Page 234 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Free Radical Chain Polymerization 197

where ΔH is the heat of polymerization defi ned by
p
ΔH = E – E dp (6.38)
p
p
where E is the activation energy for propagation and E is the activation energy for depoly-
dp
p
merization.
The entropy term is negative so that it is the enthalpy or energy term that “drives” the polymer-
ization. At low temperatures the enthalpy term is larger than the TΔS term so that polymer growth
p
occurs. At some temperature, called the ceiling temperature, the enthalpy term and entropy term are
the same and ΔG = 0. Above this temperature, depolymerization occurs more rapidly than polymer
p
formation so that polymer formation does not occur. At the ceiling temperature depolymerization
and polymerization rates are equal. The ceiling temperature is then defi ned as
∆H p
T c = (6.39)
∆S p

since ΔG = 0.
p
The ceiling temperature for styrene is about 310°C, for ethylene it is 400°C, for propylene it
is 300°C, for methyl methacrylate it is 220°C, for tetrafluoroethylene it is 580°C, and for alpha-

methylstyrene it is only 61°C.
It is interesting to note that due to their industrial importance, free radical polymerizations are
the most studied reactions in chemistry. Furthermore, the kinetic approaches taken in this chapter
are experimentally verified for essentially all typical free radical vinyl polymerizations.

There is some tendency for the formation of stereoregular sequences, particularly at low tem-
peratures, but ionic and coordination catalysts are far superior in this aspect and are used to create
stereoregular macromolecules.
Several additional comments are appropriate concerning chain type polymerization and termi-
nation. First, since the slow step is the initiation step and the other steps, and especially termina-
tion, have very low energy of activations and so are very fast, how does polymer form? Consider
the relative concentrations of the various species. The concentration of polymer monomer is very
high relative to the concentration of the free radical species favoring the formation of chain growth.
The rate of termination is proportional to the square of the concentration of growing chain for both
coupling (Equation 6.18) and disproportionation (Equation 7.20), and as a consequence of the con-
centration of growing chain being relatively quite small, polymer is allowed to form. Thus, chain
growth resulting in polymer formation is the consequence of the high concentration of monomer
and low concentration of growing chains. Second, in general, as the steric hindrance increases the
tendency for termination occurring through disproportionation increases since coupling requires
the approach of the ends of two growing chains. This approach of growing chain ends becomes, in
general, less favorable as the steric hindrance increases.

6.3 CHAIN TRANSFER
Transfer of the free radical to another molecule serves as one of the termination steps for general
polymer growth. Thus, transfer of a hydrogen atom at one end of the chain to a free radical end of
another chain is a chain-transfer process we dealt within Section 6.2 under termination via dispro-
portionation. When abstraction occurs intramolecularly or intermolecularly by a hydrogen some
distance from the chain end, branching results. Each chain-transfer process causes the termination
of one macroradical and produces another macroradical. The new radical sites serve as branch
points for chain extension or branching. As noted above, such chain transfer can occur within the
same chain as shown below:

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