1. Introduction

Free radical copolymerization is a valuable tool for tailoring the properties of polymeric materials for a broad range of applications. For statistical copolymers, the monomer sequence distribution and compositional heterogeneity in and among the copolymer chains have a profound influence on the behavior and physical properties of the materials[1-7].

The acrylic monomer, methyl methacrylate (MMA; CH₂=C(CH₃)COOCH₃) has been widely used for applications in various fields[8-12]. The MMA based polymers have good biocompatibility, low toxicity, good film forming and adhesive characteristics[13]. It is also used as a comonomer in surface-active copolymerization mainly because of its good amphilic character[8-9]. Methyl methacrylate as a monomer contains a highly polar ester group, which confirms the hydrophilic nature while the methylene and methane groups in the main chain and side chain support the hydrophobic nature, respectively[10]. Methyl methacrylate and acrylate based copolymers are extensively studied by many investigators in biomedical applications such as membranes for ultra filtration/drug delivery, contact lenses and anticoagulant films[13]. Another alkene monomer, 2-Ethoxyethyl methacrylate (2-EOEMA; H₂C= C(CH₃) CO₂CH₂CH₂OC₂H₅) is a dual functional group monomer, which has both ether and ester groups as compared to most of the vinyl acrylic monomers; these groups not only impart flexibility into the polymer, but also improve its process ability and handling[14] and improve compatibility in blends due to hydrogen bonding[15]. Because of its soft and flexible nature, 2-EOEMA based copolymers and terpolymers could be a better candidate as surfactant[16,17].

The chemical structure of a co-polymer depends on the two-monomer units, which are distributed along macromolecular chains. This distribution is a direct consequence of each monomer’s reactivity in the polymer molecules[18]. Determination of the monomer reactivity ratios with small confidence intervals requires sensitive analytical techniques, careful planning of experiments, and the use of statistically valid methods of estimation[19-20]. In order to calculate the rate of polymerization or polymer productivity and copolymer composition, monomer reactivity ratios must be known[19]. Reactivity ratios were determined by F-R[21] and K-T[22] methods using the data obtained by IR analysis. The present investigation describes the synthesis of the MMA/EOEMA copolymer by free-radical polymerization and the polymer structure was characterized by IR, ¹H-NMR and ¹³C-NMR spectroscopic techniques. Reactivity ratios calculated using the IR data are in good agreement with each other.

2. Experimental

2.1. Materials

Methyl methacrylate (MMA), 2-ethoxyethyl methacylate (2-EOEMA), and 2, 2-Azo-bis-isobutyronitrile (AIBN) were obtained from Sigma-Aldrich Chemical Co., USA. Both MMA and 2-EOEMA, were washed with a dilute aqueous alkali solution (5% NaOH) followed by water, dried over anhydrous CaCl₂ in a desiccator and stored below 0⁰C. The organic solvents such as acetone, chloroform, tetrahydrofurane, dimethylsulfoxide, dimethylformamide, dimethylacetamide, ethyl acetate, isobutyl acetate, benzene, hexane, and 1, 4-dioxane were of AR Grade with 99 % purity. They were used as received from E. Merck Chemicals, Mumbai, India.

2.2. Synthesis

Copolymerization reactions of varying compositions of monomers[MMA/EOEMA: (A) 25:75, (B) 33.3:66.7, (C) 50:50, (D) 66.7:33.3, (E) 75:25] were carried out in 1,4-dioxane solvent at 60℃ under the inert nitrogen atmosphere (Scheme I) For each reaction, calculated amounts of 2-EOEMA, MMA, 1,4-dioxane and 0.5% AIBN were taken in a 250 mL three-necked round bottom flask, placed in an oil bath maintained at 60℃ with continuous stirring. The copolymerization was allowed to proceed to high conversion (less than 15%) under nitrogen atmosphere. The polymerization reaction was stopped by cooling the reaction mixture to room temperature. The polymer product obtained was precipitated in a diethyl ether/hexane mixture to ensure complete removal of residual monomers. Copolymer samples were suction-filtered and dried in vacuum at 40℃ until constant weight.

