Copper mediated controlled radical copolymerization of styrene and 2-ethylhexyl acrylate and determination of their reactivity ratios

Copolymerization is an important synthetic tool to prepare polymers with desirable combination of properties which are difficult to achieve from the different homopolymers concerned. This investigation reports the copolymerization of 2-ethylhexyl acrylate (EHA) and styrene using copper bromide (CuBr) as catalyst in combination with N,N,N′,N″,N″- pentamethyldiethylenetriamine (PMDETA) as ligand and 1-phenylethyl bromide (PEBr) as initiator. Linear kinetic plot and linear increase in molecular weights vs. conversion indicate that copolymerization reactions were controlled. The copolymer composition was calculated using 1H NMR studies. The reactivity ratio of styrene and EHA (r1 and r2) were determined using the Finemann–Ross (FR), inverted Finemann–Ross (IFR), and Kelen–Tudos (KT) methods. Thermal properties of the copolymers were also studied by using TGA and DSC analysis.


INTRODUCTION
The homopolymer and copolymers of 2-ethylhexyl acrylate (EHA) have very good film formation characteristics and have very good low temperature flexibility, because of the presence of branched and longer alkyl pendant group in the EHA. They have also low volume shrinkage. So the copolymers of EHA are widely used in paints, coating and adhesive applications (Skeist, 1977;Plessis et al., 2001;Webster and Crain, 2002). Copolymerization is an important synthetic tool which can control the thermal and mechanical properties of the polymers (Kavousian et al., 2004). In the copolymers of styrene and EHA, the properties of the copolymers can be monitored by controlled incorporation of the respective comonomers styrene and EHA. Polystyrene possess high glass transition temperature, T g (∼100 • C) where as poly(2-ethylhexyl acrylate) (PEHA) has low T g of −60 • C. Copolymers of styrene and EHA are important components for hard coating (Plessis et al., 2001) and also are used as blend compatibilizers (Haldankar, 2001). Conventional free radical polymerization (FRP) leads to uncontrolled molecular weight and broad dispersity (Ð). There is also gel formation tendency, because of the several side reactions during FRP. This makes them difficult to apply for paints and coating material application owing to high viscosity (Solomon and Moad, 1995;Odian, 2004). Since 1990s there have been spectacular advances in the field of controlled radical polymerization (CRP). There were several CRP techniques namely, atom transfer radical polymerization (ATRP) (Kamigaito et al., 2001;Matyjaszewski and Jia, 2001;Kavitha and Singha, 2008), nitroxide-mediated polymerization (NMP) Hawker et al., 2001), and reversible addition-fragmentation chain transfer (RAFT) (Chiefari et al., 1998;Moad et al., 2000;Moad, 2006;Barner-Kowollik, 2008). Among the different CRP techniques, ATRP is applicable to polymerize wide range of monomers and can be carried out at wide range of temperature (−20 to 200 • C) (Haloi et al., 2009). ATRP has been successfully applied to synthesize a wide range of polymers with varied molecular weights, different architectures, functionalities etc. Transition metal catalyzed CRP known as ATRP is an important CRP method (Matyjaszewski, 2000) which is carried out in presence of an active alkyl halide using a transition metal halide as catalyst in combination with a suitable ligand (Matyjaszewski and Jia, 2001). A wide variety of copolymers can be prepared via ATRP with controlled molecular weight, functionality, and low dispersity. There are several reports on the copolymerization of different acrylate monomers using ATRP and determination of their reactivity ratios. The reactivity ratios are important parameters for a set of monomer. The reactivity ratios of monomers predict the copolymer composition as well as the sequence distribution of the comonomers. It also predicts the properties of the copolymer. ATRP provides random copolymer with similar chain compositions which is very much different from FRP (Matyjaszewski, 2002). During the polymerization reaction, the polymer chains grow simultaneously and thus all the polymer chains have same composition. However, in FRP macromolecular chains start growing at different times during the polymerization and the monomer composition continuously changes. As a result, in FRP the different chains will have different compositions in the end (Solomon and Moad, 1995). The reactivity ratios of co-monomers in a CRP are somewhat different from the same in FRP. This is because in CRP processes there is intermittent activation-deactivation of the active species which results in different rates of consumption of comonomers (Matyjaszewski, 2002;Braunecker and Matyjaszewski, 2007). For example, Mignard et al. reported the reactivity ratios of the copolymerization of styrene and butyl acrylate (BA) via NMP at 120 • C in solution. They reported the reactivity ratios for styrene and BA within the range of 0.60-1.2 (r styrene ) and 0.16-0.29 (r BA ) respectively (Mignard et al., 2004). Arehart and Matyjaszewski reported the reactivity ratios of styrene and BA prepared via ATRP at 110 • C in solution. They reported the reactivity ratios for styrene and BA as 0.68 < r styrene < 0.82 and 0.22 < r BA < 0.26 respectively (Arehart and Matyjaszewski, 1999). Chambard et al. reported the copolymerization of styrene and BA prepared in bulk via FRP. The reactivity ratios of styrene and BA prepared via FRP at 90 • C were reported to be 0.95 and 0.20 respectively (Chambard et al., 1999). Ziegler and Matyjaszewski reported the variation in reactivity ratios of MMA and BA with the change in ligand from 4,4 -di(5-nonyl)-2,2 -bipyridine (dNbpy) (r MMA = 2.52, r BA = 0.26) to N,N,N ,N ,N -pentamethyldiethylenetriamine (PMDETA) (r MMA = 3.15, and r BA = 0.37) (Ziegler and Matyjaszewski, 2001). Lessard et al. reported the reactivity ratios of styrene and tert-butyl acrylate(t-BA) prepared via NMP at 115 • C in bulk as r t-BA = 0.09-0.12 and r Styrene = 0.40-0.49 (Lessard et al., 2007). We reported the ATRP of furfuryl methylacrylate and methyl methacrylate (Kavitha and Singha, 2007), 2-ethylhexyl acrylate and glycidyl methacrylate (Haloi et al., 2009). However, there is no report on the ATRP of styrene and EHA and to determine their reactivity ratios. The objective of this investigation is to study the copolymerization of styrene and EHA via ATRP and to determine their reactivity ratios. The reactivity ratios of styrene and EHA were calculated using Finemann-Ross (FR), inverted Finemann-Ross (IFR), and Kelen-Tudos (KT) methods (Fineman and Ross, 1950;Kelen et al., 1980;Makrikosta et al., 2005).

