Copolymerization of Carbon Monoxide with Alkenes - Late Transition Metal Polymerization Catalysis - page 289

fin insertion; thus, enantiomorphic site control of the syndiotactic stereochemistry
takes place.
The coordination lability of the above-mentioned bis-dihydrooxazole ligands, as
demonstrated by the easy exchange reaction, causes decomposition of the catalytic
systems at high carbon monoxide pressures (a few bars are enough). Phosphorus-
modified catalytic systems are more promising in this respect. However, diphos-
phines and monophosphines ligands were found to produce low molecular weight
ketones when styrene is the substrate [94] as a consequence of a rapid -hydrogen

elimination after secondary olefin insertion. Two notable exceptions are repre-
sented by the Binaphos system 66 [95] and, surprisingly, by the non-chelating di-
phosphine ligand Ddppi 67 [96] (Scheme 8.14). The latter system is the only one
for which the styrene copolymer is isolated, at least partially, in the spiroketal
structure [34].
As mentioned above and as demonstrated by model studies using various acetyl
complexes, the insertion of styrene usually takes place with secondary regiochem-
istry [8]. However, styrene was found to insert with both primary and secondary
regiochemistry into the metal-acetyl bond of a complex obtained by carbonylation
of 66 . It is very remarkable that primary regiochemistry only was observed
for the insertion in a homologous complex, in which a polyketone chain
(CH 3 CO{CH(CH 3 )CH 2 CO} 15 ) was substituted for the acetyl ligand. Thus, it was
proposed that, for this catalytic system, primary insertion of styrene is responsible

Scheme 8.14 Phosphorus-containing ligands
(or their corresponding catalyst precursors)
for carbon monoxide-styrene copolymeriza-

tion (prevailingly isotactic ( 66-69 ), atactic
( 70-74 ) and syndiotactic ( 75 )).