Scheme 1. General reaction for the synthesis of MMA/EOEMA copolymer

2.3. Solubility Measurement

Solubilities of polymers were tested in polar and non-polar organic solvents. To a 5.0 mL solvent, a known amount of the copolymer sample (5-10 mg) was added and kept stirred for 24 hr. The observations showed that the copolymers were completely soluble in most of the polar solvents such as acetone, chloroform, tetrahydrofurane, dimethylsulfoxide, dimethylformamide, dimethylacetamide, ethyl acetate, isobutyl acetate, and 1,4-dioxane. They were insoluble in non-polar solvents like hexane, cyclohexane, carbon tetrachloride, and benzene and in highly polar water.

2.4. Spectroscopic Analysis of Copolymers

Copolymers were characterized by: i) Fourier-Transform infrared (FT-IR) spectra in acetone recorded on a Simadzu – 1800S spectrometer at 2 cm^-1 resolution with 64 scans over the spectral range of 4000-400 cm^-1; ii) Proton-NMR) and ¹³C-NMR spectra in CDCl₃ or DMSO-d₆ as solvent on a Bruker AMX-400 spectrometer using tetramethylsilane (TMS) as the internal standard at the Indian Institute of Science, Bangalore, India.

3 Results and Discussion

3.1. Copolymers characterization

Copolymers of MMA/EOEMA having different compositions were prepared using AIBN as a free-radical initiator in 1,4-dioxane solutions under nitrogen atmosphere under 60℃.

3.1.1. Infrared Spectral Study

Infrared spectra of the new copolymers are presented in Fig. 1. A strong absorption peak at 1731.7 cm^-1 is due to the carbonyl ester group of MMA and 2-EOEMA units of the polymer. The peak in the range, 1125.4 cm^-1 - 1271.3 cm^-1, is due to a strong absorption caused by an asymmetric C-O-C stretching mode in the vinyl ether of 2-EOEMA unit. The peak at 995.95 cm^-1 is due to the alkene (C₂H₄) group of the 2-EOEMA unit. The peaks at 1386.18 cm^-1 and 1451.56 cm^-1 are due to the Alkyl (-C-H) group of both MMA and 2-EOEMA units. Peaks at 1766.75 cm^-1 and 1789.59 cm^-1 are due to ketone (C=O) group of both MMA and 2-EOEMA units.

Figure 1. IR Spectrum of MMA/EOEMA copolymer

3.1.2. Nuclear Magnetic Resonance Spectroscopy

The ¹H-NMR spectrum of a copolymer product is shown in Fig. 2. The main methylene proton signals of both the MMA and 2-EOEMA units resonate at ∂ 2.09–1.89 ppm, which overlap with the type of compositional and configurational sequences. Hence, it is difficult to assign these. Similarly, methyl proton signals of 2- EOEMA observed at ∂ 0.88–1.29 ppm are overlapping with each other. Two (–CH₂ – C = O) proton signals of the keto group of both MMA and 2-EOEMA are observed at ∂ 2.79 ppm while a multiplet of three methylene (CH₂, CH₂,CH₂) protons for 2-EOEMA appear at ∂ 3.5 – 3.69 ppm. The absorption of methylene protons (–CH₂–O–CH₂–) of the ether group of 2-EOEMA occurs at ∂ 4.12 ppm. The multiplet for protons of conjugated olefins of the copolymer resonates at ∂ 7.35–8.10 ppm, whereas the methyl ester group protons of MMA (COO-CH₃) units resonate around 3.35 ppm, respectively. The structure of the copolymer was further confirmed by ¹³C-NMR spectroscopy (Fig. 3). The carbonyl carbon (>C=O) signals of both the MMA and 2-EOEMA units are appearing at ∂ 205.2 ppm. The signals of ester group carbons (R-COO-R¹) of both the MMA and 2-EOEMA units appear at ∂ 162.8 ppm. The olefinic carbon absorptions of 2-EOEMA appearing in the region, ∂ 134.7 ppm – 125.5 ppm are quite complex. The ether (C - O - C) group carbons of 2-EOEMA occur in the region, ∂ 67.4 ppm – 64.1 ppm, whereas carbon absorptions of the – CH and -O-CH₃ groups of MMA and 2-EOEMA appear at ∂ 51.14 ppm. The side chain methylene carbons of both the MMA and 2-EOEMA absorb at ∂ 29.48 ppm – 28.34 ppm. The spectral region of ∂ 67.63 ppm – 14.76 ppm is quite complex and overlapping may be due to aliphatic carbon resonance in the backbone and side chain of the copolymer. Furthermore, two methyl carbons of 2-EOEMA resonate at ∂ 14.76 ppm.

