Ambient temperature Atom Transfer Radical copolymerization of tetrahydrofurfuryl methacrylate and methyl methacrylate: Reactivity ratio determination

European Polymer Journal 45 (2009) 2685–2694

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Ambient temperature Atom Transfer Radical copolymerization of tetrahydrofurfuryl methacrylate and methyl methacrylate: Reactivity ratio determination Kannapiran Rajendrakumar, R. Dhamodharan * Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India

a r t i c l e

i n f o

Article history: Received 26 February 2009 Received in revised form 10 May 2009 Accepted 21 May 2009 Available online 27 May 2009

Keywords: Ambient temperature ATRP Tetrahydrofurfuryl methacrylate Cross-linking Reactivity ratio Monomer–catalyst interaction

a b s t r a c t Tetrahydrofurfuryl methacrylate (THFMA), a heterocyclic monomer was polymerized by ambient temperature Atom Transfer Radical Polymerization (AT ATRP) using CuX/PMDETA/EBiB system. THFMA was found to undergo very rapid polymerization, in bulk. For a target DP > 200, bulk polymerization results in cross-linking as evidenced by (CH2)n,wag peaks (IR spectroscopy). Atom Transfer Radical copolymerization (ATRcP) of THFMA with MMA was performed and the reactivity ratios were calculated from the copolymer composition, as determined by 1H NMR, using Fineman–Ross and Kelen–Tudos methods. The reactivity ratios determined for ATRP were found to be significantly different from the literature values for conventional free radical polymerization (CFRP). This may be due to the coordination of copper catalytic system with the oxygen atom of the tetrahydrofurfuryl group that could lead to the variation in reactivity ratios. 1H NMR evidence for catalyst–monomer interaction is also provided. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction ATRP [1,2] is one of the most widely practised polymerization techniques among controlled radical polymerizations (CRP). ATRP allows the preparation of a wide range of polymeric materials with controlled molecular weights and well-defined chain architectures (Scheme 1). ATRP can be performed at high to ambient temperature as well as sub-ambient temperatures [3–5]. Many monomers can be polymerized using ATRP. Though ATRP can be utilized for the polymerization of many monomers, its usage with the monomers functionalized with carboxylic acid, amide and amino groups is rather restricted due to the possible coordination of these groups with the transition metal catalysts employed. The evidence for such coordination was provided through 1H NMR by Haddleton et al., for an 3° amine functionalized monomer, N,N-dimethylaminoethyl methacrylate [6]. * Corresponding author. Tel.: +91 44 2257 4204; fax: +91 44 2257 4202. E-mail address: [email protected] (R. Dhamodharan). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.05.025

ATRP of a heterocyclic monomer, furfuryl methacrylate was thoroughly investigated and reported by Singha et al. [7–9]. ATRP of a familiar heterocyclic monomer, glycidyl methacrylate (GMA) is well documented [10,11]. In the present work, we have studied the polymerization of tetrahydrofurfuryl methacrylate (THFMA), another heterocyclic functional monomer, with a bulky pendant tetrahydrofurfuryl group, under ATRP conditions. It was envisaged that the coordination of hetero oxygen atom of THFMA with the metal catalyst would have an influence on ATRP. In addition, THFMA is a very interesting monomer as its polymer has a wide range of applicability in the development of biomaterials. Poly(ethyl methacrylate)– THFMA mixture (dough) was studied as an alternative to PMMA–MMA mixture for dental applications, owing to its lower shrinkage [12] upon polymerization. The development of isoprene elastomer and tetrahydrofurfuryl methacrylate mixtures for soft prosthetic applications has also been studied [13]. Copolymerizations of THFMA with hydroxyethyl methacrylate [14] (HEMA), styrene [15], p-methylstyrene [15] and methyl methacrylate [16] by

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rator. The concentrated solution was added in drops into a large excess of hexane. Finally, the polymer was dried in vacuo until a constant weight was obtained. The polymer yields were determined by gravimetry. Scheme 1. Mechanism of ATRP.

