Catalysis in Acyclic Diene Metathesis (ADMET) Polymerization - Late Transition Metal Polymerization Catalysis - page 196

Scheme 6.5 (a) Back-biting metathesis of ADMET polybutadiene to produce cyclooctadiene (b)
RCM of an ADMET monomer.
Scheme 6.6 Intermolecular processes occurring in concentrated solution and intramolecular
process in dilute solution (Poly =polymer chain).
highest possible concentration of olefin chain ends promotes chain extension and
degenerate intermolecular trans -metathesis, rather than intramolecular reactions
(Scheme 6.6).
6.2.4
Trans -Metathesis
Degenerative trans -metathesis refers to reaction of an active chain end with an in-
ternal olefin of another polymer chain, and is also referred to as secondary me-
tathesis. This process does not lead to increases in molecular weight or changes
in the molecular weight distribution. Like other polycondensations, the molecular
weight distribution of ADMET polymers should approach the most probable distri-
bution (Flory distribution) and the polydispersity (PDI = M w / M n ) should be ap-
proximately 2.0, both of which are observed experimentally [31-33]. Degenerative
trans- metathesis was proven experimentally by combining independently prepared
ADMET polybutadiene and ADMET polyoctenamer, and exposing the mixture to a
metathesis catalyst. The resulting polymer is identical to the random copolymer
produced by the ADMET of a mixture of 1,5-hexadiene and 1,9-decadiene
(Scheme 6.7). This result can only be explained by metathesis with the internal
double bonds of both polymers, which effectively randomizes the chemical struc-
ture to the observed copolymer [31].