Living ROMP

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. 

The drawback of the CVD method is eliminated in ROMP, which is based on a catalytic (e.g., molybdenum carbene catalyst) reaction, occurring in rather mild conditions (Scheme 2.3). A living ROMP reaction ofp-cyclophanc 3 or bicyclooctadiene 5 results in soluble precursors of PPV, polymers 4 [31] and 6 [32], respectively, with rather low polydispersity. In spite of all cis (for 4) and cis and trans (for 6) configuration, these polymers can be converted into aW-trans PPV by moderate heating under acid-base catalysis. However, the film-forming properties of ROMP precursors are usually rather poor, resulting in poor uniformity of the PPV films. 

While the majority of initial work in living ROMP procedures was performed with tungsten and molybdenum catalysts, the high reactivity of these systems... 

Living ROMP of a macromonomer with MW = 2600 yields star-like polymers with 10 to 100 arms on average possessing a somewhat broad MW distribution (i.e., 1.2—1.4). 

Besides the polymerization techniques discussed above, other polymerization methods have been used for the preparation of surface grafts. Recently, ring-opening metathesis polymerization (ROMP) became popular. This polymerization type will be discussed by Buchmeiser in Chapter 8. Recently, interesting accounts have appeared on solventiess polymerization techniques applying (living) ROMP on surfaces to prepare structured brush surfaces of conjugated polymers [331, 332]. 

These observations led to the catalytic application of well-defined ruthenium alkyUdenes, some of them freely soluble and sufficiently stable in water (Scheme 7.9) although their stability was found somewhat less in aqueous solutions than in methanol [21,27,28], With these catalysts a real living ROMP of water-soluble monomers could be achieved, i.e. addition of a suitable monomer to a final solution of a quantitative reaction resulted in further polymerization activity of the catalyst [28], This is particularly important in the preparation of block copolymers. 

Thus, the catalyst must have certain special properties, to be regarded as a living ROMP catalyst. 

D.M. Lynn, E.L. Dias, R.H. Grubbs, and B. Mohr, Acid activation of ruthenium metathesis catalysts and living ROMP metathesis polymerization in water, US Patent 6 486 279, assigned to California Institute of Technology (Pasadena, CA), November 26,2002. 

A titanacyclobutane carrier for the living ROMP of norbomene can be converted, by reaction with methanol, into an alkyl titanocene methoxide complex which can then be used in conjunction with EtAlCl2 to propagate the ZNP of ethene, so forming a block copolymer of norbomene and ethene620. 

If the living ROMP of norbomene is terminated with a 9-fold excess of terephthalaldehyde, the chains formed carry an aldehyde end-group which, when activated by ZnCl2, can be used to initiate the aldol-group-transfer polymerization of tert-butyldimethylsilyl vinyl ether621. 

Gnanou and coworkers [15,22,23] prepared several types of regular star- and comb-shaped polymers by living, ROMP of co-norbornenyl macromonomers, 8, 15 and 16. Some of them were characterized by means of the universal calibration in SEC to discuss the chain density, radius of gyration, and shrinking factor [63]. 

As will be seen later, the lack of possible side reactions such as cyclopropanation or P-hydride elimination is key to the use of titanacyclobutanes as initiators for living ROMP polymerization. 

Some well-defined ruthenium carbene complexes have been used in the living ROMP in aqueous media using a cationic surfactant to yield polymer latex [46]. 

Komiya et al. described the living ROMP synthesis of AB-type block copolymers that contain side chain liquid crystalline polymer blocks and amorphous blocks [62]. Norbornene (NBE), 5-cyano-2-norbornene (NBCN) and methyl-tetracyclododecene (MTD) were used for the amorphous polymer blocks, while I-n (n=3,6) were used for the SCLCP block (see Fig. 9). Initiator 1 was used for the ROMP. Block copolymers with monomer ratios from 75/25 to 20/80 (amor-... 

In the context of ROMP chemistry, living polymerization reaction conditions have only been observed when well-defined carbene complexes are used as the catalysts. The first catalyst to behave in this fashion was the titanocene complex (4), while more recently, complexes containing Ta, W, and Mo have been shown to be catalysts for the living ROMP of a variety of cyclic alkenes. The Mo complex (5b) is an especially promising catalyst since it is compatible with a number of functional groups and thus can be used to synthesize a variety of functionalized polymers. 

The discovery that Ti complex (4) was an effective catalyst for living ROMP chemistry resulted in the synthesis of several new types of polymers that were inaccessible with conventional catalysts. Narrow polydispersity polymer and di- and triblock copolymers were synthesized soon after living ROMP was discovered. Because polymer chains that are formed in a living ROMP reaction are terminated with metal carbene groups, fimctionalization of the chain ends is possible. Reaction of the carbene with an aldehyde occurs in a Wittiglike fashion, making a metal oxo complex that is inert to finther metathesis chemistry and terminates the chain with the alkylidene group of the aldehyde. 

The living ROMP reactions of norbomene and norbomene derivatives have been used to make a variety of polymers possessing unusual properties. Copolymerization of selected fimctionalized norbomenes with norbomene 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... 

Clearly, an enormous number of new polymers have and continue to be synthesized using ROMP reactions. The development of the new generation of single-site aUcylidene catalysts has introduced a new level of control over ROMP chemistry. Control of polymer microstructure should, in turn, result in a better understanding of the interplay between microstmcture and macroscopic properties. The use of living ROMP chemistry is still in its infancy. It will be interesting to observe whether or not useful materials can be developed from this chemistry. 

In the 1980s, well-defined metal alkylidenes were introduced as catalyst precursors for olefin metathesis [99, 109-111]. Especially for aqueous ROMP, ruthenium alkylidenes represent readily activated, well-defined, easy to handle catalyst precursors respectively initiators (for a living ROMP without chain-transfer, the term initiator appears more appropriate). Whereas in initial work vinyl-substituted carbenes (cf. 16a) were employed [112], more straightforward routes to aryl-substituted carbenes (16b) were soon developed [113]. Today, vinyl-substituted carbenes are also accessible in one-pot procedures [114], and 16a and 16b are both commercially available. 

The lessons learned from these complexes were eventually applied to the synthesis of well-defined ruthenium alkylidenes 8 and 9. Although they were insoluble in water, these alkylidenes could be used to initiate the living ROMP of functionalized norbornenes and 7-oxanorbomenes in aqueous emulsions. Substitution of the phosphine ligands in 9 for bulky, electron-rich, water-soluble phosphines produced water-soluble alkylidenes 10 and 11, which served as excellent initiators for the ROMP of water-soluble monomers in aqueous solution. These new ruthenium alkylidene complexes are powerful tools in the synthesis of highly functionalized polymers and organic molecules in both organic and aqueous environments. 

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