Polymerization of Norbornene

Assignee The Goodyear Tire and Rubber Company (Akron, OH) 

Norbornene and norbornene methyl ester have been polymerized forming a polymer that lacks backbone carbon-carbon double-bond unsaturation. The polymerization catalyst mixture consisted of palladium acetate, a phosphine such as tricyclohex-ylphosphine, a Lewis acid such as dimethyl zinc, and hexafluoroisopropanol. Polynorbornenes prepared in this manner typically had M s 200,000 daltons with polydispersies less than 2, while poly(norbornene methyl ester) had M s of roughly 100,000 daltons. 

In a typical experiment a reactor was charged with palladium acetate (8.5 x 10- mol), tricycle-hexylphosphine (8.5 x 10 mol), and norbornene (0.0213 mol). The components were dissolved in 5 ml of toluene and then treated with dimethyl zinc (2.5 X 10 mol) where a color change was observed. This mixture was next treated 

TABLE 1. Effect of varying the Lewis acid and hexafluoroisopropanol catalyst compositions on the polymerization of norbomene while keeping both palladium acetate and tricyclohexylphosphine concentrations constant at 8.5 x 10 mol.  

Entry Lewis Acid Lewis Acid (mol) Hexafluoroisopropanol (mol) Conversion (%) 

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The first documented example of the living ROMP of a cycloolefin was the polymerization of norbornene using titanacyclobutane complexes such as (207) 510-512 Subsequent studies described the synthesis of di- and tri-block copolymers of norbornenes and dicyclopentadiene.513 However, functionalized monomers are generally incompatible with the highly electrophilic d° metal center. 

Scheme 6. Ring-opening metathesis polymerization of norbornene in C02 [144,145]...

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. 

The radical or ionic polymerization of norbornene yields a saturated polymer with a rearranged structure of 2,7 linkages (16). 

The polymerization of norbornene, Eq. (19), is stopped by cooling the reaction mixture to room temperature. The active polymer 11 can be stored for long periods of time. Heating 11 to temperatures above 65 °C in the presence of monomer causes renewed chain propagation. The subsequent addition of different cyclic olefins, such as endo- and exo-dicyclopentadiene, benzonorbomadiene and 6-methylbenzonorbornadiene resulted in the formation of well-defined AB- and ABA-type block copolymers, Eq. (21) [38]. Triblock copolymers 13 with narrow molecular weight distributions (polydispersity = 1.14) were prepared. Thus, the living character enables the preparation of new uniform block copolymers of predictable composition, microstructure and molecular weight. 

Schrock, Gibson et al. [52d] found that styrene and 1,3-pentadiene could be used as chain transfer reagents for the living ring-opening olefin metathesis polymerization of norbornene with molybdenum based catalyst 35a. Renewed norbornene addition to a polymerization mixture containing initiator 35a and 30 equivalents of styrene resulted in the formation of polynorbomene with a low polydispersity and a molecular weight controlled by the number of norbornene equivalents in each of the individual monomer solutions, Eq. (38). This method allows a more efficient use of the catalyst. 

By changing the NHC ligands to NHCs possessing a hemilabile pyridine linkage, Jin and coworkers were able to use Ni(II) - NHC complexes as catalysts for the polymerization of norbornene and ethylene in the presence of methylaluminoxane (MAO) as a cocatalyst [47]. The Ni complexes were prepared via Scheme 7. Although the free carbenes of 16 could not be generated successfully, the desired Ni compounds (17) could be prepared via the... 

Polymerization of norbornene using chiral metallocenes results in insoluble polymers exhibiting a glass transition temperature of about 210 °C. Although they have been shown by oligomerization to be tactic, no melting up to 500 °C... 

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. 

FIGURE 14-25 Polymerization of Norbornene Using Carbene Catalyst. 

Ring-opening polymerization of norbornene with metathesis catalysts and with ethyl aluminium dichloride, in the absence of transition metals, has been reported by Ivin et The formation of ring-opened syndiotactic and... 

Among the most interesting examples of ROMP is the polymerization of norbornene (31, Scheme 11.7), which is also shown in part in equation 11.14 but is worth revisiting. There is much more ring strain in norbornene compared with cyclopentene, so the real driving force for this polymerization is the release of... 

Thus, for a highly active, single-component catalyst for the addition polymerization of norbornenes it seems that a necessary requirement is accessibility to the metal by the norbornene substrate. We refer to this family of catalysts as being naked since all of their ligands (olefins) are readily displaced by the norbornene monomer to afford a highly active cationic metal nucleus nickel catalysts being preferred. The envisaged active site is illustrated in Fig. 4.5. 

Tab. 4.1 Polymerization of norbornene and 5-decylnorbornene using catalyst 1 (naked nickel) with and without 1-decene as a chain transfer agent.

The polymerization of norbornene using catalyst 2 is much more sluggish than with 1. Even after 24 h (albeit at a 4000 1 molar ratio of norbornene to palladium) in 1,2-dichloroethane, only 66% conversion to poly(norbornene) was achieved. The difference in rate observed between nickel and palladium was used to our benefit in determining fhe initiation event, as explained in the following section. 

Once this new family of catalysts was proven to show excellent polymerization activity and versatility (see below) in the polymerization of norbornenes, we immediately set out to understand fhe mechanism by which the polymerization occurs. The mechanism is of importance not only in fhe design of other, and possibly better, catalysts but also in helping to establish fhe microstructure of the polymers formed. 

The Addition Polymerization of Cyclic Olefns 129 Tab. 4.2 Polymerization of norbornene using multicomponent catalyst systems. 

Thus, we synthesized and tested this toluene complex and found that it does indeed effect the polymerization of norbornene-type monomers. Polymerization of norbornene-type monomers is not restricted to nickel complexes containing CgFs ligands. We have found that the electron-withdrawing tris(2,4,6-trifluoromefhyl-phenyl) ligand [57] is also quite effective in polymerizing both norbornene and 5-triethoxysilylnorbornene, for example. At a 4000 1 monomer to nickel ratio, Ni[2,4,6-tris(trifluoromethyl)phenyl]2(l,2-dimethoxyethane) (Fig. 4.27) gave 37% conversion into polymer from an 80 20 norbornene 5-triethoxysilylnorbornene monomer mixture. 

Thus, we discovered the unusual activation of nickel toward the polymerization of norbornene-type monomers by CgFs transfer from B(C6F5)3 to nickel [58], a reaction pathway that is typically a decomposition route for transition metal catalysts [59]. This discovery led to the development of a class of neutral, single-site nickel complexes containing electron-withdrawing group such as CgFs that are effective for the polymerization of norbornene-type monomers. 

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