Chemistry Reference
In-Depth Information
1.3.2
Novel Ligand Control of PE Molecular Weight
[13]
GC analyses of the reaction solutions show the formation of the homologous se-
ries of ethene oligomers butene, hexene, octene, decene, etc. While the phos-
phane-induced Schulz-Flory distributions fall off rapidly, the ylide-derived catalysts
favor formation of higher oligomers. Oligomers above C
40
are detectable. Both
ylide ligands affect the ratio of the reaction rates propagation/termination. The
quotient of the corresponding rate constants can be derived from neighboring GC
peaks.
The intrinsic viscosities of the solid oligo-/polyethenes from ylide-steered nickel-
catalyzed polymerizations are higher than those produced by related Ni-phos-
phane systems. Provided the polymer structure remains unchanged, the “Ni-ylide-
steered” products consist of longer macromolecules and, accordingly, have higher
molecular weights. IR data show correspondingly fewer end groups (methyl or vi-
nyl) per 1000 C. The DSC curves of the low-melting “Ni-phosphane-steered” oli-
goethenes are contrasted by higher melt temperatures in ylide catalysis. GPC in-
vestigations confirm the expected higher molecular weights for the ylide-derived
products.
In the above-mentioned bis(ylide)nickel series [Eq. (10), top], where the intact
ylide ligand Me
3
PCH
2
is kept constant and the PO component is changed at the
-position from H
Me
Ph, substituting the formyl-methylenephosphorane at
the
-position with a methyl group (hyperconjugation in acetyl-methylenepho-
sphorane) affects the catalytic cycle with respect to turnover and polymer proper-
ties; the effect is even greater for substitution by a phenyl group (
-conjugation in
benzoyl-methylenephosphorane). The catalyst activity increases - and so does the
intrinsic viscosity of the PE formed, from approximately 0.05 to approximately
0.13 dL g
-1
. Chain propagation as opposed to chain termination and chain trans-
fer are favored by the ligand field modifications.
We therefore looked for other (R
3
P-C=C-O) components in the (ylide A/ylide
B/Ni) concept that would allow us to control the molecular weight to a greater ex-
tent.
Indeed, more marked chemical changes in the PO component dramatically al-
tered the selectivity for the PE molecular weight and led to a
novel
ligand-steered
molecular weight control. The molecular weight increases in the ligand series
1
2
3
4
5
6
7 [Eq. (11)].
A sulfonate substituent at the
-position results in high molecular weight PEs.
The already mentioned
-substituent effects on the PE molecular weight can be
detected here again, i.e. the sulfonated benzoyl derivative exceeds the acetyl deriva-
tive. Incorporating the
-carbon into a ring system also increases the
molecular weight, with phosphane-chinone adducts being at the top of the scale.
An extreme range of high to ultra-high molecular weight PE is accessible by
using an in-situ catalyst, obtainable by reacting 1 mmol of each of the three com-
ponents Ni(COD)
2
/Ph
3
P-benzochinone/Ph
3
PCH
2
in 10 ml toluene at 50 C for 2
hours. The heterogeneous reaction mixture catalyzes the polymerization of ethyl-
- and the
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