1. Introduction

Copolymerization is one of the important techniques adopted in effecting systematic changes in the properties of the commercially important polymers. The chemical structure of copolymer depends not only on the monomer units forming the macromolecule but also how such units are distributed along macromolecular chains. This distribution is a direct consequence of each monomer’s reactivity in the copolymer molecule [1]. Reactivity ratios are the most important parameters for composition equation of copolymers and they offer information about the reactivity of monomer pairs. However, the copolymer composition is important in evaluating its own utility. The copolymer composition and its distribution are dependent on their reactivity ratios, which can be evaluated either by employing linear and nonlinear methods or by using other copolymer composition equations [2-4].

Many polymers with reactive functional groups are now being synthesized, tested and used not only for their macromolecular properties, but also for the properties of functional group. These groups provide an approach to a subsequent modification of the polymer for specific end application [5].

N-vinylpyrrolidone (NVP) has been widely investigated for applications in various Fields, [6-10] as these are known to exhibit good biocompatibility due to their hydrophilic nature and low cytotoxicity [11-13]. The amide group of NVP has a high binding affinity for several small and large molecules that are known as good hydrogen-bond acceptors and has been copolymerized with a variety of monomers [14-18].

S. Sundararajan et al. synthesized the copolymer of Glycidal Methacrylate and N- vinylpyrrolidone and the reactivity ratio was determined [19]. It indicates that Glycidal Methacrylate is more reactive and the copolymer formed is random in nature. S. Vijay Kumar et al. studied the reactivity ratio of N-Vinylpyrrolidone and Ethoxy ethyl methacrylate. They found out the reactivity ratio of the copolymer and the distribution of the monomer sequence along the copolymer chain was also calculated [20]. A.S. Brar et al. synthesized the copolymer of N-Vinylpyrrolidone – Methylmethacrylate by free radical bulk polymerization and they determined the reactivity ratio [21]. Dilek Solpan et al. synthesized copolymer N-Vinyl Pyrrolidone – Methylmethacrylate and they studied the swelling property of these hydrogels [22]. Mitesh.B.Patel et al. synthesized the copolymer of N-Vinylpyrrolidone – 2, 4 – Dichlorophenyl methacrylate and they determined that the reactivity ratio of both the monomers is less than unity [23]. They have also determined the Azeotropic composition of the copolymer. Nicolas Gatica et al. synthesized the copolymer of N-Vinylpyrrolidone – Vinyltrimethylsilane and N-Vinylpyrrolidone – Vinyltrimethoxysilane and reported that the copolymers formed are random [24].

In the present work a copolymer of NVP with acrylic acid (AA) is synthesized. The prepared copolymer is characterized by FTIR, H¹-NMR and C¹³ NMR. Reactivity ratios of monomer are calculated using Fineman- Rose (F-R) [25] and Kelen - Tudos(K-T) [26] methods at lower conversion, using the data obtained from Acid Value studies. Distribution of the monomer sequences along the copolymer chain is determined by the statistical methods based on the reactivity ratio obtained by K-T method.

2. Experimental

2.1. Materials

N-Vinylpyrrolidone (Sigma-Aldrich), Acrylic acid (SD Fine Chemicals) are washed with Fuller’s Earth and is kept for a night to settle. Hydrogen peroxide (Spectro Chem) 30% and other solvents are of AR Grade which are of 99% purity. They are used as received from SD Fine Chemicals, Mumbai, India.

2.2. Synthesis of Copolymer

As both the monomers are soluble in water, the copolymerization is carried out in presence of water. Known amounts of monomer NVP, AA and water are taken in a three necked 100ml round bottom flask. When the mixture is heated up to 45℃, 0.5% hydrogen peroxide is added and heated for 3 hours in nitrogen atmosphere with continuous stirring. At this juncture the process is stopped and the reaction is allowed to cool down to the room temperature. The high viscous reaction mixture is poured into the Petri dish and evaporated in vacuum.

2.3. Structure of Copolymer

2.4. Copolymer Characterization

Copolymers are characterized by FTIR spectra using Shimadzu-1800S spectrometer on KBr pellets in the range of 400-4000cm^-1. The H¹-NMR and C¹³-NMR (Brucker AMX-400, at Indian Institute of Science, Bangalore) spectra of copolymers are recorded in methanol as solvent and Tetramethylsilane (TMS) as an internal standard.

3. Results and Discussion

Different homo and copolymers were obtained by the free radical solution polymerization technique. The copolymer with different compositions of NVP –AA is prepared using hydrogen peroxide as initiator in the presence of water as solvent under nitrogen atmosphere at 45℃. The data on the composition of the feed and copolymers are given in Table-1 and Figures 1&2. Figure 1&2 show the copolymerization behavior of monomer as a function of their reactivity ratios and monomer feed ratios.

Table 1. Monomer and Copolymer composition

Figure 1. For NVP monomer

Figure 2. For AA Monomer

3.1. Characterization of Polymers

3.1.1. Solubility Studies

Solubility of the homo and copolymer is tested in various polar and non polar organic solvents. They are soluble in solvents like tetrahydrofuran, dimethyl sulfoximide, dimethyl formamide, dimethyl acetamide, isopropylalcohol and methanol. But they are insoluble in ethylacetae, chloroform, 1,4-dioxane, chlorobenzene, toluene, n-hexane, n-heptane, cyclohexane, carbontetrachloride, diethyl ether and benzene.

