using OmniSEC 4.2 software. Differential scanning calorimetry (DSC) analysis was carried out using a Pyris Diamond DSC, Perkin-Elmer (UK) instrument at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The temperature against heat flow was recorded. Thermogravimetric analysis (TGA) was carried out on a TA (TGA Q50 V6.1 Build 181) instrument. In this case, a sample weight of ∼10 mg was heated from room temperature to 600 °C at a heating rate of 20 °C min⁻¹ under nitrogen atmosphere. ¹H NMR spectra of the polymers were recorded on a 400 MHz Bruker NMR spectrometer. For all NMR analysis, unless stated otherwise, deuterated chloroform (CDCl₃) was used as the solvent containing a small amount of TMS as an internal standard. FT-IR spectra were recorded on a Perkin-Elmer, Inc. version 5.0.1 spectrophotometer. Mass spectra were acquired using a Perceptive Biosystems Voyager Elite MALDI-TOF mass spectrometer, equipped with a nitrogen laser (wavelength 337 nm). For MALDI-TOF sample preparation, the polymer samples were dissolved in THF (1 mg mL⁻¹) and mixed with sodium trifluoroacetate (1 mg mL⁻¹) and 2,5-dihydroxybenzoic acid (40 mg mL⁻¹) in the volume ratio 10 : 1 : 10, respectively. All the spectra were averaged over 128 laser shots. UV-vis spectroscopic measurements were performed at 25 °C on a Shimadzu (model no. UV-2450) UV-vis spectrometer, using a quartz UV cell. UV and visible spectra were recorded at a concentration of 5.0 × 10⁻³ mol L⁻¹ in tetrahydrofuran at λ = 200–800 nm.

Ambient temperature atom transfer radical polymerization (AT-ATRP) of DCPMA

In a typical ATRP of DCPMA, CuBr (0.01 g, 7.63 × 10⁻⁵ mol) was taken in a Schlenk tube. The tube was sealed with a silicone rubber septum and then nitrogen gas was passed through the Schlenk tube to drive off oxygen present in the tube. Then PMDETA (0.0132 g, 7.63 × 10⁻⁵ mol) and DCPMA (1 g, 3.81 × 10⁻³ mol) were added into the Schlenk tube and finally EBiB (0.0148 g, 7.63 × 10⁻⁵ mol) was added to start the polymerization reaction. The polymerization was carried out at ambient temperature under nitrogen atmosphere. After a specific time, the reaction was stopped and was diluted by adding 10 mL solvent (THF). The polymer solution was passed through a column of activated alumina to remove the copper catalyst. The polymer was precipitated into a large amount of methanol and then was dried in a vacuum oven at 40 °C. Yield: 0.92 g (92%). ¹H NMR (CDCl₃): δ (ppm) = 5.4–5.6 (2H, –CH=CH–), 4.0 (2H, –COO–CH₂–), 3.4–3.6 (3H, –CH₂–O–CH<), 0.9–2.6 (different aliphatic protons). The M_n(GPC) and PDI of the polymer were 15 100 g mol⁻¹ (M_n(Theo) = 12 240 g mol⁻¹) and 1.39 respectively.

Free radical polymerization of DCPMA

The monomer, DCPMA (1 g, 3.81 × 10⁻³ mol) was taken in a reaction tube equipped with a silicone rubber septum. The polymerization was carried out at 70 °C by adding AIBN (0.0125 g, 7.63 × 10⁻⁵ mol). The polymerization reaction was conducted for 10 min. The polymer was precipitated into methanol and was dried in a vacuum oven at 40 °C. Yield: 0.9 g (90%).

General procedure for thiol–ene reaction

In a Schlenk tube, the polymer (0.3 g, 5 × 10⁻⁵ mol), thioglycolic acid (0.368 g, 4 × 10⁻³ mol, 80 equiv.), and AIBN (0.0328 g, 2 × 10⁻⁴ mol, 4 equiv.) were dissolved in 3.5 mL toluene. The tube was sealed with a silicone rubber septum and then nitrogen gas was passed through the Schlenk tube to drive off oxygen present in the tube. The reaction was carried out at 90 °C for 24 h. The polymer was purified by precipitating several times in a methanol–water mixture (1 : 1 v/v) and dried in a vacuum oven at room temperature for 24 h. ¹H NMR (acetone-d₆): δ (ppm) = 5.5–5.7 (remaining –CH=CH–), 4.1 (2H, –COO–CH₂–), 3.4–3.6 (3H, –CH₂–O–CH<) 3.3 (–S–CH₂–), 0.9–2.7 (different aliphatic protons).

