Well-Defined Transition Metal Catalysts for Metathesis Polymerization - Late Transition Metal Polymerization Catalysis - page 168

Scheme 5.11 Cyclopolymerization of heptadienes. X=C(COOEt) 2 , C((1 S ,2 R ,5 S )-(+)-menthyloxy-
carbonyl), C(COOEt)((1 S ,2 R ,5 S )-(+)-menthyloxycarbonyl).

resulting conjugated polymer contains both 5- and 6-membered rings, resulting
from competitive

-insertions (Scheme 5.11) [115, 116].
In accordance with the concept of “small” alkoxides elaborated for

- and

-insertion
[117, 118], a catalyst containing large carboxylate groups, (Mo(N-2- t -butylphenyl)
(CH- t -Bu)(O 2 CCPh 3 ) 2 ), was found to produce a dipropargylmalonate-polymer
which solely contained 6-membered rings [119]. Vice versa, catalytic systems that
allow the synthesis of polymers consisting solely of 5-membered rings have been
elaborated by our group [120]. Polyacetylenes with pendant metallocenyl side
groups have been prepared by metathesis polymerization of the corresponding 1-
alkynes [121-123]. Additionally, using 4-(ferrocenylethynyl)-4

-ethynyltolan [124,
125], the living character of an alkyne metathesis polymerization which proceeded
via -insertion was demonstrated for the first time [123]. In the case of linear n -al-

kyl-substituted alkynes the mode of insertion was reported to be governed solely
by the electronic nature of the alkoxide. Thus, fluorinated alkoxides led to -inser-

tion, while the t -butoxide ligand favored -insertion [126, 127].

Molybdenum and Tungsten Alkylidynes [128, 129]
The tungsten alkylidyne complex ( t -BuO) 3 W(C- t -Bu) [130-132] is capable of cata-
lyzing alkyne metathesis polymerization of cyclic alkynes. In addition, it can be
used in acyclic diene metathesis (ADIMET) [133, 134]. The synthesis of the corre-
sponding Mo-compound Mo(CCH 2 SiMe 3 )(OAd) 3 (Ad=adamantyl) has been re-
ported by C. C. Cummins et al. [135]. Though the active species is not known yet,
a mixture of Mo(CO) 6 and 4-chlorophenol is nowadays used for purposes of con-
venience [136].