Radical Polymerization Kinetics of Hexyl Methacrylate in Dimethylsulfoxide Solution

: In this study, we conducted a series of polymerization studies of hexyl methacrylate in dimethyl sulfoxide with (0.1 - 0.4) mol dm -3 of monomer and (1  10 -3 – 4  10 -3 ) mol dm -3 of benzoyl peroxide as initiators at 70 °C. Using the well-known conversion vs. time technique, the effects of initiator and monomer concentration on the rate of polymerization (R p ) were studied. An initiator of order 0.35 was obtained in accordance with theory and a divergence from normal kinetics was detected with an order of 1.53 with respect to monomer concentration. The activation energy was determined to be (72.90) kJ mol -1 , which does not correspond to the value of most thermally initiated monomers. The observed value of activation energy suggests that propagation and termination reactions have equal activation energy and the difference between them is nearly zero. The average degree of polymerization (DP n ) decreased as benzoyl peroxide concentration increase whereas an increase in solvent polarity has slightly increased rate of polymerization value.


Introduction
The term rate of polymerization is given by the rate at which the monomer disappears [1]. It can be expressed by: This equation shows a square-root dependency on the initial initiator concentration and a firstorder dependence on the rate of polymerization in relation to the monomer concentration [2]. This reliance has been demonstrated by numerous polymerization systems. For example, 7-(methacroyloxy)-2-oxo-heptylphosphonic acid follows this equation when it was polymerized for kinetic study [1]. It's also noteworthy to observe that the rate of polymerization doubles when the monomer concentration is doubled [3].
When choosing how to carry out a polymerization, the impact of temperature on the degree and Rp is crucial. Increasing the reaction temperature often causes the polymerization rate to increase while the molecular weight of the polymer drops [4,5]. The quantitative impact of temperature is nevertheless complicated because Rp depends on the interaction of the ki, kp and kt. An Arrhenius type relationship can be used to express the rate constants for termination, propagation and initiation: Where Ea is the Arrhenius activation energy, A is the collision frequency factor, and T is the Kelvin temperature. Numerous researchers [6][7][8][9][10] have looked into how the polymerization energy deviates from the fundamental set of rules for free radical polymerization. They found that the free radical polymerization end rate constant, kt, is a diminishing capacity of the responding radical's span and that the end rate between polymer radicals isn't always independent of their chain length. Unless the effects of the chain length dependence on end rate constants are taken into account, regular tactics for the dynamic study of radical polymerization appear to result in incorrect ends. In addition, there are many studies discussed kinetic investigation of different monomers.
For example, Kumar et al. studied the kinetic of free radical polymerization of methyl methacrylate using cyclohexanone/water mixture as a solvent and persulfate potassium as an initiator. The order (0.5) was found with respect to phase transfer catalyst, (1) with respect to monomer concentration and (0.5) with respect to initiator concentration [11]. Victoria-Valenzuela and coworkers [12] have reported the comparison between the kinetic behavior of free radical polymerization of vinyl acetate and methyl methacrylate monomers. Based on the results of this study, the Rp of methyl methacrylate monomer is higher than Rp of vinyl acetate monomer. The aim of this study is to study the kinetic of polymerization of hexyl methacrylate include the influence of the monomer and initiator concentrations, polarity of solvent and average degree of polymerization on Rp. The overall activation energy was also determined.

Materials
Commercial hexyl methacrylate monomer was shaken 2-3 times with 10% NaOH to eliminate hydroquinone inhibitor and then dried over anhydrous CaCl2 for 5 h. After that, the monomer was passed through a 2.5x15 cm aluminum oxide (Al2O3) column until colorless products were obtained. Potassium persulfate and benzoyl peroxide were recrystallized twice from methanol prior to use. The monomers and initiators were filtered and refrigerated prior to use to prevent degradation or polymerization of the monomers. The filtered monomers must be kept in a dark, tight as well closed container. All solvents of 98% purity grade were used as received.