Results and discussion

AT-ATRP of DCPMA

ATRP of DCPMA was carried out in bulk at ambient temperature using ethyl 2-bromoisobutyrate (EBiB) as an initiator using different catalyst systems (Scheme 1). Table 1 shows a summary of the results of ATRP of DCPMA using different catalyst systems. It was noted that ATRP of DCPMA was extremely fast with a CuX/PMDETA/EBiB catalyst system, and very high conversion (∼90%) was achieved in only 2 minutes. The polydispersity index (PDI) of the resultant polymer was relatively narrow (PDI = 1.39), indicating the controlled nature of the polymerization. When ATRP of DCPMA was carried out using CuBr as a catalyst in the presence of dNbpy as ligand, the rate of polymerization was relatively slow but the molecular weight was more controlled with a lower PDI (1.22) compared to ATRP of DCPMA using PMDETA as a ligand. The lower redox potential of the copper–PMDETA complex than that of the copper–dNbpy complex and the greater complex formation strength of the tridentate ligand, PMDETA, made the polymerization rate faster for DCPMA.^31,32

Scheme 1 Ambient temperature ATRP (AT-ATRP) of dicyclopentenyloxyethyl methacrylate (DCPMA).

Table 1 Ambient temperature ATRP (AT-ATRP) and free radical polymerization (FRP) of DCPMA

Entry	Catalyst system (EBiB/CuX/ligand) (mol ratio)	Time (min)	Conv. (%)	Mn(Theo) (g mol^−1)	Mn(GPC) (g mol^−1)	PDI	Remarks
a AT-ATRP, temperature = 30 °C, [DCPMA]0 = 3.81 × 10^−3 mol, [EBiB]0 = 7.63 × 10^−5 mol, [CuX]0 = 7.63 × 10^−5 mol, [PMDETA]0 = 7.63 × 10^−5 mol, [dNbpy]0 = 1.52 × 10^−4 mol.b Solution ATRP; toluene was used at the ratio 1 : 1 (w/w) to DCPMA.c FRP, temperature = 70 °C, [AIBN]0 = 7.63 × 10^−5 mol.
1	EBiB/CuBr/PMDETA^a (1 : 1 : 1)	2	92	12 240	15 100	1.39	No gelation
2	EBiB/CuCl/PMDETA^a (1 : 1 : 1)	3	88	11 700	12 100	1.30	No gelation
3	EBiB/CuBr/dNbpy^a (1 : 1 : 2)	10	85	11 300	11 600	1.22	No gelation
4	EBiB/CuBr/PMDETA^b (1 : 1 : 1)	60	42	5700	6000	1.20	No gelation
5	AIBN^c	10	90	—	—	—	Polymer was gelled

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However, in general it was observed that the ATRP of DCPMA was tremendously fast (even at ambient temperature) compared to any conventional (meth)acrylate monomers. Dhamodharan et al. reported a very fast ATRP of tetrahydrofurfural methacrylate (THFMA) and they illustrated this due to the interaction of CuBr with THFMA, the monomer.³³ In the present case controlled experiments were carried out to investigate the interaction between CuBr and DCPMA. UV-vis spectrometry analysis showed that the UV spectrum of CuBr/DCPMA was very broad with a small hump at 280 nm indicating the interaction between CuBr and DCPMA, which in turn activates the monomer. (The UV spectra of CuBr and DCPMA/CuBr are shown in the ESI,† Fig. S1.) When the monomer was polymerized using conventional FRP in bulk as well as in toluene using AIBN as an initiator, the resultant polymers were gelled and were insoluble in different organic solvents like THF, CHCl₃ and toluene. Hence, it was not possible to determine their molecular weights. Sohxlet extraction showed gel formation in the PDCPMA. Table 1 shows that there is no gel formation in the PDCPMA prepared by ATRP. Though the ATRP of DCPMA in bulk was exceedingly fast, its solution ATRP was relatively slower. So its polymerization kinetics was studied in solution at ambient temperature using toluene as a solvent. Fig. 1 shows the ATRP of DCPMA in toluene using CuBr/PMDETA as the catalyst. A semilogarithmic plot of conversion vs. polymerization time is linear and R² is closed to unity. Thus this linear kinetic plot indicates that the concentration of the active species was constant throughout the polymerization reaction.

Fig. 1 Kinetic plot for ATRP of DCPMA in toluene at ambient temperature using a CuBr/PMDETA catalyst system (x = conversion (%) of DCPMA, R² = regression coefficient, k_app = apparent rate constant).