conventional free radical polymerization (CFRP) were reported. Copolymers of hydroxyethyl methacrylate (HEMA) with tetrahydrofurfuryl methacrylate were studied as drug delivery release systems [17]. PTHFMA has been suggested as a sealant for amalgam restorations [18], as well as a basis for low shrinkage temporary crown and bridge materials [19]. PTHFMA resin is also used in synthetic finger nails [20]. All the above references confirm the compatibility of poly(tetrahydrofurfuryl methacrylate) towards biological systems. The tetrahydrofurfuryl ring in THFMA is highly heatand-light sensitive and is prone to radical-induced ring opening [21], which restricts its usage in conventional free radical polymerization (CFRP). Therefore, ambient temperature ATRP (AT ATRP) would be a better alternative to conventional free radical polymerization or any other CRP techniques that require protecting the tetrahydrofurfuryl functionality. To the best of our knowledge, ATRP of THFMA has not been studied so far. This study involves the homopolymerization and copolymerization of THFMA with MMA using ambient temperature ATRP. The conditions required for the synthesis of narrow dispersed homopolymers were explored and the monomer–catalyst interactions were also studied using 1H NMR spectroscopy. Furthermore, the reactivity ratio of the copolymer systems was determined using Fineman–Ross [22] and Kelen–Tudos [23] methods.

2.2.2. Solution copolymerization of THFMA with MMA The copolymerization of THFMA with MMA was done in solution (50% anisole) with a lower catalyst (CuCl/PMDETA) concentration of 0.5 equivalent relative to the initiator, EBiB. Copper(I) chloride was accurately weighed and taken in a Schlenk tube. Calculated molar quantities of monomers, THFMA and MMA, PMDETA in anisole were added through a gas-tight syringe, followed by three freeze– pump–thaw – fill back with argon cycles. After complete exclusion of any air present in the tube, EBiB initiator is added through a gas-tight syringe. The polymerizations were carried out at ambient temperature. After the desired period of time, the polymerizations were stopped by adding the solution into a large excess of hexane. For reactivity ratio determination, the conversion was kept below 10%. 2.3. Polymer characterization The FT-IR spectra of the polymers were recorded in the film form, cast from polymer solution on CsCl cell. GPC measurements were done at ambient temperature using Waters GPC system with a Waters 515 HPLC pump, three Phenomenox columns in series (guard column, 500, 103, 0 4 A; 5 lm particle size) and a Waters 2487 dual k absor10 Å bance UV detector and 2414 RI detector with Empower software data analysis package supplied by Waters (USA). THF (HPLC grade, Merck) was used as a solvent at a flow rate of 1 mL/min. Narrow molecular weight polystyrene standards were used for calibrating the GPC. 3. Results and discussion

2. Experimental 3.1. Bulk ATRP 2.1. Materials and methods Tetrahydrofurfuryl methacrylate (THFMA, 98%, Aldrich) and methyl methacrylate (MMA, 98%, Alfa Aesar) were passed through an alumina column and vacuum distilled. CuBr (99.999%), CuCl (99.999%), N,N,N0 ,N00 N00 -pentamethyldiethylenetriamine (PMDETA, 99%), ethyl-2-bromoisobutyrate (EBiB, 99%) were purchased from Aldrich. Anisole was purchased from SD fine Chemicals (India). 2.2. General procedure 2.2.1. Homopolymerization of THFMA Polymerizations were carried out in a Schlenk tube. The monomer, CuBr, anisole, PMDETA, EBiB mixture were taken in a Schlenk tube and were degassed by freeze–pump–thaw – fill back with argon cycle. This was done thrice using a Schlenk vacuum line. Polymerizations were carried out at ambient temperature, for the desired period of time. Following this, the reaction mixture was diluted with THF and passed through an alumina column to remove the copper catalyst and was then concentrated by using a rotary evapo-

The ATRP of THFMA, in bulk, was done at ambient temperature (30 °C ± 1) with the CuBr/PMDETA/EBiB system (Scheme 2). The experimental details of bulk polymerizations are given in Table 1. A very rapid polymerization was noted for a DP of 100 (Entry 1). For low target DP, the polymers obtained were soluble in THF. Polymerizations with high target DP (>200) and catalyst:initiator ratio (1:1) resulted in insoluble cross-linked polymers (Entries 4 and 5), which could be due to possible radical-induced ring opening of tetrahydrofurfuryl group, arising out of rapid evolution of heat and associated higher rate of polymerization and viscosity build up. Only a very low catalyst concentration (0.1 equivalent relative to the initiator (Entry 6) gave relatively narrow dispersed polymers. Thus it was evident that soluble PTHFMA of a reasonably high molecular weight and narrow PDI could be synthesized only by lowering the polymerization rate. 3.1.1. Cross-linking Ring opening and cross-linking during free radical polymerization was already reported [21]. Herewith, we provide

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Scheme 2. AT ATRP of tetrahydrofurfuryl methacrylate.

