Norbornene microstructure

We have reported the first example of a ring-opening metathesis polymerization in C02 [144,145]. In this work, bicyclo[2.2.1]hept-2-ene (norbornene) was polymerized in C02 and C02/methanol mixtures using a Ru(H20)6(tos)2 initiator (see Scheme 6). These reactions were carried out at 65 °C and pressure was varied from 60 to 345 bar they resulted in poly(norbornene) with similar conversions and molecular weights as those obtained in other solvent systems. JH NMR spectroscopy of the poly(norbornene) showed that the product from a polymerization in pure methanol had the same structure as the product from the polymerization in pure C02. More interestingly, it was shown that the cis/trans ratio of the polymer microstructure can be controlled by the addition of a methanol cosolvent to the polymerization medium (see Fig. 12). The poly(norbornene) prepared in pure methanol or in methanol/C02 mixtures had a very high trans-vinylene content, while the polymer prepared in pure C02 had very high ds-vinylene content. These results can be explained by the solvent effects on relative populations of the two different possible metal... 

The microstructures of these polymers have been investigated by using oligomers as models. Norbornene was shown to polymerize via cis exoinsertion (232,233), whereas cyclopentene was shown to give quite unusual cis and trans-1,3 enchainment of the monomer units (234-236). 

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. 

Figure 25 Alternative concatenation microstructures in ethylene-norbornene copolymers.

ButG5H3, G9H6) have been synthesized either by the standard salt metathesis or the amine elimination procedure. These compounds are used as pre-catalysts for norbornene homopolymerization and ethylene-norbornene co-poly-merization. The influence of the catalyst symmetry and structure on the activity, norbornene incorporation, and polymer and co-polymer microstructure has been studied.706... 

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 NMR spectra of the poly(norbornene) produced by catalyst 1 (naked nickel) and 2 (naked palladium) are given in Fig. 4.9 a and b, respectively. Based on the different NMR signatures of these two polymers, their microstructures are obviously different However, the absence of resonances upheld of 28 ppm (see 2,3-dimethylnorbornane model compounds above) in either spectrum proves that subsequent monomer insertions occur on the exo face of norbornene. Therefore, any microstructural differences between the poly(norbornene)s must be attributed to differences in tacticity. 

Using metallocene catalysts, characterization of oligomers has been used successfully to facilitate the microstructural analysis of poly(cyclic olefins) [45], in the cases of both cyclopentene and norbornene. In these cases hydrogen was applied as chain transfer agent and the resulting products were christened hydrooligo-mers . 

Moreover, the microstructure of the norbornene homopolymer that is formed using this multi-component catalyst is identical to that produced by naked nickel as evidenced by and NMR spectroscopy. Based on these observations, it is clear that the active catalyst formed in situ from the multi-component system is very similar to the naked nickel single component catalyst [50]. 

However, it was also clear, based on NMR studies, that the type of addition polymer formed in run 1 was quite different from that produced using, for example, [(z7 -crotyl)Ni(l,5-COD)]PF5. A comparison of the polymer ll NMR spectra is given in Fig. 4.25. Note the dissimilarity between the two spectra. The same is true of their NMR spectra. The formation of a catalyst unique and distinct from [(/7 -crotyl)Ni(l,5-COD)]PF6 by the reaction of nickel(2,2,6,6-tetramethyl-3,5-hep-tanedionate)2 or Ni(dprri), AlEts, and BlC Fsls would be consistent with the production of poly(norbornene) exhibiting different NMR spectra and apparently different microstructures. 

Finally, the ethylene/norbornene copolymers obtained using the nickel catalysts are essentially indistinguishable from those obtained using metaUocene-based early transition metal catalysts, both in terms of the microstructure and such physical properties as Tg and tensile modulus. For the ethylene/norbornene polymers synthesized, the glass transition temperature (Tg) increases smoothly with increasing norbornene content. 

Albeit these catalysts display a limited stability to water, activities of 3 X lO TO h at room temperature are observed. As with the polymers obtained in traditional polymerization in organic solvents, the materials obtained in water with sal-icylaldimine-based nickel(II) complexes possess a moderate number of methyl branches. Overall, as in the case of the linear polyefhylenes obtained with 6 to 8, the presence of water has no effect on the basic polymer microstructure. Polymer crystallinity can be influenced by employing norbornene as a co-monomer. High molecular weight, amorphous efhylene-norbomene copolymers, which form films at room temperature, can be obtained in aqueous polymerizations [71]. 

