Page 255 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

764 Polymers, Synthesis

SCHEME 14 Tacticity. Free-radical polymerization is most commonly atactic. Low temperature favors syndiotactic
placement. Coordination polymerization produces stereoregular chains.

1. Thermodynamics of Polymerization heat of polymerization will help determine whether the
polymerization process can proceed. Another useful term
The possibility of polymerizing monomers such as
in connection with polymerization thermodynamics is the
α-olefins and their derivatives and various other vinyl
so-called ceiling temperature. The ceiling temperature can
monomers depends on whether the free energy of such be defined either by the equation in Table VI or by the
a process will be favorable. The free energy in turn
idea that at the ceiling temperature the propagation rate
is dependent on the values of the enthalpy or heat of
will be essentially equivalent to the depropagation rate.
polymerization. Table V lists values for the H of
The ceiling temperature phenomenon is not of particular
polymerization for a number of vinyl monomers. In
significance for monomers like ethylene, propylene, or
general the transformation of an alkene to a polymeric
even styrene where the ceiling temperature is far removed
species is quite exothermic. The exact value is dependent
from normal polymerization temperatures; however, it
on the detailed structure of the monomer. One important
becomes quite significant for monomers such as methyl
correlation between monomer structure and heat of
methacrylate and extremely important for systems such
polymerization is related to whether the tertiary carbon
as α-methylstyrene or acetaldehyde. Methyl methacrylate
contains a hydrogen or a methyl group. For example,
the value for α-methylstyrene is approximately one-half
that of styrene. This structural correlation can be seen TABLE VI Heats of Polymerization and Ceiling
a
more clearly in Table VI. As already mentioned, the Temperatures
Heat of Ceiling
TABLE V Heats of Polymerization a polymerization temperature
◦
Monomer ∆H (kcal/mole) T c (bulk) ( C)
Monomer −∆H p (kcal/mole)
Styrene 16 235
Ethylene 22.7 α-Methylstryene 7 61
Propylene 20.5 Methyl acrylate 20 —
Isobutene 12.3 Methyl methacrylate 13 164
1,3-Butadiene 17.4 Formaldehyde 13 −26
Isoprene 17.8 Acetaldehyde 0 −11
Styrene 16.7 Acetone 6 —
α-Methylstryene 8.4 Vinyl chloride 22 —
Vinyl chloride 22.9 Vinylidene chloride 14 —
Vinylidene chloride 18.0 Ethylene 26 407
Tetrafluoroethylene 37.2 Propylene 21 300
Methyl acrylate 18.8 Isobutene 17 50
Methyl methacrylate 13.5 T c = H/( S + R ln[M e ])
Vinyl acetate 21.0 k p
R n∗ + M (R n + 1)

k D
a
H p refers to the conversion of liquid monomer to
amorphous or slightly crystalline polymer. At T c , propagation and depropagation are equally probable.