Chemistry Reference
In-Depth Information
The same
T
c
applies to all polystyrenes of given molecular weight and tacticity,
for example, regardless of whether they were polymerized by anionic, cationic, or
free-radical initiation.
It is evident that
T
c
will be lower if
H
p
Δ
is small. It is found, in fact, that
S
p
varies little between monomers so that the ceiling temperature is dominated
by the magnitude of
Δ
H
p
.
Vinyl monomers with 1,1-disubstitution generally have lower standard enthal-
pies of polymerization and lower ceiling temperatures than the corresponding sin-
gly substituted analogs. Thus, the ceiling temperatures of polystyrene and poly
(alpha-methylstyrene) are 310 and 61
C, respectively, for high-molecular-weight
polymer in equilibrium with pure liquid monomer. It is interesting that poly
(alpha-methylstyrene) unzips to monomer if degradation is initiated by free radi-
cals at temperatures above its ceiling temperature. Poly(methyl methacrylate)
with
T
c
165
C exhibits the same behavior, and depolymerization is a valuable
method for recovering scrap quantities of this polymer as monomer. Polystyrene
and most other polymers that have fairly high ceiling temperatures and contain
hydrogen atoms bonded to tertiary carbon atoms do not produce major quantities
of monomer when they are thermally degraded. This is because other modes of
molecular scission occur at temperatures lower than
T
c
.
The concepts developed in this section can be used to calculate how much
monomer will be in equilibrium with high-molecular-weight polymer at any tem-
perature. This is useful information because many monomers are toxic or have
offensive odors and it is often necessary to limit their concentrations in their poly-
mers.
Equation (8-126)
is valid at any temperature since [M]
e
will vary along
with
T
. The equilibrium monomer concentration is given by
Δ
H
p
S
p
R
e
5
Δ
RT
2
Δ
ln
½
M
(8-126a)
100 J/
deg mol. From
Eq. (8-126a)
, only a negligible concentration of monomer will
be in equilibrium with high-molecular-weight polymer at 25
C. If unreacted
monomer can be purged from the polymer, no significant concentration will
develop thereafter at room temperature because of the equilibrium of reaction
(8-121).
In contrast,
For vinyl chloride,
ΔH
p
52
96 kJ/mol and
ΔS
p
can be taken to be
2
Δ
H
p
for methyl methacrylate is only
56 kJ/mol, with a
Δ
S
p
of
2
117 J/deg mol. The concentration of monomer in equilibrium with high poly-
mer at 25
Cis2
2
10
2
4
M
and [M]
e
at 100
C is calculated to be approximately
0.05
M
[from
Eq. (8-126a)
assuming no significant
3
temperature dependence
of
S
p
]. If a free-radical polymerization of methyl methacrylate were
being carried out at 100
C it would not be possible to convert more than 95%
(1
Δ
H
p
or
Δ
[M]
e
) of the monomer to polymer. For reasons given above, however, one can
synthesize the polymer at a lower temperature and use the stabilized product for a
2