¹H NMR study

The chemical structure of the polymer was elucidated by ¹H NMR. The ¹H NMR spectra of the monomer as well as of the homopolymer are shown in Fig. 2 and Fig. 3 respectively. The ¹H NMR spectrum of dicyclopentenyloxyethyl methacrylate (DCPMA) shows resonances at 6.1 and 5.5 ppm (designated as ‘a’ in Fig. 2) which are due to the methacrylic double bond. The resonances at δ = 5.4 to 5.6 ppm are due to the olefinic protons designated as ‘f’ and ‘f′’ in the pendant group. The resonance at 4.2 ppm is attributed due to the –OCH₂– protons and is designated as ‘c’. The resonances at 3.3 to 3.6 ppm are due to the –CH₂–O–CH< protons and are designated as ‘d’ and ‘e’, the resonances at 1.1 to 2.5 ppm are due to different methine (>CH–), methylene (–CH₂–) and methyl (–CH₃) protons present in the polymer backbone and in the pendant group. In ¹H NMR spectrum (Fig. 3) of PDCPMA there are no resonances at 6.1 and 5.5 ppm (designated as ‘a’ in Fig. 2) indicating the complete polymerization of the methacrylic double bond. The resonances at 5.4 to 5.6 ppm (‘f’ and ‘f′’), which are due to the pendant double bond, were not affected during the polymerization reaction.

Fig. 2 ¹H NMR spectrum of dicyclopentenyloxyethyl methacrylate.

Fig. 3 ¹H NMR spectrum of poly(dicyclopentenyloxyethyl methacrylate).

MALDI-TOF mass spectrum

The matrix assisted laser desorption ionization time-of-flight mass spectrum (MALDI-TOF-MS) was carried out to elucidate the chemical structure of PDCPMA prepared via AT-ATRP using CuBr/PMDETA as the catalyst. For the MALDI-TOF experiment 2,5-dihydroxybenzoic acid and sodium trifluoroacetate were used as the matrix and cationic agent respectively. In the MALDI-TOF-MS spectra the different mass series (M) can be expressed as follows,

M = M_ini + nM_r + M_{end group} + M_cation

where, M_ini, M_r, M_{end group} and M_cation are the molar masses of the initiator without Br [i.e. (EBiB − Br) = 115], monomer (DCPMA = 262), repeating unit (n), end group (Br = 80) and cation (Na = 23) respectively. The MALDI-TOF-MS of PDCPMA shows intense peaks at a regular interval of about 262, which is the molar mass of DCPMA (Fig. 4). The peaks at 3026.25 and 3288.79 can be assigned to n = 11 and 12 respectively relating to the chemical structures, as shown in Fig. 4. Each PDCPMA macromolecular chain has an ethyl isobutyryl group [–C(CH₃)₂CO₂C₂H₅], but the –Br end group cannot be detected. Loss of the –Br group during the MALDI-TOF-MS analysis of polymethacrylate is reported in the literature.³⁴ However, the MALDI-TOF mass spectrum showed that the bicyclo-alkenyl pendant group of PDCPMA was not affected during the polymerization reaction.

Fig. 4 MALDI-TOF mass spectrum of PDCPMA prepared by CuBr/PMDETA catalyst (entry 4 in Table 1).

Thermal properties of PDCPMA

Thermogravimetric analysis was carried out to study the thermal stability of the polymers. TGA of PDCPMA prepared by FRP shows that the onset temperature, T_i (temperature at 5% weight loss) is 225 °C, whereas the T_i value of PDCPMA prepared by ATRP is 237 °C (Fig. 5). The T_max value of the polymer prepared by ATRP is much higher (389 °C) than the polymer prepared by FRP (369 °C), indicating higher thermal stability of the polymer prepared by ATRP. The DTG curve of PDCPMA prepared by FRP shows a broad bimodal curve, indicating the presence of different modes of linkages, like head to head linkage etc. in addition to conventional head to tail linkage.^35–37 For these reasons polymethacrylates prepared by FRP show lower T_i and T_max in TGA analysis. It is reported that PMMA obtained by Cu-catalyzed ATRP has only a single decomposition temperature at about 400 °C, whereas PMMA obtained by free radical polymerization shows several stages of decomposition.^35–37 In the present case the PDCPMA prepared by ATRP shows a single decomposition peak. This indicates that it has controlled head to tail linkages.

Fig. 5 TGA and DTG thermograms of PDCPMAs prepared by ATRP and FRP.