Figure 2. 1H-NMR Spectrum of MMA/EOEMA copolymer

Figure 3. 13C-NMR Spectrum of MMA/EOEMA copolymer

3.1.3. Copolymer Composition

The composition of monomers in the polymer product was determined by IR spectroscopy through recorded absorption bands for comonomers[23,24]. Pure carbonyl group of the MMA/2-EOEMA copolymer has a characteristic, broad infrared absorption band at 1732 cm-1, while the ether group of 2-EOEMA is at 1152 cm-1. These carbonyl absorbance peaks in the copolymers of different composition are shown in Fig. 4. It is evident that with an increase in 2-EOEMA content, peak-1 increases correspondingly. Using the absorbance of carboxyl groups of MMA and EOEMA of all copolymer compositions, the concentrations of 2-EOEMA and MMA in the copolymer were obtained. The product compositions obtained by the IR method were analyzed and the results calculated according to the following equations[25].

Figure 4. FT-IR spectra of MMA/2-EOEMA copolymers of different compositions of MMA/EOEMA: (A) 25:75, (B) 33.3:66.7, (C) 50:50, D) 66.7:33.3, (E) 75:25

Figure 5. Plot of mol% MMA (M1) in the reaction mixture vs. mol% MMA (m1) in copolymer

Figure 6. Plot of mol% of 2-EOEMA (M2) in the reaction mixture vs. mol% 2-EOEMA (m2) in copolymer

where absorbance, A= log(1/T), T= % transmittance; M_MMA and M_2-EOEMA are molecular weights of monomer units. Compositions obtained by the IR method were compared (Table 1). Plots of mol% of MMA (M₁) and 2-EOEMA (M₂) in the reaction mixture vs. mol% of MMA (m₁) and 2-EOEMA(m₂) in copolymers are shown in Figs. 5 and 6.

Table 1. Monomer and copolymer compositions Monomer (mol %) Copolymer composition (mol %) MMA (M1) EOEMA (M2) MMA(m1) EOEMA (m2) 25.0 75.0 23.0 77.0 33.3 66.7 30.1 69.9 50.0 50.0 42.2 55.8 66.7 33.3 59.2 40.8 75.0 25.0 67.5 32.5

3.1.4. Monomer Reactivity Ratio

The comonomer composition sequence is one of the main factors that influences copolymer behavior and properties. The copolymer composition depends on the monomer feed composition and on the relative monomer reactivity. Therefore, it is very important to study the comonomer reactivity in these systems[26]. Copolymerization reactivity ratios of MMA and 2-EOEMA in the products were determined using the standard Fineman-Rose (F-R)[21] and Kelen-Tudos (K-T)[22] methods based on IR spectroscopy. The equations (1) and (2) were used for F-R and K-T methods, respectively.

(1)

where r1 and r2 are the reactivity ratios relating to the monomers MMA and 2-EOEMA, respectively. G and F are F-R parameters. A F-R plot of G vs. F is shown in Fig. 7.

