Chemistry Reference
In-Depth Information
220
C to around 130
C. On the other hand the molecular weight increases sub-
stantially.
Fig. 4.38 shows a wider variety of di-imine ligands exemplified in the Goodrich
patent [89]. Exactly the same trends are seen in terms of polymer properties; in
this figure the effect of ligand backbone substitution on molecular weight is dis-
played. Again, no substitution on the backbone (A-C) gives low molecular weight
while any substituent (D-G) results in a dramatic increase in
M
w
.
Our rationale for the influence of backbone substitution on the copolymeriza-
tion process is that the catalyst center is extremely sterically-hindered because of
the size of the growing polymer chain (with large bicyclic norbornanes incorpo-
rated) and the norbornene monomers about to insert in the growing chain. When
there is no substituent on the backbone, the phenyl ring (with its ortho-substitu-
ent) can twist around to accommodate an incoming norbornene unit even when
the last inserted monomer unit was a norbornene also. Therefore in this case nor-
bornene-norbornene linkages can be formed and high norbornene content (up to
>90%) can be attained with corresponding high
T
g
. However, the ability to rotate
also permits chain transfer and results in lower molecular weights. The introduc-
tion of any substituent prevents free rotation and therefore a norbornene unit can
only be accommodated after ethylene insertion, resulting in a limit of 50% nor-
bornene incorporation (alternating copolymer) and therefore
T
g
.
4.4
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