Table 1 Experimental details of bulk AT ATRP of THFMA. Entry

DP

Cux:PMDETA:EBiB

Time (mm)

% conv.

Mn(exp)

Mn(GPC)

PDI

Solubility

1 2 3 4 5 6

100 100 200 300 1800 200

1:1:1 0.5:0.5:1 1:1:1 1:1:1 1:1:1 0.1:0.1:1

5 30 13 20 480 60

46 89 40 20 – 20

8105 15,680 14,096 – – 7050

9500 16,300 14,800 – – 7650

1.66 1.45 1.32 – – 1.24

Soluble Soluble Soluble Insoluble Insoluble Soluble

the evidence for cross-linking of THFMA under ATRP conditions using infrared spectroscopy. For recording IR spectra, the polymers were either dissolved in THF or in 1:1 benzene/chloroform mixture (in case of slightly cross-linked polymers). The IR spectra of cross-linked polymers show a band around 750 cm1. This may be due to wagging of adjacent methylene groups of tetrahydrofurfuryl ring, [(CH2)n,wag] rocking in phase after ring opening. The THFMA monomer and its

polymers with no cross-linking show no band around 750 cm1 (Fig. 1(a–c)). The intensity of this band increased for high target DPs (Fig. 1(d) and (e)), possibly, due to increase in the extent of monomer conversion and associated cross-linking. For lower DPs, addition of monomers to the growing polymer chain radical will be very fast leading only to linear chain polymers without any cross-linking, as the exotherm of the polymerization can be rapidly dissipated through

Fig. 1. FT-IR spectra of, monomer THFMA, (a), absence of (CH2)n(wag) peak around 750 cm1 for uncross-linked poly(THFMA) samples [Entry 1 and 3 , Table 1] (b and c), presence of (CH2)n(wag) peak around 750 cm1 for cross-linked poly(THFMA) samples [Entry 4 and 5, Table 1] (d and e).

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Scheme 3. Schematic representation of formation of cross-linking during AT ATRP of THFMA (thermal scission of the C–O bond of the five membered ring).

efficient stirring of the low viscous medium. But for higher DPs, the rate of heat generation (DH of polymerization is negative) may be higher than the rate of removal of heat through efficient stirring of the viscous medium resulting in ring opening. Relatively longer reaction time and higher monomer concentration for high target molecular weights, would have facilitated the ring opening of some THFMA monomeric units, resulting in cross-linking. This phenomenon will be more dominant for higher DPs (DP 1800 to DP 300). A possible way of ring opening in the tetrahydrofurfuryl group, leading to cross-linked polymers, is shown in Scheme 3. It is worth mentioning here that no cross-linking was reported for the ATRP of furfuryl methacrylate (FMA, CuX/PMDETA/EBiB system) at 90 °C [9]. But cross-linking was reported in the same paper for conventional free radical polymerization (CFRP) of FMA. We too obtained highly cross-linked polymers for the CFRP of THFMA even at 60 °C(THFMA:AIBN = 100:1, bulk/toluene) within 10– 30 min. As ring opening and subsequent cross-linkings are noticed in THFMA with a stable five membered ring system, it is obvious to expect such cross-linkings with a much strained oxirane ring of glycidyl methacrylate (GMA) too. But no such report was found in the well documented literature on the ATRP of GMA [10,11]. Krishnan et al. has reported ATRP of GMA in ambient temperature using Haddleton’s ligand for a maximum DP of 250 in bulk as well as in solution [10]. High polymerization rate with less control was reported by Garcia et al., for GMA with more active CuX/PMDETA/EBiB catalytic system at ambient temperature (target DP = 200, 85% conversion in 5 min) [11]. No report was found in the literature for bulk ATRP of GMA for target DPs > 250. We can anticipate ring opening and subsequent cross-linkings with GMA too, for high target DPs, particularly with a highly active CuX/ PMDETA system. More detailed study on the mechanism of ring opening of THFMA and comparison with other heterocyclic counterparts, FMA and GMA is under progress in our laboratory. 3.2. Kinetic study of homopolymerization As the bulk polymerization was very fast and uncontrolled, the kinetics of ATRP of THFMA was done in 50% anisole solution, at ambient temperature, with 0.25 equivalent catalyst (CuCl/PMDETA) relative to the initiator

