Chemistry Reference
In-Depth Information
were claimed to result in formations of covalent perchlorate esters that are solvated and stabilized by
monomer. The propagation in stage two, therefore, is pseudo cationic and covalent because it consists
of monomer insertion into the C-O bond of a perchlorate ester [
109
,
110
]:
Cl
O
O
2
ClO
2
O
O
When there are insufficient amounts of monomer present to stabilize the covalent esters, such as at
high conversions, ionizations take place. This leads to rapid ionic, stage three polymerizations. The
propagation by free ions and ion pairs in stage three is between 10
4
and 10
5
L/mole s at low
80
C. By comparison, polymerizations in stage two are much
slower, somewhere between 0.1 and 20 L/mole s. The rates depend, of course, upon the solvent and
upon the temperature. While ion-ion pair type propagations yield polymers with molecular weights
equal to 10
4
, the covalent propagations only yield oligomers. Differences in molecular weights and
molecular weight distributions of the products from the three stages of polymerization can be detected
by size exclusion chromatography. The chromatogram shows bimodal distributions. This was taken
as supporting evidence for two modes of propagation, ionic and covalent [
110
].
Similar results were reported in polymerizations of styrene with CH
3
COClO
4
,CF
3
SO
3
H,
CF
3
COOH, ClSO
3
H, FSO
3
H[
109
]. Pseudo-cationic mechanism was also claimed in polymerizations
of some styrene derivatives, like
temperatures, between
60 and
p
-methyl styrene,
p
-methoxystyrene, and
p
-chlorostyrene with these
protonic acids [
109
].
Although the concept of pseudo-cationic mechanism may be accepted by some, it is not accepted
by many. An alternative mechanisms based on ion pairs were offered instead in the past [
111
-
113
]. A
publication by Scwarc claims that all detailed re-examinations of the evidence for pseudo-cationic
polymerizations shows that all the features of styrene polymerization initiated by acids are accounted
for satisfactorily and convincingly by the orthodox ionic mechanism. To support his claim, he
reported that optical absorption and electric conductance of the styrene solutions undergoing poly-
merization induced by perchloric acid in stopped-flow studies demonstrated protonations of the
monomer by the acid and formation of ionic species [
114
].
Further proof against pseudo-cationic polymerization mechanism came from an earlier work by
Matyjaszewski who successfully synthesized the styryl perchlorate and demonstrated that
it
hydrolyzes rapidly in reaction with water [
115
]:
H
2
O
2CH
3
CHPh
OClO
3
!
(CH
2
CHPh)
2
O
รพ
2HClO
4
This contradicts the basic claim of pseudo-cationic polymerization mechanism, as originally
proposed by Gandini and Plesch, that the hydrolysis of the ester should be negligible [
109
].
4.3.3.3
Isomerization Polymerization
In some cationic polymerizations the monomers may rearrange in the process of placements into the
polymeric chains. They isomerize into energetically preferred configurations. The result is that the
