Page 40 - Engineering Plastics Handbook

14 Introduction

(1) fluidized bed, (2) stationary bed, and (3) vacuum. Polymerization sys-
tems influence the molecular architecture and configuration, such as
aliphatic or aromatic, semicrystalline or amorphous, linear or branched;
and other important molecular characteristics such as MW and MWD.
Bulk (mass) polymerization is the polymerization of monomers to
polymers without the presence of solvents or dispersants. Bulk poly-
merization is typically exothermic. Polymerization requires only
monomer and a catalyst or accelerator. Bulk polymerization is used for
many polycondensation polymerizations and chain-growth polymeriza-
tions. It is used with liquid monomers and also with gases and solids. Heat
stabilizers can be introduced during high-temperature polymerizations.
The Tromsdorf effect, also called the Norrish effect or gel effect, is
associated with exothermic reactions during bulk polymerization.
Autoacceleration of the polymerization rate can occur with medium to
high polymerization conversions. This phenomenon inhibits termination.
Strength of the Tromsdorf effect is calculated as the gel effect index [12]

R
γ= p −1
R
p0
where γ= gel effect
R = rate of polymerization with no gel effect (theoretical)
p
R p0 = rate of polymerization with gel effect
The Tromsdorf effect can occur in suspension polymerization of PVC, but
it is not as significant as it is with bulk polymerization.
During conversion from monomer to polymer in bulk polymerization,
the volume of the reaction mixture shrinks. There is less volume shrink-
age with increasing number and length of side chains. To calculate the
specific volume of a reaction mixture during bulk polymerization at a
given temperature [12], use
ν=ν (1 −εχ)
n
3 3
where ν= specific volume of a reaction mixture, cm /g (in /oz)
3 3
ν = specific volume of monomer, cm /g (in /oz)
n
ε= contraction constant for a given polymer
χ= degree of conversion, %
Ring-opening polymerization
Ring-opening polymerization can be a chain-growth polymerization or
condensation polymerization, catalyzed by metal catalysts such as a
tungsten, molybdenum, or rhenium catalyst. With a cyclic polymer, ring
opening is caused by ring strain, which is due to the angle between ring