Catalysts Norbornene polymerization

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

The sheer activity of these new nickel norbornene polymerization catalysts and their very rapid polymerization rates makes them potential catalysts for a new nor-bornene-RIM (reaction injection molding) technology to afford a saturated, oxidatively stable RIM part rather than the unsaturated ROMP (ring-opening metathesis polymerization) polymer that is the current commercial Telene or Metton . Fig. 4.6 illustrates the concept and also exemplifies the very rapid polymerization rates that are observed. 

Naked nickel (1) is a very effective catalyst for norbornene polymerization. Addition of 1 to 1,2-dichloroethane solutions of norbornene (norbornene Ni molar ratio of 2000 1) gave conversions of norbornene homopolymer typically exceeding 95% after about 1 h. 

Scheme 4.1 Multi-component catalyst for norbornene polymerization.

Ni(fJ,l r)2. In an effort to lower the molecular weight of the polymer chain and simplify analytical characterization, homopoiymerization of norbornene was carried out in the presence of ethylene pressure. Norbornene polymerizations catalyzed by cationic nickel catalysts such as (// -crotyl)Ni(1,5-fX)l)) ld, are quite susceptible to acyclic olefins as chain transfer agents and produce substantially reduced molecular weight vinyl-terminated poly(norbornenes) (Section 4.2.3.5). 

All of this data suggest strongly that (// -toluene)Ni(C6F5)2 should be thought of as an initiator of norbornene polymerization rather than a catalyst. A full paper describing the chemistry and polymerization performance of this initiator system will be pubhshed shortly [62]. 

In 1993, Perez et al. reported norbornene polymerization in aqueous emulsion at 70 °C using PdCl2 as a catalyst precursor [92, 93]. A stable latex consisting of low-molecular-weight oligomeric material (degree of polymerization DP ca. 10) was obtained with low catalyst activities (70 TO h ). Very small latex particles of 10 to 20 nm diameter were reported. In the free radical polymerization of olefinic monomers such small particles are only obtained by microemulsion polymeriza-... 

Another example, using a different ruthenium catalyst is polymerization of cyclohexenyl norbornene to form high molecular weight products [171] ... 

Turnover frequencies for a given ROMP catalyst are very dependent on the monomer type. Table 10. For example using 5 as catalyst, NBE polymerizes ca. 700 times faster than DCPD, 4000 times faster than the 2-norbomene-5-tert-butylester (NBE-TBE) and >10" times faster than the 2-norbornene-5,6-dimethyldiester (NBE-DME). The reason for this large reactivity differences of ruthenium carbenes towards different monomers is not yet completely understood but steric constraints and complexation of the catalyst with certain functional groups of the monomer (ester groups, double bonds, etc.), which is in large excess relative to the catalyst, may play an important role. 

FIGURE 16.3 Naked -type nickel and palladium catalysts for norbornene polymerization developed by the group at BF Goodrich (M = Ni, Pd). 

FIGURE 16.4 Active species for norbornene polymerization by naked -type nickel catalysts according to Goodall (P = polymer chain). 

SCHEME 16.6 Multicomponent catalysts for norbornene polymerization based on cationic palladium... 

The present paper reports the ring-opening polymerization of norbornene derivatives substituted by nitrile, amide, imide, ester, pyridyl and acid anhydride groups by the above-mentioned catalysts. The polymerization behavior of these monomers and the physical properties of the polymers of norbornenenitriles will be described. 

Various unicomponent catalysts based on n complexes of transition metals from groups IV-VII of the Periodic Table, e.g., (7i-allyl)4Zr, (7i-allyl)3Cr, (ti-allyl)2Ni, (7i-allyl)3Co, (7i-allylPdX)2, (7i-allylRhX)2, have been found to be very active in polymerization of a large number of cycloolefins like cyclobutene, cyclopentene, cyclooctene, cyclooctadiene and norbornene [36], Some of these catalysts induce polymerization of the cycloolefin totally to vinyl polymers while other catalysts of this class give preferentially vinylic polymers accompanied in a large extension by ring-opened polymers. 

Eleuterio [57], using several heterogeneous catalysts formed from oxides of chromium, molybdenum, tungsten or uranium on alumina, titania or zirconia showed that norbornene polymerizes by ring opening with... 

Figure 6 Mechanism of the formation of 2-exo,7 -sy/7-enchained units during norbornene polymerization by raf>[Me2C(lnd)2]ZrCl2/MA0 catalysts according to Karafiiidis etal. (R=H, Me bis(indenyi) iigand and charge on Zr center are not shown).

Similar micro structures are obtained when norbornene is replaced by norbomadiene. 5-and 6-substituted norbornene derivatives when polymerized by ROMP give more complex micro structures (Figure 11). By ROMP it is possible, more easily than by Ziegler-Natta catalysts, to polymerize norbor-nenes with polar substituents such as carbomethoxy, carboethoxy, or trifluoromethyl groups. " ... 

---