MATERIALS AND METHODS
The monomers, 2-ethylhexyl acrylate (EHA) (Aldrich, USA) and styrene (Jyoti Chemicals, Mumbai) were purified by vacuum distillation. CuBr (Aldrich, USA) was purified by washing with glacial acetic acid, and then it was washed thoroughly with diethyl ether and was finally dried under vacuum. Phenyl ethylbromide (PEBr) (97%) and N,N,N ,N ,Npentamethyldiethylenetriamine (PMDETA) (97%) were purchased from Aldrich, USA and were used as received.

CHARACTERIZATION
Number average molecular weight (Mn, GPC) and dispersity (Ð) were determined by Gel Permeation Chromatography (GPC). GPC analysis was carried out at room temperature using a Viscotek GPC equipped with a refractive index detector (Model VE3580), two ViscoGEL GPC columns (model GMHHR-M # 17392) connected in series. GPC analysis was carried out using tetrahydrofuran as eluent at a flow rate of 1 ml/min. Linear and narrow disperse polystyrene was used as calibration standard and Viscotek OMNI-01 software was used for data processing.
1 H NMR spectra of the polymers were recorded on a 200 MHz Brucker NMR spectrometer using CDCl 3 as solvent which had a small amount of tetramethylsilane (TMS) as an internal standard. Differential scanning calorimetry (DSC) analysis was carried out by using TA Instrument (DSC Q100 V8.1 Build 251) under nitrogen atmosphere at a heating rate of 10 • C/min within a temperature range of −100 • C to +150 • C. The baseline calibration was done by scanning the temperature domain with the help of an empty pan. The enthalpy was calibrated by using indium standard and the heat capacity was calibrated by using the sapphire disc that was supplied by TA instrument. The glass transition temperature (T g ) was determined from the plot of heat flow vs. temperature in the second heating scan in the DSC analysis.
Thermogravimetric analysis (TGA) was carried out by using a TA Instrument (Q50) at a heating rate of 20 • C/min in the temperature range of 30-600 • C in nitrogen atmosphere. TGA analyzer consists of high precision balance with a pan which was placed in a small electrically heated oven. The temperature was measured accurately with the help of a thermocouple. From the plot of weight percent vs. temperature the polymer degradation temperature was determined.

SYNTHESIS OF COPOLYMERS OF STYRENE AND EHA VIA ATOM TRANSFER RADICAL COPOLYMERIZATION (ATRcP)
The polymerization reaction was carried out in a Schlenk tube. In a typical ATRP reaction EHA (4.82 g, 26.1 mmol), styrene (0.909 g, 8.7 mmol) and CuBr (0.050 g, 0.35 mmol) were accurately weighed and transferred to the Schlenk tube. The PMDETA ligand (0.0604 g, 0.35 mmol) was then added to the reaction tube. Oxygen was removed from the reaction mixture by passing nitrogen through the reaction tube. The polymerization was started by adding PEBr (0.0646 g, 0.35 mmol) and was carried out at 90 • C. Aliquot samples were taken out at different time intervals and were used to calculate the conversion by gravimetric method. The samples were also used to find out the molecular weight by GPC. The final product was diluted with THF and was purified by passing through alumina column to remove the copper catalyst. The same procedure was adopted for other feed ratios of ATRcP (atom transfer radical copolymerization) of EHA and styrene.

