Synthesis and Characterization of Organotin Containing Copolymers: Reactivity Ratio Studies

Organotin monomers containing dibutyltin groups – dibutyltin citraconate (DBTC) as a new monomer and dibutyltin maleate (DBTM) – were synthesized. Free radical copolymerizations of the organotin monomers with styrene (ST) and butyl acrylate (BA) were performed. The overall conversion was kept low (≤15% wt/wt) for all studied samples and the copolymers composition was determined from tin analysis using the Gillman and Rosenberg method. The reactivity ratios were calculated from the copolymer composition using the Fineman-Ross (FR) method. The synthesized monomers were characterized by elemental analysis, 1H-, 13C-NMR and FTIR spectroscopy.


Introduction
Copolymerization is one of the most important means to improve the performance of polymers. Copolymers are extensively used in industrial processes, because their physical properties, such as elasticity, permeability, glass transition temperature (T g ) and solvent diffusion kinetics can be varied within wide limits [1,2]. Knowledge of a copolymer's composition is an important factor in the evaluation of its utility [3,4]. Controlling the polymer property parameters, such as copolymer OPEN ACCESS composition, copolymer sequence distribution and molecular weight averages, is of particular importance in copolymerization processes. This is because copolymer density and viscosity, which are two of the most important property measures used by polymer manufacturers, depend on these parameters [5]. Reactivity ratios are among the most important parameters for the composition equation of copolymers, which can offer information such as the relative reactivity of monomer pairs and help estimate the copolymer composition [3]. To calculate the polymerization rate or polymer productivity and copolymer composition, monomer reactivity ratios must be known. The method which is used most often nowadays for estimating monomer reactivity ratios is to perform a low conversion copolymerization at various initial monomers feed compositions. Subsequently, the copolymer composition is determined for each reaction [5]. Reactivity ratio values may be evaluated by various procedures: linear procedures, nonlinear procedures, and other copolymer composition equations [6][7][8][9].
The present article investigates the synthesis, and structural characterization of copolymers of dibutyltin citraconate (DBTC), and dibutyltin maleate (DBTM), with styrene (ST) and butyl acrylate (BA) as well as the reactivity ratios in the copolymerization. For this purpose, reactivity ratios for the classical copolymerization model were determined using the linearization methods of Finemann-Ross (FR method) [19].

Copolymerization Method
Copolymerization of DBTM or DBTC with styrene (ST) and butyl acrylate (BA) was done in solution using benzoyl peroxide as initiator via the free radical technique (Schemes 3 and 4). Copolymerization was done at 70 ºC in benzene with a total concentration of 2 mol/L at different time intervals. The formed copolymer was precipitated in an excess amount (20 fold), of the corresponding solvent, and was purified by washing with excess precipitation solvent or by reprecipitation from benzene, chloroform, or acetone, depending on the type of copolymer. All samples were dried in an oven under vacuum at 40-60 ºC. Different copolymers with different ratios were prepared and the percentage of tin was determined in each sample [20] (Table 2).
So the copolymerization of DBTM or DBTC with BA showed the highest overall conversion, compared to ST. Based on the percentage of tin (Table 3), the copolymer composition showed that the content of DBTM in (III) is higher than in (IV) and the copolymer composition showed that the content of DBTC in (V) is lower than in (VI) after 4 h [21]. The structural characterizations of (III), (IV), (V) and (VI) were done by FTIR and 1 H-NMR spectroscopy. The FTIR spectrum of (III), (IV), (V) and (VI) with overall conversion after 8 hrs of 13.89, 36.80, 16.55 and 38.40%, respectively, was characterized by the disappearance of C=C stretching bands at 1,582 and 1,610-1,640 cm -1 of DBTM or DBTC with ST or BA, respectively which confirm the formation of the copolymer.
Generally, the FTIR spectrum showed peaks at 1,452, 1,492, 1,583, and 1,601 cm -1 assigned to C=C stretching of the aromatic ring of ST [8]. The FTIR spectrum also showed peaks at 3,024, 3,058, and 3,080 cm -1 assigned to C-H stretching of the aromatic ring, and peaks at 2,848, 2872, 2931, 2,914, and 2957cm -1 assigned to aliphatic C-H stretching. On the other hand, the FTIR showed characteristic peaks at 1,605 and 1,736 cm -1 assigned to C=O stretching of DBTM and BA, respectively.
The 1 H-NMR spectrum of (III) and (IV) was characterized by the disappearance of peaks at δ 5.49-6.23 ppm (-CH=CH-and CH 2 =CH-) or (-CH=CCH 3 -and CH 2 =CH-) of DBTM or DBTC with ST or BA, respectively, which confirm the formation of the copolymer.