Scheme 24. Microstructures of Maximum Order for ROMP Poly(norbornene)...

For the copolymerization of norbornene and ethylene, different metallocene catalysts having C, C2, C2v, and Cs symmetry were studied by the authors (1-3, Table 16.1, and 4-15, Figure 16.11). The copolymers obtained differ not only regarding the incorporation of norbornene, but also in microstructure and stereostructure, depending on the symmetry of the catalyst used. 

Ethylene/norbornene copolymerization with the CGC catalysts could be described by a Markov first order model. No temperature effects on incorporation and microstructure were observed. 

The best method for E/NB copolymer micro- and stereostructural analysis is NMR spectroscopy,which can be used to determine if the polymer chain contains dyads and triads (short norbornene blocks) or only isolated norbornene units. This section compares the different microstructures of E/NB copolymers made with the different types of catalysts discussed in this chapter. Details can be found in the literature. ... 

Alternating E/NB copolymers synthesized by CGC catalysts show a range of different microstructures and tacticities (Figure 16.15). Catalysts 16 and 18 are not able to make copolymers with norbornene-norbornene dyads, even with a high molar excess of norbornene in the feed. Small amounts of meso norbornene dyads can be observed if catalyst 17, which contains a relatively flexible cyclododecyl ligand N-substituent, is used. 

Bergstrom, C. H. Sperhch, B. R. Ruotoistenmaki, J. Seppala, J. V. Investigation of the microstructure of metallocene-catalyzed norbornene-ethylene copolymers using NMR spectroscopy. J. Polym. ScL, Part A.- Polym. Chem. 1998, 36, 1633-1638. 

Nickel-0- and palladium-O-complexes are very active catalysts for the polymerization of norbornene and also for cyclopentene [552-554], Nickel catalysts produce soluble polymers with a molecular weight of over one million while polymers obtained with palladium or metallocene complexes are insoluble. The soluble polymers have an atactic structure. The microstructure of the polynorbornene depends on the catalyst used and is isotactic by synthesis with chiral metallocenes. 

Most of the microstructure investigations beyond these simple cycloalkenes were carried out on polymers of norbornene, norbornadiene and their derivatives (Table 6) [284]. 

INVESTIGATION OF THE MICROSTRUCTURE OF METAL CATALYZED CYCLOPENTENE CO-NORBORNENE POLYMERS BY SPECTRAL METHODS... 

Notably, copolymers obtained with various catalysts substantially differed in properties. For example, copolymer of NBSi(OFt)3 with norbornene obtained on [(fl -toluene)Ni(C6F5)2] catalyst, exhibited suitable mechanical properties (a tensile modulus of 1.4 GPa, an elongation-to-break of 15%) while catalyst [(fl -crotyl)Ni(l,4-COD)]PFe gave completely brittle polymers, despite the fact that they had equal molecular weights [231]. Ludovice et al. suggested that these differences could be connected with polymers microstructure a more atactic polymer was brittle, and a more stereoregular one had better mechanical properties [232]. However, determination of microstructure of substituted polynorbornenes is a very intricate... 

Tcrt-butylmethacrylate-co-norbornene Effect of catalyst on microstructure [41]... 

Since the homopolymers of cyclic alkenes are insoluble in hydrocarbons, it is difficult to study their microstructure therefore oligomers are produeed. In the case of poly(cyclopentene), the configurational base units are cis- and tru s -l,3-enchained (27) while poly(norbornene) shows cis-exo insertion. 

All methyl isomers of norbornene, 1-, 2-, 5-, and 7-methylnorbornene have been reacted in the presence of catalysts based on tungsten, rhenium, ruthenium, osmium and iridium compounds [7]. The polymers corresponded to ring-opened products having various microstructures. Racemic mixtures or pure enantiomers have been used as starting materials. Differences in reactivities as a function of the methyl position (1, 2, 5 or 7) and steric configuration (endo-exo and syn-anti) have been reported. 

It is obvious that in all cases, the catalytic systems discriminate in favor of norbornene, a strained monomer with particularly high reactivity. Copolymer microstructure is well documented from detailed NMR spectroscopy studies, from which valuable information concerning the reaction mechanism and stereochemistry or the nature of the initiating and propagating species can be inferred. 

The homopolymerization of norbornene using early transition metal catalysts drew new attention with the discovery of metallocene/MAO catalysts. " It was found that most of the obtained polymers are insoluble in organic solvents, are crystalline, and show extremely high melting temperatures. Information about the microstructure was gathered by Arndt et who used the hydrooligomerization technique to... 

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