Modern Polymer Spect..

3.12 CIS-trans Opening @the Double Bond 139
The question which remained to be answered for inore than a decade was
whether the opening of the double bond occurs in cis or trans. The understanding of
this reaction mechanism was of fundamental importance for the development of
other similar reactions for other classes of materials.
One way to solve the problem was to polymerize deuterated derivatives of ethylene
with the same Ziegler-Natta catalysts. If cis di-deutero ethylene (monomer A)
is polymerized and the opening of the double bond is CIS. the resulting polymer
should be rich in the so-called ‘cridzro’ units
HH
c-c 7
DD
c-c-c-c
HDDH
DHHD
a 1 2
If the opening is cis for the case of monomer B (tram-dideuteropolyethylene) the
reaction should yield a polymer rich in the so-called ‘threo’ units.
DH
c-c 7
HD
c-c-c-c
HHDD
DDHH
b 1 2
The opposite should occur if the catalyst forces the double bond to open in trans
configuration.
The polymers were made by chemists, but the identification of the configuration
of the deuterium and hydrogen atoms remained unsettled for a long time. The
vibrational spectra were recorded, but their interpretation was impossible on the
basis of the classical techniques of polymer spectroscopy.
An example of the procedure followed in the identification of the spectra is the
following.
Let the configurations of monomeric units be indicated as follows
HD DH HH DD
c-c- 7
-c-c-
- -c-c- i -c-c- 4
DH HD DD HH
I I1 111 IV
From the viewpoint of calculations a computer program which generates a sequence
of 1-andoni numbers was used in order to generate a random sequence g( 1’)