Poly(DBTM-co-ST) (III)
Poly(DBTM-co-ST) (III) was prepared using different ratios of DBTM and ST with BPO as initiator and the polymerization was stopped at an overall conversion ≤15 wt/wt%. The copolymer was precipitated in excess methanol. The percentage of tin was calculated according to the Gilman and Rosenberg method [20], and subsequently the copolymer composition (f) was determined as shown in (Table 4). The monomers reactivity ratios and the content of the reaction mixture and the copolymer was calculated according to the FR method [22][23][24] (Table 5). The FR parameters for DBTM and ST (Table 6) were calculated by plotting the relation between F(f-1)/f and F 2 /f. From the values of the experimental reactivity ratio, r 1 (k 11 /k 12 ) is smaller than r 2 (k 22 /k 21 ), it is evident that monomer DBTM (r 1 = 0.099) is less reactive towards the addition of its units compared to the addition of ST units. On the other hand, the route of ST (r 2 = 9.9065) is more reactive towards the addition of its units compared to the addition of DBTM units. As r 1 r 2 < 1, so the copolymer tends to random distribution of its monomer units [15].

Poly(DBTM-co-BA) (IV)
Poly(DBTM-co-BA) (IV) was prepared using different ratios of DBTM and BA with BPO as initiator and the polymerization was stopped at an overall conversion ≤15 wt/wt%. The copolymer was precipitated in excess methanol and was purified by reprecipitation in chloroform. The percentage of tin was calculated according to the Gilman and Rosenberg method [20], and subsequently the copolymer composition (f) was determined as shown in Table 7. The monomers reactivity ratios and the content of the reaction mixture and the copolymer was calculated according to FR method [12][13][14] ( Table 8). The FR parameters for DBTM and BA (Table 6) were calculated by plotting the relation between F(f-1)/f and F 2 /f. From the values of the experimental reactivity ratio, r 1 (k 11 /k 12 ) is smaller than r 2 (k 22 /k 21 ), it is evident that monomer DBTM (r 1 = 0.0248) is less reactive towards the addition of its units compared to the addition of BA units. On the other hand, the route of BA (r 2 = 24.431) is more reactive towards the addition of its units compared to the addition of DBTM units. As r 1 < 1 and r 2 > 1, so the copolymer will contain blocks of BA with low random units of DBTM due to the high reactivity of BA with its high reactivity ratio compared to DBTM [25]. Moreover, as r 1 r 2 < 1, so the copolymer tends to random distribution of its monomer units.

Poly(DBTC-co-ST) (V)
Poly(DBTC-co-ST) (V) was prepared using different ratios of DBTC and ST using BPO as initiator and the copolymerization was stopped at overall conversion ≤15 wt/wt%. The copolymer was precipitated in excess methanol. The percentage of tin was calculated according to the Gilman and Rosenberg method [20], and subsequently the copolymer composition (f) was determined ( Table 9). The monomers reactivity ratios and the content of the reaction mixture and the copolymer was calculated according to FR method [22][23][24] (Table 10). The FR parameters for DBTC and ST (Table 6) were calculated by plotting the relation between F(f-1)/f and F 2 /f.
From the values of the experimental reactivity ratio, r 1 (k 11 /k 12 ) is smaller than r 2 (k 22 /k 21 ), it is evident that the monomer ST prefers the addition of its units compared to the addition of DBTC units. As r 1 < 1 and r 2 > 1, so the copolymer will contain blocks of ST with low random units of DBTC due to the high reactivity of ST with its high reactivity ratio compared to DBTC [25]. Finally, when r 1 < 1, the copolymerization is preferred and when r 2 > 1, ST will tend to homopolymerizations.

Poly(DBTC-co-BA) (VI)
Poly(DBTC-co-BA) (VI) was prepared using different ratios of DBTC and BA using BPO as initiator and the polymerization was stopped at overall conversion ≤15 wt/wt%. The copolymer was precipitated in excess methanol and was purified by reprecipitation from acetone. The percentage of Tin was calculated according to Gilman and Rosenberg method [20], and subsequently the copolymer composition (f) was determined as shown in (Table 11). The monomers reactivity ratios and the content of the reaction mixture and the copolymer was calculated according to FR method [22][23][24] ( Table 12). The FR parameters for DBTC and BA (Table 6) were calculated by plotting the relation between F(f-1)/f and F 2 /f.
From the values of the experimental reactivity ratio, r 1 (k 11 /k 12 ) is smaller than r 2 (k 22 /k 21 ), it is evident that monomer DBTC (r 1 = 0.2727) is less reactive towards the addition of its units compared to the addition of BA units. On the other hand, the route of BA (r 2 = 33.611) is more reactive towards the addition of its units compared to the addition of DBTC units. As r 1 < 1 and r 2 > 1, so the copolymer will contain blocks of BA with low random units of DBTC due to the high reactivity of BA with its high reactivity ratio compared to DBTC [25]. Moreover, when r 1 < 1, the copolymerization is preferred and when r 2 > 1, BA will tend to homopolymerization.

