Bulk Polymerization

Bulk Polymerization

Dilatometery:-

Dilatometery is used to determine the rate of polymerization and the effects of initiator concentration and chain transfer agent on the rate. In this experiment the rate of polymerization will be measured by the use of dilatometer. Dilatometery utilizes the volume change that occurs upon polymerization to follow conversion versus time. The conversion is conveniently followed in a dilatometer whose volume includes a capillary region. The dilatometer is placed in a constant temperature bath and the volume change of the polymerizing system, is followed with time. Dilatometery is not useful for most step polymerizations where there is a small molecule by-product that results in no appreciable volume change upon polymerization.

                        As the dilatometer is placed into the constant temperature bath, initial meniscus movement is due to two factors:

1. Thermal expansion of the monomer

2. Contraction due to polymerization.

                        The reaction mass is heated for initiating the polymerization. As the polymerization proceeds, the viscosity of medium increases. The disadvantage of bulk polymerization is that as the medium gets viscous, the diffusibility of the growing polymer chains becomes restricted, the probability of chain collision becomes less, termination becomes difficult, and the rate of polymerization increases. This whole phenomenon is called auto acceleration, the gel effect or the trommosdorff effect. Trommosdorff effects complicate the kinetic study of polymer formation.

                        This involves an increase in viscosity which leads to a decrease in the rate of termination, since the bulky growing polymer radical cannot diffuse easily through the medium. Thus, the possibility for two polymer radicals to approach each other and participate in a termination process becomes lessened. The rate of termination reaction is limited by the rate at which the reactants can diffuse together to react. Thus lower the effective rate constant Kt, the net effect is to increase the rate of polymerization.

Bulk Polymerization:-

 The polymerization of a monomer in the absence of any medium other than a catalyst or accelerator. The monomers are usually liquids, but the term also applies to the polymerization of gases and solids in the absence of solvents, Also known as Mass Polymerization.

Polymerization of the undiluted monomer. Viscosity increases dramatically during conversion.

Advantages	Disadvantages
Pure products	heat control
Simple equipment	Dangerous
No organic solvents	Molecular weights very disperse

Rate of polymerization:-

The rate of polymerization is defined as the rate at which monomer is consumed.

The rate of polymerization is the same as the rate of disappearance of monomer. Monomer disappears faster when there are more particles. Now suppose the concentration of initiator is left the same. This will give us more particles and fewer radicals. In other words, the rate of termination will be low since there are fewer radicals.