Aqueous Polymerization of Acrylonitrile with Cerium ( IV )-p-Hydroxyacetophenone Redox System

Aqueous polymerization of acrylonitrile initiated by Ce(IV)/ p-hydroxy acetophenone (Ce(IV) – HAP) was studied in aqueous solution of sulfuric acid at 40 °C. The rate of polymerization was investigated at various concentrations of monomer, initiator, activator, sulfuric acid and the effect of temperature of 30-70 °C range was studied. The rate of polymerization is governed by the expression Rp = Kp [M] [Ce(IV)] [HAP]. The activation energy of polymerization was found to be 17.9 kJ/mol. A probable mechanism consistent with the observed results is proposed and discussed.


Introduction
Ce(IV) ion either alone or in combination with various organic reducing agents such as alcohols 1 , acids 2 , amides 3 , aldehydes 4 and ketones 5 has been used to initiate vinyl polymerization.The polymerization of acrylonitrile by Ce(IV)-glucose system was studied by Padhi and Singh [6][7][8] .Earlier studies by different investigations indicate that the order of oxidant of ceric ion was found to be ceric perchlorate> ceric nitrate> ceric sulphate [9][10] .It is known that Ce(IV) is an one electron oxidant capable of oxidizing compound in sulfuric acid.Oxidation of any substrate by Ce(IV) occur through a single electron transfer from the substrate to the oxidant 11,12 .The mechanism of oxidation of cerium in many cases is simpler than the cases with other transition metal ions such as chromate, manganite ion etc.This articles reports the result on the polymerization of acrylonitrile in sulfuric acid with the redox system of Ce(IV) -HAP.

Experimental
The monomer (AN) was purified by the usual procedures 13 .Sulfuric acid AR grade was used without further purification.p-HAP (AR) were distilled under reduced pressure.Water doubly over alkaline permanganate were employed in the preparation of the reagents and solutions.The appropriate quantities of a reaction mixture that containing ceric ammonium sulfate, reducing agent and sulfuric acid were placed in clean conical flasks mounted in a rocking thermostat.The monomer and reducing agent were introduced under N 2 atmosphere and the reaction flasks were sealed.The polymerization was allowed to proceed at 40 °C in the dark in a thermostat.The polymerization reactions started almost instantaneously, indicated by the appearance of turbidity that increased with the progress of polymerization ultimately the polymers precipitated out.After desired intervals the polymerization were halted by adding 10 mL of 1% ferrous ammonium sulfate.The precipitated polymer were filtered off on sintered glass crucible, washed repeatedly with water and dried to constant weight at 60 °C in a vacuum oven.The polymer yield was determined gravimetrically.The rate of polymerization (Rp) was calculated from the weight of the polymer formed by using the equation: Rp =1000W/vtM Where W = weight of the polymer in grams, v = volume of the reaction mixture in milliliters, t = reaction time in seconds, M = molecular weight of the monomer

Results and Discussion
The polymerization of acrylonitrile was carried out with Ce(IV) in the absence of the activator, the polymerization process markedly decreased.The polymerization of acrylonitrile by Ce(IV)-HAP redox system occurred at a measurable rate at 40 °C.

Effect of monomer concentration on polymerization rate
The rate of polymerization was investigated by varying the concentration of monomer, acrylonitrile (AN) from 0.10 to 0.50 mol/L at 40 o C, keeping the concentrations of initiator (Ce(IV)), ligand (p-hydroxyacetophenone) and sulfuric acid constant.The initial slope as well as the percentage conversion increases steadily with an increase of monomer concentration.The order dependence of the rate of polymerization on monomer concentration was determined by the double logarithmic plot of Rp versus [AN], the slope of is 1.44 (Figure 1).The reaction order greater than unity with respect to monomer concentration, It may be described to a dependence of the initiation rate on monomer concentration.The order nearly 1.5 dependence of the rate of polymerization on monomer concentration as confirmed by the double logarithmic plot has widely been reported in redox initiated polymerization systems.Generally, the order with respect to initiator is 0.5 when termination is bimolecular in free radical polymerization processes and it drops to nearly zero when primary radical termination predominates.The initiator rate exponent of nearly 0.5 confirms bimolecular termination in the present study.The rate of polymerization rises rapidly with [Ce(IV)] until a concentration of approximately 0.05 M is reached; further increases in the concentration have a retarding effect on the rate of polymerization.The decrease of rate of polymerization at higher [Ce(IV)] can be accounted for by considering that at high concentrations, the oxidation and termination process increases.

Effect of ligand concentration on polymerization rate
The rate of polymerization was measured with different concentrations of p-hydroxyacetophenone at a fixed concentration of monomer (0.25 M), initiator (0.0025 M), sulfuric acid (0.01M), and at constant temperature 40 °C for 60 min duration.The rate of polymerization increases with increasing ligand concentration from 0.1 to 0.5 M. The order of reaction was found to be nearly half (0.51) from the double logarithmic plot of Rp versus [HAP] (Figure 3) and this suggests a bimolecular mechanism of growing polymer radical termination.This fractional order dependence on [HAP] indicates that the polymerization is initiated by primary radical with termination by mutual reaction of two growing polymer radicals.

Effect of concentration of acid on rate of polymerization
The rate of polymerization was measured with different concentrations of sulfuric acid at fixed concentration of monomer (0.25 M), initiator (0.0025 M), ligand (0.002 M) and at constant temperature 40 °C for 60 min duration.The effect of concentration of sulfuric acid on the conversion is shown in Figure 4.It is clear that the addition of sulfuric acid causes a substantial increase in rate of polymerization at lower concentration ranges and there after the rate of polymerization decreases.It is evident that at low acid concentrations, the reducing agent may be deprotonated so that the coordination becomes easier for the ceric salt whereas at higher acid concentrations the Rp decreases because the complexation may be high.The increase in sulfuric acid concentration causes the formation of less reactive complexes of Ce(IV) ion, which is responsible for decreasing the rate of polymerization.

Effect of reaction temperature on rate of polymerization
The rate of polymerization increases with an increase in temperature from 30-60° C.This is because as the reaction rate of Ce(IV) reduction of ligand increases with temperature, there will be slow and steady generation of free radicals, and hence the polymer yield increases gradually in the temperature range 60°-80° C. At above 60 °C reduction becomes very fast, resulting in a local over concentration of free radicals.As high temperature causes an increase on the rates of chain termination reactions, a decrease was observed form 60 °C to 80 °C (Figure 5).The increase in the Rp with increasing temperature may be attributed to: increase in the mobility of monomer and initiator molecules in the polymerization medium increase in the initiation and propagation rates of polymer.The decrease in the maximum conversion as the temperature increases may be due to the side reaction.This rate expression explains clearly the dependence of Rp on HAP and monomer concentrations.