SYNTHESIS OF DIBLOCK COPOLYMER OF POLYSTYRENE WITH EHA VIA ATRP
The homopolymer of polystyrene (i.e., PS-Br) was used as macroinitiator for the synthesis of diblock copolymer of styrene and EHA. The macroinitiator, PS-Br (1.0 g, 0.066 mmol) was taken in a Schlenk tube and was dissolved in THF solvent into which CuBr (0.009 g, 0.069 mmol) and PMDETA (0.017 g, 0.10 mmol) were added followed by EHA (1 g, 5.4 mmol). The polymerization reaction was carried out at 90 • C for 6 h. The polymer obtained was dissolved in THF and was purified by passing through basic alumina column and then precipitated in methanol.

RESULTS AND DISCUSSION
Copolymerization of styrene and 2-ethylhexyl acrylate was carried out via ATRcP (shown in Scheme 1) at different feed ratios by using phenylethylbromide (PEBr) as initiator, CuBr as catalyst in combination with PMDETA as a ligand. From the kinetic plot of ln(1/1-X) (where X is the percent conversion of monomer) vs. time, it was observed that the value of ln(1/1-X) was linearly increased with time (Figure 1). This linear dependency is the characteristics of the controlled polymerization reaction which follows the first order kinetics. Figure 1 showed that with increase in styrene content in the feed the rate of polymerization increased (Jianying et al., 2006). Figure 2 infers that there was linear increase in molecular weight with conversion and dispersity (Ð) was relatively narrow. GPC traces of poly(styrene-co-EHA) (50:50) (sample 3 of Table 1) is shown in supplementary section ( Figure S1). It indicates the controlled nature of the copolymerization reaction. Table 1 summarizes the feed ratio as well as the copolymer composition of the different copolymerization reactions.

STRUCTURAL CHARACTERIZATION AND COPOLYMER COMPOSITION
The structural characterization and copolymer composition were determined by 1 H NMR spectroscopy. Figure 3 shows the 1 H NMR spectra of poly(styrene-co-EHA) of 40:60 feed ratio. The resonance at δ = 0.8 ppm is due to the -CH 3 protons of PEHA part. The broad resonances at δ = 1.0 to 2.1 ppm are due to the different -CH 2 -and >CHprotons of pendant group of PEHA part as well as those of the main chain backbone of the copolymer. The resonances at δ = 6.5-7.1 ppm are due to the different aromatic protons of polystyrene part. The resonances at 3.8 ppm are due to -OCH 2 -protons of PEHA part. The distinct resonances at 6.5-7.3 for five aromatic protons of styrene and 3.8 ppm for two protons of EHA were used to calculate the composition of the copolymer of styrene and EHA. The copolymer composition was determined by the equation (1) as shown below where, A and B represent the integral area at δ = 3.7 and δ = 6.5-7.3 ppm for -OCH 2 -protons in PEHA and aromatic protons of polystyrene unit respectively.

REACTIVITY RATIO DETERMINATION
In the copolymerization of styrene (1) and EHA (2) the reactivity ratio r 1 is defined as k 11 /k 12 , where k 11 is the rate constant of the reaction between the growing polymer chain carrying free radical of styrene as the terminal unit and styrene (homo propagation) and k 12 is the rate constant of the reaction between the same reactive chain end and the EHA monomer. Similarly the reactivity ratio r 2 is also defined as k 22 /k 21 , where k 22 is the rate constant of homo propagation reaction between the growing

