Transition metal catalysts, initiation ring-opening polymerization

The development of living ring-opening metathesis polymerization (ROMP cf. Section 6.10) catalysts was greatly influenced by the initial use of hydrated late transition metal salts. In the aqueous polymerization of 7-oxanorbornene derivatives, RuC13 and [Ru(H20)6]2+ were found to produce polymer with the latter having a smaller induction period [68], A key intermediate and product in the reaction is the alkene adduct, [Ru(H20)5(alkene)]2+ with the use of RuC13 and... 

The mechanism of the ring-opening polymerization effected by transition metal catalysts has been investigated and an insertion of the transition metal between the Cp-Si bond is believed to be the key initiation step. This is followed by successive insertion reactions leading to the formation of the pol5mer [7]. 

The observation that the metal carbene complex, (CO)5W = C(Ph)2 [22], catalyzed the polymerization of cyclic olefins to ring opened polymers containing the diphenylmethylene unit of the catalyst provided additional evidence that carbenes were involved in the catalytic cycle. The formation of the initiating metal carbenes in the classic systems that consist of transition metal halides and alkylating agents was proposed to involve metal alkylation followed by oc-hydrogen loss, Eq. (6). Methane and propene were detected in the early stages of these reactions [23]. 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

Transition-Metal-Catalyzed Polymerizations. Metal-catalyzed polymerizations (76) have been performed in SCCO2 (77,78). The ring-opening metathesis polymerization (ROMP) of norbornene has been performed with CO2 and C02/methanol mixtures using a Ru(H20)6(03SC6H4CH3-p)2 catalyst as the initiator (78) (eq. (6)). 

No unusual initiators, such as transition metal complexes or heterogeneous catalysts, are needed for perhaloacetaldehyde polymerizations, the polarization of the carbonyl group of the aldehyde monomer is well defined and does not cause the formation of head to head linkages in the polymer. The shorter carbon oxygen single bond (1.43 A) which is formed by ring opening of the carbonyl double bond (1.21 A) has a beneficial effect for the formation of a helical structure for the isotactic polymer and should, consequently, favor the formation of isotactic polymer. 

---