(2)

Figure 7. F-R plot of G vs. F

Figure 8. K-T plot of . η vs. ξ

where η, ξ and α are K-T parameters, which are mathematical functions of G and F as indicated and presented in Table 2. A K-T plot of η vs. ξ is shown in Fig. 8. The rate of polymerization depends on the nature of the acrylate. This can be understood from the value of 1/r that gives a measure of the reactivity of the acrylate towards 2-EOEMA radical. The value of 1/r1 copolymer is 1.187.

3.1.5. Azeotropic Composition

The shapes of the azeotropic compositions of the copolymer systems and distribution of monomer units are random. The azeotropic composition (N₁) was determined by equation (3).

(3)

For the system, MMA/2-EOEMA, the mean value of 0.5898 obtained indicates that the copolymer is richer in MMA below this point and richer in 2-EOEMA above this point. This behavior also suggests the random distribution of monomers in the polymer. In the present case, the product r₁r₂ < 1 indicates that the copolymer is random.

3.1.6. Copolymer Microstructure

The copolymer microstructure and sequence distribution of monomers in the formation of the copolymer were evaluated. The probability of finding the sequence of n EOEMA and n MMA units is calculated using the following equations:

(4)

(5)

where P₁₁ and P₂₂ are probabilities of MMA and EOEMA units. These probabilities in Table 3 are calculated using equations (6) and (7).

(6)

(7)

where R is the mole ratio of MMA-to-2-EOEMA in the feed.

The values of N_MMA(n) and N_EOEMA(n) shown in Figs. 4 and 5 were calculated for the copolymers at an equimolar feed. In the present investigation, it was observed that the MMA/EOEMA copolymer contains predominantly a sequence of 2-EOEMA, which is in agreement with higher reactivity of 2-EOEMA. The mean sequence lengths μMMA and μEOEMA were also calculated using equations (8) and (9).

(8)

(9)

Table 2. F-R and K-T parameters for MMA/E-OEMA copolymers from IR data

Table 3. Monomer reactivity ratios for MMA/E-OEMA Copolymer by F-R and K-T methods Method r1 (EOEMA) r2 (MMA) 1/ r1 1/ r2 r1 r2 N1 F-R method 0.87640 1.005 1.1410 0.9953 0.8805 0.04774 K-T method 0.81081 0.54556 1.2333 1.8330 0.4423 0.7061 Mean 0.84361 0.7751 1.1872 1.4141 0.661 0.5898

Table 4. Statistical data for MMA/EOEMA copolymers Sample no. P11 P22 μEOEMA μMMA A 0.954697 3.100793 1.281203 3.32542 B 1.093555 2.325583 1.421171 2.552608 C 1.843608 1.55028 1.843608 1.77514 D 4.855617 1.16213 2.689749 1.386989 E 9.843608 1.03352 3.530823 1.25838

The values of μMMA and μEOEMA presented in Table 4 show that the μEOEMA value increases from 1.281203 to 3.530823 in the copolymer with an increase of 2-EOEMA concentration in the feed. The higher 2-EOEMA content found in the synthesized copolymer correlated with the calculated r₁ and r₂ values indicating that the MMA/EOEMA composition is enriched with the EOEMA monomer.

4. Conclusions

Copolymers of MMA and 2-EOEMA were synthesized using AIBN as a radical initiator in solution. The copolymers obtained show good solubility in most of the polar solvents and insoluble in non-polar solvents. The characterization of copolymers performed by IR, ¹H-NMR and ¹³C-NMR techniques confirms the chemical structure and composition.

The reactivity ratios determined by two standard methods using IR data are in good agreement with each other. Calculated reactivity ratios indicate that 2-EOEMA is more reactive than MMA. The monomer sequence distributions calculated from the statistical method also indicates the copolymer formation with predominately-random distribution of the monomers with a higher content of 2-EOEMA units in the polymer chain. However, reactivity of the monomer depends on its strength and nature of the functional groups present.