Norbornene functional derivatives

A series of [2.2.1]bicycloheptenyl (norbornene) functional prepol)nners have been prepared via the cycloaddition reaction of cyclopentadiene monomer with corresponding acrylics. When these materials are formulated with an appropriate multifuntional thiol crosslinker and photoinitiator and irradiated, a rapid, exothermic, crosslinking reaction takes place. When the acrylic precursors are organic resins, the derived polymers behave like toughened plastics. The choice of a norbornene functional polydimethylsiloxane precursors gives elastomeric products. 

Another group active in this field of research is led by Michael Buchmeiser, who has reported NHC-supported, catalyst systems anchored to both monolithic polymer and silica supports. The first example from this group involved the preparation of the norbornene-functionalized NHC salt 13, and the subsequent tethering of this molecule to a Runorbornene-derived monolithic material (14 Scheme 5.1) using ROMP [25]. The free carbene was then generated on the support and treated... 

The molecular weight of the polymers obtained depends upon the phosphine used, and the addition of a-olefins can be used to induce chain transfer nevertheless, the system is less susceptible to the a-olefin chain transfer method than the aforementioned Ni catalysts. A major advantage of this catalyst system is its tolerance toward polar groups, enabling its use in the copolymerization of functional derivatives of norbornene and for polymerization in suspension or emulsion. 

COPOLYMERIZATION OF ETHYLENE WITH NORBORNENE AND ITS FUNCTIONAL DERIVATIVES CATALYZED BY NICKEL COMPLEXES WITH PHOSPHORYLIDE LIGANDS... 

Figure 24 Palladium allyl complexes used for the copolymerization of norbornene or the functional derivatives N-C02Et, N-CH20C(0)Me, N-CH2OH, and N-COOMe with ethene.

An alternative synthesis of a thermally stable cyclopentadienyl functionalized polymer involved ring bromination of poly(oxy-2,6-diphenyl-l,4-phenylene), followed by lithiation with butyl lithium to produce an aryllithium polymer. Arylation of 2-norbornen-7-one with the metalated polymer yielded the corresponding 2-norbornen-7-ol derivative. Conversion of the 7-ol to 7-chloro followed by treatment with butyl lithium generated the benzyl anion which undergoes a retro Diels-Alder reaction with the evolution of ethylene to produce the desired aryl cyclopentadiene polymer, 6. 

A carbazole-functionalized norbornene derivative, 5-CN-carbazoyl methy-lene)-2-norbornene, CbzNB, was polymerized via ROMP using the ruthenium catalyst Cl2Ru(CHPh)[P(C6Hii)3]2 [100]. The polymerization was conducted in CH2C12 at room temperature, to afford products with polydispersity indices close to 1.3. Subsequent addition of 5-[(trimethylsiloxy)methylene]-2-norbornene showed a clear shift of the SEC trace of the initial polymer, indicating that a diblock copolymer was efficiently prepared in high yield. 

Chloromethyl polystyrene can be converted to a free-radical initiator by reaction with 2,2,6,6-tetramethylpipcridinc-/V-oxyl (TEMPO). Radical polymerization of various substituted alkenes on this resin has been used to prepare new types of polystyrene-based supports [123]. Alternatively, cross-linked vinyl polystyrene can be copolymerized with functionalized norbornene derivatives by ruthenium-mediated ringopening metathesis polymerization [124],... 

Figure 3.3 Effect of neighbouring functional groups on the epoxidation of norbornene derivatives.

ROMP behaviour of bimetallic complexes Figures 6a. Complex 6a readily catalyzes the ROMP of norbornene, but is not active enough for ROMP of functionalized norbornene derivatives. In contrast, complexes 6b and 6c are active for the functionalized substrates, but suffer from the side reactions shown in Equations 1 and 2 when norbornene is the substrate. 

