Catalytic addition reactions of styrene

Abstract

Hydrogen chloride inhibits the long chain polymerisation of styrene catalysed by stannic chloride. Styrene reacts with hydrogen chloride in the presence of stannic chloride and in carbon tetrachloride medium to form [alpha]-phenylethyl chloride and low molecular weight polymers. To support the main kinetic investigations the products to the reaction have been examined. The reactions are compared with the short chain polymerisation of styrene in the presence of strong acids and are consistent with the mechanism in which a proton,derived from the interaction of hydrogen chloride and stannic chloride,attaches itself to the styrene molecule or may grow by the accretion of monomeric styrene until stabilised to form short chain polymers.The reaction rate is measured by following the fall in styrene concentration. It is shown that, provided the hydrogen chloride is initially in excess over styrene and stannic chloride, some simple kinetic relationships are discernible. The effects of varying the styrene and catalyst concentrations are investigated by 'flow' experiments.It appears that the addition is first order with respect to styrene and there is good evidence to show that over the range of styrene concentration 0.05 to 0.2 M the rate of consumption is proportional to the concentration of styrene. Also up to 0.015 M stannic chloride the first order reaction constant k is directly proportional to the catalyst concentration. However above 0.015 M this simple relationship does not hold.At high styrene concentration the experimental evidence is conflicting. There is some evidence that there is a lowering of the reaction order, but on the occasions when the reaction obeys the usual first order reaction, k, is proportional to the square of the catalyst concentration.Experiments varying the initial hydrogen chloride concentration show that the rate of consumption of styrene is proportional to the hydrogen chloride concentration provided the latter is in excess over styrene and stannic chloride. However when the stannic chloride chloride becomes equal to or falls below the acid concentration there is again a lowering of the reaction order. The simple thoery mentioned earlier is discussed in the light of kinetic evidence. The catalyst may form a complex with (a) hydrogen chloride (b) styrene but kinetic evidence does not clearly between the alternatives, (a) is favoured when the hydrogen chloride is in considerable excess over styrene and stannic chloride. However,its application is limited and as styrene or stannic chloride increase in concentration it is probable that the styrene complex plays a part. Also the reaction is complicated by the formation of low polymers.