Chemistry Reference
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where B represents boron and Me aluminum.
A different mechanism, however, was offered by Nakano and coworkers [
47
]. They felt that there
must be a relationship between the crystal structures of the heterogeneous catalysts and the resultant
stereoregularity of the polymers. If the crystal structures of the catalysts are tetrahedral and the
crystals have active edges, stereoregular polymers should form even at room temperature. In addition,
shorter active edges make the catalysts more suitable for stereospecific polymerization. The follow-
ing mechanism was, therefore, proposed [
47
].
If the terminal end of the growing chain ends:
O
R
have sp
2
type configurations, vacant orbitals on the terminal carbon atoms of the growing polymeric
chains are in a state of resonance with the lone pair of electrons on the adjacent oxygen atoms. This means
that the positive charges are distributed to the adjacent oxygens and are not localized on the carbons:
δ
δ
Counterion
O
R
The monomer can potentially add in four different ways:
1.
2.
δ
3.
δ
4.
δ
δ
δ
O
δ
O
δ
O
δ
O
RO
OR
R
OR
R
R
R
OR
Reaction 3 yields isotactic polymers and should be the mode of addition [
47
] when isotactic
polymer forms.
4.3.3.2 Pseudo-Cationic Polymerization
Most cationic polymerizations of olefins proceed through carbon cation carriers. There are, however,
instances of cationic polymerizations where the evidence was interpreted as suggesting that the
propagating species are not carbon cations. Instead, the reactions were said to proceed through
covalently propagating species. Such reactions were termed
pseudo cationic
[
109
,
110
]. In these
polymerizations the propagating species may be combinations of ionic (free ions and ion pairs) and
covalently bonded species. Reaction conditions were claimed to determine the relative amounts of
each. Examples of such polymerizations are cationic polymerizations of styrene or acenaphthene with
protonic acids like HClO
4
or iodine as the initiators. The propagations were suggested [
110
] to take
place in three successive stages when the reactions are carried out in methylene chloride at
20
C. In
the first one, rapid, short-lived ionic reactions take place. In the second stage, the ions can no longer
be detected by spectroscopy or conductivity measurements. In the third one, rapid increase in the
presence of ionic species was shown (detected by conductivity measurements and spectroscopy). At
temperatures between
20 and 30
C there is effectively no stage one and stage three is shorter. On
the other hand, at temperatures as low as
80
C there is only stage one. In stages one and three,
propagation takes places through combinations of free ions and ion pairs. These combinations of ions