Homo polymerization of Hexyl Methacrylate
For homo polymerization of hexyl methacrylate, the following kinetic experiments were carried out: The test tubes were charged with the specified amounts of monomer HMA (0. were filtered out and dried to a consistent weight at 40 °C. The well-known gravimetric approach was used to quantify monomer to polymer conversion.
The replication of runs ensured that the experiments were repeatable. At a given time, the residual monomer concentration [M] can be calculated using the following equation: Where, C is the fraction conversion and [M]0 is the initial concentration of the monomer. Thus, from the slope of the time-conversion curve, Rp could be determined. Since the free-radical initiator is frequently present in low quantities in kinetic investigations, its concentrations and thermodynamic activity are likely to be similar.

Viscosity Measurement
Ubbelohde viscometer was calibrated according to ASTM D446 and ISO 3105 standard method.
It was used to calculate the viscosity of the Poly HMA solutions. The measurements were carried out at 30 °C using DMSO as standard solvent. Intrinsic viscosity was determined by plotting C versus Ƞsp/c [11,12]. By using the obtained intrinsic viscosity results, Mark-Houwink equation was employed to calculate the average degree of polymerization [13].

Determination of the Rate Equation
RP for a variety of initiator and monomer concentrations might be determined from the lineslope of typical time-conversion curves. Typical time-conversion curves are shown in Figurs (1 and 2) for (variable initiator concentration and fixed monomer) and Figurs (4 and 5) for (variable monomer concentration and fixed initiator).

Rp= [M]0 /100 × slope
[M]0 :Feed monomer concentration, slope should be calculated from the initial lines of % conversion vs time. The relationship between Rp and the concentrations of the initiator and monomer in such a system can be expressed as follows: The logarithmic form of the equation presented above can be used to experimentally determine the order of the reaction with regard to the concentrations of initiator and monomer: log Rp = log K + α log[monomer]+ β log[initiator] The following results can be obtained: From the dependence of Rp, on the monomer concentrations from 0.1 to 0.4 mol dm -3 ; at a fixed initiator concentration: Plot of log Rp, verses log [monomer] has been fitted by linear least-square analysis, allowing both K and α to be calculated from the intercept and slope respectively, as shown in Figure 5 and the rate of polymerization was proportional to the 1.53 power of the monomer concentration. which give rise to exponent < 0.5. A situation may also arise when the order of dependence of R, on [1] will be greater than 0.5 [14].

Overall Activation Energy of Polymerization
Rate of polymerization was calculated from the slope of typical time-conversion curves by using equation (4.1). Typical time-conversion curves and plot of the overall rates of polymerization Rp are shown in Figures 7 and 8 respectively. The activation energy of polymerization of HMA is determined to be (72.90) kJ mol -1 . However, this value is significantly lower than that of typical acrylate monomers such as methyl methacrylate [15,16].  [17,18].

Because most polymerizations initiated by thermal initiators have an overall activation energy ER
of around 70-90 kJ mol -1 , the value of (72.94) kJ mol -1 proposes that the variance between the termination and propagation activation energies is quite modest. For methyl -acetylaminoacrylate polymerization, similar results have been obtained in the literature [19]. The increase in the rate of polymerization could be explained in terms of the organic medium's increased polarity. Thus, greater transfer of benzoyl peroxide radicals to the organic phase may occur.  Table 2).

Conclusions
In this study, we conducted a series of polymerization studies of hexyl methacrylate in distilled water with (0.1 -0.4) mol dm -3 of monomer and (1 10 -3 -4 10 -3 ) mol dm -3 of benzoyl peroxide as initiator at 70 °C. An initiator of order 0.38 was obtained in accordance with theory and a divergence from normal kinetics was detected with an order of 1.68 with respect to monomer concentration. The calculated activation energy was (72.94) kJ mol -1 , which is lower than the value for the 95% of thermally started monomers. The observed activation energy value indicates that propagation and termination reactions have equal activation energy and the difference between them is nearly zero.