A distinct advantage of the nickel catalysts, when compared to the corresponding early transition metal-based analogs (metallocenes and the like), is their ability to copolymerize norbornene derivatives bearing oxygen functionalities. However, compared to norbornene itself the level of incorporation of these functionalized norbomenes is lower, as are the reaction rates and polymer molecular weights. Also, substantially higher amounts of catalyst are required to copolymerize functionalized norbomenes. 

A final illustration of the effect of coordinating functionalities on the polymerization of olefins by cationic metal complexes involves norbornene derivatives. The species, [Pd(PR3)(Me)]+, generated in situ by halide abstraction from [Pd(PR3)(Me)(Cl)]2, is an extremely active catalyst for the insertion polymerization of norbornene with a rate exceeding of 1000 tons norbornene/mol Pd hour at... 

C [10] However, the polymerization rate was found to decrease dramatically for norbornene derivatives with pendant oxygen functionalities on the side opposite to the C = C bond (Tab. 9.1) [10]. 

Because they are synthesized by Diels-Alder reaction, functionalized norbornene derivatives sold commercially consist of exo and endo isomers with the latter predominating. The drop in polymerization rate for functionalized norbornene derivatives may be ascribed to the formation of a chelate by coordination of the metal to the functionality and the C = C bond along the endo face (see Fig. 9.2). 

This has two detrimental effects on polymerization. First, chelation strengthens the metal-olefin interaction, thereby raising the barrier for the insertion step. Second, it forces insertion through the endo face, in sharp contrast to the known propensity for norbornene to insert into metal-carbon bonds through the less hindered exo face [3 a, 5]. Consistent with this hypothesis has been our observation of the preferential uptake of the exo isomer in the polymerization of functional norbornene derivatives by Pd(PRj)(Me). For example. Fig. 9.3 shows the uptake profile versus time for the polymerization of 5-norbornene-2-carboxyhc acid ethyl ester starting with a monomer isomer ratio of 22% exo to 78% endo. Indeed, under certain conditions a polymer can be obtained from the exo isomer but not the endo isomer [10]. 

Given the published studies that indicate that the insertion of nonfunction-alized norbomenes into metal-carbon bonds occurs with exo, exo- stereochemistry (e.g., Fig. 9.1) we sought to determine whether the presence of an endo functionality would change the insertion stereochemistry. Because of the relative instability of the catalytically active palladium species, detailed studies encompassing the coordination and insertion of norbornene derivatives were carried using the model... 

These complexes also functioned well in benzene, although small amounts of ethanol were necessary to initiate polymerization. The order of activity for these catalysts was Ir(III) > Os(III) > Ru(III), and they were found to polymerize monomers with exo substituents more readily than endo isomers. Rinehart and Smith later demonstrated that these complexes initiated the aqueous polymerization of a substituted norbornene derivative in the presence of anionic emulsifiers and suitable reducing agents [24]. This reaction gave particularly low yields of polymer (typically less than 9%), but the overall tolerance of these complexes to polar and protic functionalities made them ideal candidates for further study. 

The living ROMP reactions of norbornene and norbornene derivatives have been used to make a variety of polymers possessing unusual properties. Copolymerization of selected functionalized norbornenes with norbornene has been used to synthesize star polymers and side-chain liquid crystal polymers. " This chemistry has also resulted in the preparation of phase separated block copolymers that contain uniform sized metal or semiconductor nanoparticles. The... 

J.P Mathew, A. Reinmuth, J. Melia, N. Swords, and W. Risse, (ti AUyl)palladium(II) and palla dium(II) nitrile catalysts for the addition polymerization of norbornene derivatives with functional groups, Macromol. 29, 2755 (1996) W. Risse and S. Breunig, Transition metal catalyzed vinyl addition polymerizations of norbornene derivatives with ester groups, Makromol. Chem. 193, 2915 (1992). 

Although very few catalysts are effective with molecules containing functional groups as well as double bonds, (diphenylcarbene)penta-carbonyltungsten and (phenylmethoxycarbene)pentacarbonyltungsten do initiate metatheses of ester derivatives of norbornene, such as the methyl norbomene-5-carboxylates (62a). 

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