Catalysis in Acyclic Diene Metathesis (ADMET) Polymerization - Late Transition Metal Polymerization Catalysis - page 219

is easily done with catalyst 11 . ADMET with catalyst 11 could give higher molecu-
lar weight polymers than that with catalyst 10 because the methylidene I d gener-
ated from 11 is less likely to be trapped by PPh 3 than that from 10 with PCy 3 ,ifit
follows the same trend as the benzylidenes shown in Tab. 6.5 [101]. This question
is currently being probed.
It should be mentioned that although the methylidene displays anomalous ki-
netic behavior, complex 10 is active for ADMET and produces higher molecular
weight polymers than 6 , in many cases. In other cases, the two complexes pro-
duce polymers of similar molecular weights. This observation was recently re-
ported for the ADMET of phosphazene-containing monomers, which could be due
to the relative inactivity of the methylidene [104].

6.6.4
Stereochemical Aspects of ADMET

The stereochemical outcome of the metathesis reaction in ADMET with Grubbs
catalysts determines the ratio of cis and trans olefins in the polymer, which affects
the physical properties of the polymer. In addition, consideration of the formation
of metallacyclobutane shows that a great majority of the metathesis events in an
ADMET reaction are non-productive, and it is the occurrence of the less favored
pathways that cause productive ADMET.
The cis to trans ratio of most ADMET polymers is approximately 3 : 7 to 1 : 4
after long reaction times. The preference for trans olefin formation is easily ratio-
nalized by consideration of both approach of the olefin to the metal center for pro-
ductive chain extension and the conformation of the resulting metallacyclobutane
(Scheme 6.29).
The trans microstructure is not only a reflection of the kinetic product rationa-
lized by the metallacyclobutane conformation and approach of the olefin to the
metal carbene, but is also a reflection of the eventual thermodynamic preference
for trans olefins resulting from trans -metathesis olefin equilibration (see Scheme
6.11).
It has been demonstrated, however, that the kinetic product of reaction of 6 or
the propylidene analog with a terminal olefin is the alkylidene, and not the
methylidene. This preference could be explained by the greater stabilization of the
formal Ru(IV) metallacycle by the electron-donating alkyl groups at the

-position

compared to the -position [103]. However, the , -disubstituted metallacycle is re-

quired for productive metathesis (Scheme 6.30).

Scheme 6.29 Stereochemical rationalization of the preference for trans
olefins in ADMET with Grubbs catalysts.