Polynorbornenes, ring-opening polymerization

The polynorbornenes X bearing cyclotriphosphazenes with -0(CH2CH20)x-CH3 (x=l, 2,3,7.2) side groups have been prepared via ring opening metathesis polymerization (ROMP) and complexed with LiS03Cp3 and LiN(S02Cp3)2 (10-60% molar ratios) by Allcock [619,620]. 

The first report of ROMP activity by a well-characterized Mo or W species was polymerization of norbornene initiated by W(CH-t-Bu)(NAr)(0-f-Bu)2 [122]. In the studies that followed, functionality tolerance, the synthesis of block copolymers, and ring-opening of other monomers were explored [30, 123]. Two important issues in ROMP concern the cis or trans nature of the double bond formed in the polymer and the polymer s tacticity. Tacticity is a consequence of the presence of two asymmetric carbons with opposite configuration in each monomer unit. The four ROMP polymers (using polynorbornene as an example) that have a regular structure are shown in Scheme 3. 

Scheme 3. The four possible regular structures of polynorbornene prepared by ring-opening metathesis polymerization...

Functionalized polynorbornene derivatives, (V), prepared by Liaw et al. (4) using ring-opening metathesis polymerization exhibited strong carbazole fluorescence. Monomer emissions occurred in the near ultraviolet (UV) at approximately 380 nm and extended into the blue-violet region at 330 nm. 

A patent by A. W. Anderson et al. (Du Pont) disclosed the first transition metal catalyzed polymerization of a cyclic olefin in 1955 [1]. Subsequent to the discovery of Ziegler-Natta polymerization [2] norbornene was found to polymerize in the presence of the catalyst systems TiCl /EtMgBr. Eleuterio [3 a] and Truett et al. [3 b] obtained polynorbornene by using transition metal catalysts based on Mo and Ti, respectively. IR-studies and ozonolysis of the polymer revealed the presence of carbon-carbon double bonds, indicating that polymerization had occured by unexpected ring opening Eq. 1 [3]. 

Schrock, Wrighton, and coworkers have reported that ring-opening metathesis polymerization (ROMP) of norbomene monomers fhnctionalized with ferrocenyl groups allowed for the isolation of polymers such as 55-58. Alternatively, polynorbornenes capped with ferrocenyl units could be produced by using a molybdenum initiator functionalized with ferrocene. When this polymerization reaction was terminated through the addition of octamethyl-ferrocenecarboxaldehyde, low molecular weight polymers such as 58 were isolated. 

Ring-opening metathesis polymerization has also been exploited to build blocks from cyclic olefins, especially polynorbornene [5]. The development of ROMP for block copolymer synthesis has recently been facilitated by the introduction of functional-group-tolerant metathesis catalysts by Grubbs [21]. 

---