DSC analysis was carried out to determine the glass transition temperature (T_g) of PDCPMA. DSC analysis shows that PDCPMA has a T_g at +28 °C (see ESI,† Fig. S2). The lower T_g of PDCPMA compared to PMMA (T_g = +100 °C) is due to better segmental mobility of the ether linkage.³⁸ For example, poly(isobornyl methacrylate) has a high T_g of about 190 °C as in this case the bulky isobornyl group is directly attached to the ester linkage.³⁹ But in PDCPMA the bicycloalkenyl is not directly attached to the ester linkage (–COO–). In this case the bulky group is attached to the ester linkage with a flexible ether linkage of ethylene glycol. Due to this flexible linkage the chain mobility increases and T_g decreases.

Thiol–ene reaction

The thiol–ene reaction of PDCPMA was carried out in toluene using thioglycolic acid in the presence of AIBN as a thermal initiator (Scheme 2). Fig. 6 shows the ¹H NMR spectrum of the thiol–ene modified PDCPMA. It shows the emergence of a sharp resonance at around 3.3 ppm (compared to the ¹H NMR of PDCPMA, Fig. 3), indicating the presence of –SCH₂– protons, which is designated as ‘m’. There is a drastic reduction in the resonances at 5.5 to 5.7 ppm compared to the ¹H NMR of PDCPMA (Fig. 3). This indicates that a thiol moiety has been included in the cycloalkenyl functionality via the thiol–ene reaction. The extent of thiol–ene modification was calculated by comparing the integral ratio of the resonances at 5.5 to 5.7 ppm of the remaining double bond and of the same at 4.1 due to –OCH₂– protons in thiol modified PDCPMA. The extent of thiol–ene modification was determined to be about 80%.

Scheme 2 Thiol–ene reaction of PDCPMA with thioglycolic acid.

Fig. 6 ¹H NMR spectrum of thiol modified PDCPMA.

The thiol–ene reaction in PDCPMA was also characterized by FT-IR analysis. Fig. 7 shows the FT-IR spectra of thiol modified PDCPMA (A) and PDCPMA (B). The comparison of the two spectra indicates that the absorption band at 1618 cm⁻¹ for the [double bond splayed left]C=C[double bond splayed right] stretching in PDCPMA diminished and new absorption bands emerged at ∼3450 cm⁻¹ (broad) and at 670 cm⁻¹ respectively for the –OH group in –COOH⁴⁰ and the C–S bond of the attached thioglycolic acid moiety (Fig. 7). FT-IR analysis complements the successful thiol–ene modification of PDCPMA. Benzyl mercaptan was also explored as a thiol reagent in this thiol–ene modification of this cycloalkenyl group in PDCPMA. In this case the extent of the thiol–ene reaction was about 40% (Table 2). The high conversion (80%) using thioglycolic acid as a thiol may be due to the higher reactivity of thioglycolic acid compared to benzyl mercaptan.⁴¹

Fig. 7 FT-IR spectra of (A) thiol modified PDCPMA and (B) PDCPMA.

Table 2 Thiol–ene reaction of PDCPMA with different thiols^a

Entry	Thiol	Feed ratio [thiol]/[PDCPMA]	Conversion (%)
a Temperature = 90 °C, time = 24 h. All reactions were performed in toluene using 4 equiv. AIBN (w.r.t. the mol of PDCPMA).
1	Thioglycolic acid	80	80
2	Benzyl mercaptan	80	40

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Conclusions

In conclusion, DCPMA was successfully selectively polymerized by ATRP at ambient temperature (AT-ATRP) using CuBr/dNbpy as well as a CuBr/PMDETA catalyst system. The polymerization was exceedingly fast, but the resultant polymer had controlled molecular weights and narrow molecular weight distributions. Conventional radical polymerization of DCPMA using AIBN as a thermal initiator leads to gel formation, but PDCPMA obtained by ATRP was gel free and soluble in organic solvents like toluene, THF and CHCl₃ etc. The alkenyl functionality in the pendant bicyclic ring was not affected during the AT-ATRP of DCPMA, as evidenced by ¹H NMR and MALDI-TOF-MS analysis. The alkenyl functional group was successfully modified via the thiol–ene reaction, which was confirmed by FT-IR and ¹H NMR spectroscopy. This finding opens new possibilities for the incorporation of a pendant bicyclo-alkenyl group into tailor-made polymers which can be potential materials for post-polymerization modification like the thiol–ene reaction.

Acknowledgements

PM is grateful to the University Grants Commission (UGC), New Delhi, India, for a Senior Research Fellowship. We are thankful to DST, New Delhi for the financial assistance.

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Footnote

† Electronic supplementary information (ESI) available: UV-vis spectra and DSC traces. See DOI: 10.1039/c3ra43801e

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