(EBiB) for a target DP of 200. The plots of ln[M]0/[M]t vs time and Mn (GPC) vs % conversion are shown in Fig. 2(a) and (b), respectively. Even in 50% anisole solution, crosslinking was observed at higher conversions. It can be concluded at this stage, that the ring opening side reaction predominates at higher conversions both in bulk as well as in solution. 3.3. Kinetic study of copolymerization of THFMA with MMA The copolymerization of THFMA and MMA was performed as shown in Scheme 4. For an initial feed ratio of 60:40, the kinetic plot in Fig. 3(a) shows the linearity between ln[M]0/[M]t vs time, consistent with the nature of controlled radical polymerization. The increase in the molecular weight of the copolymer with conversion is shown in Fig. 3(b). Although the Mn increases linearly with monomer conversion, the PDI obtained are greater than 1.5 suggesting that the rate of propagation could be higher than the rate of initiation, although chain termination and transfer cannot be ruled out. 3.3.1. Structural characterization and copolymer composition The structural characterization of the copolymer was done using NMR spectroscopy. Fig. 4(a) and (b) shows the 1H and 13C NMR spectrum of the copolymers of p(THFMA–co-MMA). The identification of downfield protons of tetrahydrofurfuryl group in this copolymer was difficult, due to the multiple overlapping of signals. It was resolved by using 2D HSQC spectra as shown in Fig. 4(c). All the protons of poly(tetrahydrofurfuryl methacrylate) are assigned readily using 2D spectrum. It was confirmed that the signal at 4.1 d is only due to the proton attached to the 3° carbon atom of the tetrahydrofurfuryl ring (designated as ‘d’ in Fig. 3(a)). The copolymer compositions were then determined by comparing the peak area of this proton with the –OCH3 proton of MMA (designated as ‘j’, Fig. 4 (a)). 3.3.2. Reactivity ratio determination For reactivity ratio determination, copolymerizations were performed with different initial feed ratios while maintaining the monomer conversion below 10%. Fineman–Ross (FR) and Kelen–Tudos (KT) methods were employed. The catalyst concentration was kept at 0.5 equivalent relative to EBiB initiator in 50% anisole.

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Fig. 2. ln[M]0/[M]t vs time and Mn(GPC) vs % conversion plots (a and b) for the ATATRP of THFMA with catalytic system, CuCl:PMDETA:EBiB (0.25:0.25:1) for a target DP of 200.

Scheme 4. AT ATRcP of tetrahydrofurfuryl methacrylate with methyl methacrylate.

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Fig. 3. ln[M]0/[M]t vs time and Mn(GPC) vs % conversion plots (a and b) for the ATATRcP of THFMA and MMA (40:60), with catalytic system, CuCl:PMDETA:EBiB (0.25:0.25:1) for a target DP of 200.

The copolymer composition (C) was determined by using the following expression:

IHðdÞ m1 C¼ ¼ m1 þ 3ð1  m1Þ IHðdÞ þ IHðjÞ

Eq. (1) on further simplification leads to

m1 ¼ ð1Þ

where m1 is the mole fraction of THFMA in the copolymer and (1  m1) is the mole fraction of MMA in the copolymer. Here (IHðdÞ ) and (IHðjÞ ) are defined as the integrated peak areas of Hd and Hj.

3C 1 þ 2C

ð2Þ

From Eq. (2), the mole fractions of THFMA in copolymers were determined by measuring the integrated peak areas of Hd (IHðdÞ ) and Hj (IHðjÞ ). The FR parameters were found by using the following relations.

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Fig. 4. (a) Partial 1H NMR spectrum of poly(tetrahydrofurfuryl methacrylate–co-methyl methacrylate). (b) 13C NMR spectrum of poly(tetrahydrofurfuryl methacrylate–co-methyl methacrylate). (c) 2D HSQC spectrum of poly(tetrahydrofurfuryl methacrylate–co-methyl methacrylate).

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Fineman–Ross method Kelen–Tudos method

r2(k11/k12)

r2(k22/k21)

0.34 0.39

1.97 2.21

4. Transition metal catalyst–monomer interactions

Fig. 5. Fineman–Ross plot for ATRcP of THFMA with MMA (r = 0.77).