Characterization
1 H-and 13 C-NMR Spectra were recorded on a Jeol (400 MHz) instrument. FTIR Spectra were recorded on a Perkin Elmer 883. Elemental analyses were performed at Perkin Elmer Series II CHN/O Analyzer 2400. Thin-layer chromatography (TLC) was performed using the ascending technique with silica gel 60F 254 precoated aluminum sheets.

Synthesis of Dibutyltin Maleate (DBTM, I)
In a 500 mL round bottom flask, maleic anhydride (4.90 g, 50.0 mmol) was added to dibutyltin oxide (12.44 g, 50.0 mmol) in dry benzene (170 mL). The mixture was heated under gentle reflux for 9 h. The formed precipitate was removed by filtration, and the solvent was totally evaporated on a rotavapor to give an oily residue. The oily residue was dissolved in diethyl ether (100 mL) under gentle heating with stirring. The solution was filtered and the filtrate was concentrated to one third its volume on a rotavapor. The solution was cooled to room temperature and then was kept in freezer (-10 ºC) for 72 h to give a white precipitate. The formed precipitate was filtered, recrystallized from diethyl ether [26] and was dried under vacuum at 40 ºC for 24 h to give 10.20 g, 58.8% yield and m.p. 127-129 ºC. The product I was characterized by elemental analysis (Table 1), FTIR and 1 H-and 13 C NMR spectroscopy.

General Procedure for Copolymerization
Copolymerizations were carried out in a three neck round bottomed flask. Copolymerization was done in solution by dissolving benzoyl peroxide (BPO) (1% mol) in 2 mL of the corresponding solvent, and then the calculated molar quantities of the monomers were added. The reaction mixture was bubbled with nitrogen to expel oxygen. Copolymerization was done at 70 ºC for the desired period of time. The formed copolymer was precipitated in excess amount (20 fold), of the corresponding solvent. All samples were dried in oven under vacuum at 40-60 ºC. For reactivity ratio determination, the copolymerization was stopped at overall conversion below 15% wt/wt [25] from the total weight of both monomers by changing the time of polymerization.

Overall Conversion
The overall conversion in copolymerization [25,27,28] of monomers DBTM and DBTC with ST and BA was studied by taking a fixed number of moles (20 mmol), and composition of 20% mol of monomer DBTM or DBTC and 80% mol of ST and BA, in benzene with a total concentration of 2 mol/L at different time intervals. The overall conversion by weight (wt/wt%) was determined using Equation (1)

Reactivity Ratios Determination
For reactivity ratio determination, copolymerizations were performed with different initial feed ratios while maintaining the monomer conversion below 15%. The Fineman-Ross (FR) method was employed. The initiator concentration was kept at 1% relative to the total monomers concentration in benzene. Monomer reactivity ratios can be calculated from the experimental results depending on the copolymer composition. Copolymer composition can be expressed as following; Moreover, the feed composition of the reaction mixture is known in advance, so feed composition was used in the calculations of the reactivity ratios and can be expressed as follows: In this research, the calculations were based on the tin content in the copolymer composition [29]. The Fineman-Ross (FR) [30] method is based on the use of copolymer composition and the content of the polymerization mixture. Based on the calculations of the copolymer composition and feed composition and according to equation (2): A plot of (F 2 /f) on X-axis vs {F/f(f-1)} on Y-axis gave a straight line, the intercept is r 2 and the slope is r 1 .

Conclusions
The organotin monomers dibutyltin citraconate (DBTC) as a new monomer, and dibutyltin maleate (DBTM), were synthesized. The organotin monomers were copolymerized with styrene (ST) and butyl acrylate (BA) using a free radical technique. The overall conversion was kept low (≤15% wt/wt) for all studied samples and the copolymers composition was determined from tin analysis. From the values of the experimental reactivity ratio, r 1 (k 11 /k 12 ) is smaller than r 2 (k 22 /k 21 ), it is evident that poly(DBTMco-ST) (III), poly(DBTM-co-BA) (IV) tend to random distribution of its monomer units as r 1 r 2 < 1. For poly(DBTC-co-ST) (V), r 1 < 1 and r 2 > 1, so the copolymer will contain blocks of ST with low random units of DBTC due to the high reactivity of ST with its high reactivity ratio compared to DBTC. For poly(DBTC-co-BA) (VI), r 1 < 1 and r 2 > 1, so the copolymer will contain blocks of BA with low random units of DBTC due to the high reactivity of BA with its high reactivity ratio compared to DBTC.