3.1.2. Fourier Transform Infrared (FTIR) Spectral Studies

The structures of all the copolymer are confirmed by FTIR spectra as shown in Figure – 3

Figure 3. FTIR spectra of Copolymer

Only important peaks are assigned. A strong absorption band observed in the region 1680 cm^-1 to 1724 cm^-1 is due to the –C = O of NVP and carboxylic acid of AA. The peak at 1440 cm^-1 is due to OC-OH stretching vibration. The peak at 3440cm^-1 is due to –OH bond of AA. The peak at 1330 cm^-1is due to-C-N- stretching in lactam.

3.1.3. Nuclear Magnetic Resonance Spectroscopy (H^1_ NMR) :

H^1_ NMR spectrum of copolymer is as shown in Figure – 4

Figure 4. spectra of the Copolymer

The main chain methylene proton of both NVP and AA units resonate at δ 4.2, 3.5 and 2.5 ppm. The –CH group of NVP appears at δ 4.8 ppm. The ring methylene proton in NVP are assigned at δ =2.2, δ = 1.5 and δ = 2.8 ppm respectively. The proton present in carboxylic group of AA resonates at δ = 11.2 ppm.

3.1.4. Nuclear Magnetic Resonance Spectroscopy (C^13_ NMR) :

The structure of copolymer is also confirmed by C^13_ NMR spectrum which is shown in Figure – 5.

Figure 5. NMR of Copolymer

The carbonyl carbon (- C=0) signals of both NVP and AA appear between δ 177 to 181 ppm. The carbon atoms present in the parent chain appear at δ = 42, 30 and 35 ppm region. The side chain ring methylene carbon signals are assigned at δ = 68, δ =25 and δ = 58 ppm respectively.

3.2. Copolymer Composition and Monomer Reactivity Ratio

Reactivity ratios of NVP and AA were determined by F- R and K-T methods using the data obtained by Acid Value method. The values are given in Table - 2. The equations used for F-R and K-T are

(1)

(2)

Where r₁ and r₂ are the reactivity ratios relating to the monomer NVP and AA respectively. G and F are F-R parameters, ŋ ξ and α are K-T parameters. The plot of G vs. F as shown in Fig-6 gave a straight line with r₁ as slope and- r₂ as the intercept. ŋ, ξ and α are mathematical functions of G and F as indicated and presented in Table-2 for the K-T method. The K-T plot of ξ vs. ŋ is shown in Fig-7 using Acid value data.

Figure 6. Plot for F-R Method

Figure 7. Plot for K- T Method

Table 2. F-R and K- T parameter for NVP /AA copolymer by Acid value method

In the present case of the copolymer, the product r₁r₂ < 1 indicating that the type of copolymer is random.

3.3. Copolymer Microstructure

3.3.1. Probability of Finding a Monomer

Simona et al. calculated the copolymer microstructure and sequence distribution of monomer in the formation of the copolymer. Here, we have attempted to calculate the sequence distribution of the resulting copolymer. The probability of finding the sequence of n NVP and n AA units is calculated as follows.

(3)

(4)

Where P11 and P22 are the probability of an NVP (or AA) unit to be followed by a NVP (or AA) unit. These probabilities Table-(3) are calculated with the following equations and tabulated in Table - 4.

Table 3. Monomer reactivity ratios for NVP/AA Copolymer by F-R and K-T methods

Table 4. Probability values of finding the monomer

(5)

(6)

Where R is the mole ratio of NVP to AA in the feed.

In our case we observed that the NVP/AA copolymer contains predominantly a sequence of NVP. As the concentration in the feed increases the sequence length P₁₁ is also increasing considerably, which is in agreement with higher reactivity of NVP.

3.3.2. Mean Sequence Length

The mean sequence lengths μNVP and μAA are calculated using the following equation.

(7)

(8)

The value of μNVP and μAA are presented in Table - 5.

The μNVP values increase from 0.2720 to 22.050 in the copolymer NVP/AA with an increase of NVP concentration in the feed. Higher NVP content found in the synthesized copolymer correlated with the calculated r₁ and r₂ values indicates that the NVP/AA composition is enriched with the NVP monomer.

Table 5. Mean sequence lengths of monomers in copolymer

3.3.3. Statistical Distribution of Monomer

Statistical distribution of the monomer sequence 1-1, 2-2 and 1-2 are calculated using the following relations

(9)

(10)

(11)

Where r₁ and r₂ are the reactivity ratios, m₁ (NVP) and m₂ (AA) are mole fractions in the copolymer. The mole fractions of 1-1, 2-2 and 1-2 sequences are shown by S_1-1, S_2-2 and S_1-2 respectively. Structural data are given in Table 6.

Table 6. Structural Data for the copolymer

The mole fraction of 1-1 sequence increases as the mole fraction of NVP increases, indicating a higher and higher incorporation of NVP in the copolymer. The sequence length of AA linking to AA S_2-2 is very less, indicates that there is possibility of AA forming homopolymer but it is very less. The values of S_1-2 indicate that there is incorporation of AA in the copolymer but the extent is less.

4. Conclusions

Copolymer of N-Vinyl Pyrrolidone and Acrylic acid is synthesized by free radical solution polymerization technique. The formed copolymer is soluble in most of the polar solvents and insoluble in non polar solvents. The copolymer is characterized by FTIR, H¹-NMR and C¹³ NMR. The reactivity ratio of NVP (r₁) is greater than that of AA (r₂) and the product r₁r₂ is less than one. This shows that the monomers are distributed in the copolymer chain in random fashion. The statistical data calculated - Probability of finding the monomer and Mean frequency length indicate that the copolymer is rich in NVP. This is confirmed by determining the sequence distribution of monomers by statistical method.

ACKNOWLEDGMENTS

I thank faculty of SSIT for their support in conducting the experiments.