H ¼ F 2 =f

and G ¼ Fðf  1Þ=f

where F is the initial monomer feed ratio (M1/M2) and f is the copolymer composition (m1/m2). The plot of G vs H gave a straight line and the reactivity ratios r1 and r2 were calculated from the slope and intercept, respectively (Fig. 5). In the Kelen–Tudos method, g (ETA) was plotted against n (SHI), where

Reactivity ratios for copolymerization of THFMA with MMA by conventional free radical copolymerization [16] at 60 °C were found to be r1 = 1.06; r2 = 0.81. Although considerable evidence exists to confirm the radical nature of ATRP, under appropriate conditions, dissimilarities do occur between CFRP and ATRP. The radicals in ATRP systems may be in an environment different from CFRP. There are reasons to get into such arguments, as many components (transition metal complex, solvent, initiator and monomer) in the ATRP system have influence on the polymerization. Some reports explain the increase in rate of polymerization as arising due to the presence of coordinating groups, like, oxyethylene [24], phenols [25], DMSO [26], acetonitrile [27], ethylene carbonate [28] in the ATRP system. With a heterocyclic monomer like THFMA, the possibility of coordination of transition metal with the heteroatom cannot be neglected. With such interactions, the radicals produced would not be a free and isolated, but under the constant influence of complexing entities.

g ¼ G=ða þ HÞ; n ¼ H=ða þ HÞ; a ¼ ðHmin  Hmax Þ1=2

4.1. NMR evidence for catalyst–monomer interactions

A plot of ETA vs SHI gave a straight line, the ordinate intercept giving r2/a and the slope giving r1 + r2/a (Fig. 6) Knowing r2 from the intercept, r1 can be calculated. All the FR and KT parameters calculated for different mole ratios of THFMA and MMA are tabulated in Table 2. From the reactivity values calculated, it is evident that monomer 1 (THFMA) prefers cross propagation while monomer 2 (MMA) prefers self-propagation. The reactivity ratio values thus calculated are shown in Table 2.

The interaction of THFMA monomer with the CuBr/ PMDETA catalytic system was demonstrated using 1H NMR spectroscopy. 1H NMR (CDCl3) spectrum of a 1:1:1 mixture of CuBr/PMDETA/THFMA, at ambient temperature, is shown in Fig. 7(b). A clear upfield shift (0.11 ppm) in the absorptions of both the olefinic protons and the ring protons of THFMA were noted. Haddleton et al. has reported a similar upfield shift in the absorptions of one of the olefinic protons lying cis to the ester group of a heteroatom containing monomer, dimethylaminoethyl methacrylate, (DMAEMA) [6]. In the present study, with THFMA, an upfield shift was noted for all the protons in the ring along with the olefinic protons. Though the reason for this observation is not clear at this stage, it can be safely concluded that copper coordination with the heterocyclic monomer, THFMA, is so strong that it has strong influence in the polymerization, under ATRP conditions. A simple, sonicated, 1:1 mixture of THFMA/CuBr too showed exactly a similar extent of upfield shift (0.1 ppm) for all the protons of THFMA system (not shown here). Thus the radical intermediates and the growing polymer chain end formed may not be similar to that of simple ATRP system. The possible way of coordination of THFMA with the catalyst is shown in Scheme 5. To further extend the present work, we have made a comparison among various heterocyclic methacrylates under ATRP conditions. As a preliminary result, we observed an exactly similar catalyst–monomer interaction for GMA as like THFMA, showing an upfield shift of 0.11 ppm for olefinic protons and ring protons. The results will be discussed elsewhere.

Fig. 6. Kelen–Tudos plot for ATRcP of THFMA with MMA (r = 0.94).

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Fig. 7. (a) Partial 1H NMR spectrum (CDCl3, 30 °C) of THFMA and (b) 1:1:1 mixture of CuBr/PMDETA/THFMA.

[4]

[5]

[6]

[7]

[8] Scheme 5. Possible way of interaction of copper complex with the heterocyclic monomer.

[9]

5. Conclusions

[10]

Atom Transfer Radical Polymerization of a heterocyclic monomer, tetrahydrofurfuryl methacrylate was performed at ambient temperature. Polymerizations targeted for high conversions, both in bulk as well as solution, resulted in cross-linking as evidenced by IR spectroscopy. Copolymerization of THFMA with MMA was studied and reactivity ratios were determined. Coordination of the copper catalyst with the monomer was established using 1H NMR spectroscopy. This suggests a possible reason for the difference in reactivity ratio obtained in comparison with conventional free radical polymerization.

[11]

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