Scheme 1 | Copolymerization of styrene and EHA via atom transfer radical polymerization (ATRP).
www.frontiersin.org October 2014 | Volume 2 | Article 91 | 3 macromolecular chain having EHA active radical as the terminal unit and EHA and k 21 is the rate constant of the reaction between EHA active radical and styrene monomer (cross propagation). For determining the r 1 and r 2 , copolymerization of styrene and EHA was carried out at different feed ratio of styrene (1) and EHA  (2). In this case copolymerization was carried out at low conversion (∼10%) and its molar composition was determined by 1 H NMR spectroscopy. Composition of the low conversion copolymer was used for the determination of monomer reactivity ratios. In this case FR, IFR, and KT methods were used to determine the reactivity ratio of the monomers. In the FR method the following equation  (Figure 4). From this the slope and intercept were calculated to be r 1 = 1.24 and r 2 = 0.71 respectively.
In the IFR method the equation used is So, from the plot of G/H and 1/H ( Figure 5) the reactivity ratios, r 1 and r 2 were calculated as 1.34 and 0.76 from the intercept and the slope respectively. In KT method the reactivity ratio was determined by using the equation  In the plot of η vs. ξ, (Figure 6) the slope gives the value of (r 1 + r 2 /∝) and intercept provides r 2 /∝. From these two values r 1 and r 2 were calculated as r 1 = 1.30 and r 2 = 0.73 respectively. All the parameters used for the three methods are given in Table 2. The reactivity ratios, r 1 and r 2 calculated by the different methods are tabulated in Table 3. They are quite comparable. The values; r 1 > 1, r 2 < 1 and r 2 < r 1 indicate that the styrene has much influence on the copolymer formation during the reaction (Jianying et al., 2006). Srivastava et al. (Srivastava and Rai, 1995) reported the copolymerization of styrene and EHA initiated by azobisisobutyronitrile (AIBN) in bulk in the presence of anhydrous ZnCl 2 . They reported the reactivity ratios for styrene and EHA as 0.10 and 0.175 respectively. Moreover, Kavousian et al. reported the copolymerization of styrene and EHA via conventional radical polymerization (Kavousian et al., 2004). They reported the reactivity ratio of styrene and EHA as 0.926 and 0.238 respectively. There is a difference in the polymerization mechanism of ATRP and non-ATRP processes. In ATRP, atom transfer from an organic halide to a transition metal complex occurs to generate the active radical species, which are FIGURE 6 | Kelen-Tudos plot for copolymerization of styrene with EHA.   then quickly "deactivated" by back transfer of the atom from the transition metal to the radical species (Matyjaszewski, 2002;Braunecker and Matyjaszewski, 2007). So, there is difference in reactivity ratio values in comparison to FRP. Barim et al. stud-ied the FRP and ATRP of phenoxycarbonylmethyl methacrylate (PCMMA) and styrene at 110 • C. They reported the reactivity ratios of PCMMA and styrene prepared via ATRP were 0.33 and 0.96 respectively and the same prepared via FRP were 0.47 and 1.16 respectively (Barim et al., 2007). The reactivity ratio varies with the polymerization temperature (Chambard et al., 1999;McManus et al., 2002). We did the polymerization reaction via ATRP in bulk at 90 • C. However, the reactivity ratios calculated in this work follow the same trend (r EHA < r Styrene ) as reported by the other authors. In addition, in ATRcP technique the product of the reactivity ratios is less than one. It shows the tendency of random copolymer formation, where the chances of incorporation of styrene is more in comparison to EHA. The mean sequence length (l) of the copolymers were determined by using the equations (Pazhanisamy et al., 1997) l 1 = r 1 M 1 M 2 + 1 and l 2 = r 2 M 2 M 1 + 1 where, r 1 (styrene) = 1.29 and r 2 (EHA) = 0.73 The results of the mean sequence length in the copolymer are shown in Table 4. It indicates that the length of EHA increases as its content in the monomer feed increases.

THERMAL PROPERTIES
The glass transition temperature (T g ) of the copolymer was determined by DSC analysis (shown in Figure 7). All the copolymers showed a single T g and they are shown in Table 5. It indicates that as the EHA (T g = −60 • C) content increases the T g decreases. The T g for the copolymers determined by DSC analysis was compared with the same (T g,F-F ) determined by Flory-Fox equation (Lijia et al., 1997). There is consistent difference between the T g values at different content of two comonomers ( Table 5). Table 5 shows that there is some discrepancy between the experimental T g and T g calculated by Flory-Fox equation. The Flory-Fox model is based on the free volume theory. The discrepancy in the T g values is due to the fact that the effect of the chemical nature and organization of the monomers on the mobility of a polymer chain was not considered (Fernandez-Garcia et al., 1999). The block copolymer of styrene and EHA (PS-b-PEHA) was prepared by using polystyrene-Br as macroinitiator. The shift of GPC traces of the diblock copolymer toward lower elution volume indicated the successful preparation of block copolymer. (The GPC traces are shown in Figure S2). This block copolymer showed two T g s, PEHA block at −64 • C and polystyrene at +123 • C (Figure 8). However, the copolymers, poly(styrene-co-EHA) showed only one T g indicating the copolymers were not blocky in nature. Thermal stability of the copolymer was studied by TGA (Figure 9). It is clear that as the styrene content increases, there is a slight increase in T onset . However, there was no significant change in T max as shown in Table 5.

CONCLUSIONS
Copolymers of styrene and 2-ethylhexyl acrylate were synthesized successfully in bulk by using atom transfer radical copolymerization (ATRcP). The chemical composition was studied by 1 H NMR spectroscopy and the reactivity ratios of the two monomers were calculated by using FR, IFR, and KT methods. The reactivity ratios of styrene and EHA were somewhat different from the polymerization reaction of styrene and EHA using the FRP system. DSC analysis showed that the T g of the copolymer increases on increasing styrene content.

SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online at: http://www.frontiersin.org/journal/10.3389/fchem. 2014.00091/abstract