Sequential ROMP

Amphiphilic star-block copolymers can be prepared by adding a polycyclic diene such as 238 to a living diblock copolymer made by sequential ROMP of (i) the monomer in Table 9 with R = COOSiMe3, and (ii) norbomene. The trimethylsilyl ester groups are then converted to carboxylic acids by soaking the cast film of the polymer in water for 2-3 days to give a product with a hydrophobic core of polynorbomene and a hydrophilic outer layer126,502. 

Two kinds of well-defined copolymers were synthesized by sequential ROMP of PS and PEO or PB macromonomers [4,9]. The strategy used to obtain the expected architecture is illustrated in Scheme 3. The order of polymerization of macromonomers appeared to be essential for a complete crossover to occur indeed give rise to propagating species of highest reactivity. 

Figure 5.2 Structures of block copolymers synthesized through sequential ROMP. The self-assembly of these two block copolymers was studied by SAXS. (Adapted from Refs. [14, 15].)...

The living character of the ROMP promoted by the initiator Ru(CHPh)(Cl)2 (PCy3)2 (Cy = cyclohexane) was tested with the synthesis of diblock, triblock, and tetrablock copolymers of norbornene derivatives carrying acetyl-protected glucose, [2,3,4,6-tetra-O-acetyl-glucos-l-O-yl 5-norbornene-2-carboxylate], A or maltose groups, [2,3,6,2/,3/,4/,6/-hepta-0-acetyl-maltos-1-O-yl 5-norbornene-2-carboxylate], B, shown in Scheme 41 [102]. The AB, ABA, and ABAB structures were prepared by sequential addition of monomers with narrow molecular weight distributions to quantitative conversions. 

For the synthesis of carbohydrate-substituted block copolymers, it might be expected that the addition of acid to the polymerization reactions would result in a rate increase. Indeed, the ROMP of saccharide-modified monomers, when conducted in the presence of para-toluene sulfonic acid under emulsion conditions, successfully yielded block copolymers [52]. A key to the success of these reactions was the isolation of the initiated species, which resulted in its separation from the dissociated phosphine. The initiated ruthenium complex was isolated by starting the polymerization in acidic organic solution, from which the reactive species precipitated. The solvent was removed, and the reactive species was washed with additional degassed solvent. The polymerization was completed under emulsion conditions (in water and DTAB), and additional blocks were generated by the sequential addition of the different monomers. This method of polymerization was successful for both the mannose/galactose polymer and for the mannose polymer with the intervening diol sequence (Fig. 16A,B). 

Unlike in radical or anionic polymerizations, in ROMP with single-component metathesis catalysts the growing polymer chain remains able to further grow even after consumption of the monomer. This enables the manufacture of block copolymers with interesting physicochemical properties by sequential addition of different monomers to such living systems. 

This finding is a significant improvement over aqueous ROMP systems using aqueous ROMP catalysts. The propagating species in these reactions is stable. The synthesis of water-soluble block copolymers can be achieved via sequential monomer addition. The polymerization is not of living type in the absence of acid. In addition to eliminating hydroxide ions, which would cause catalyst decomposition, the catalyst activity is also enhanced by the protonation of the phosphine ligands. Remarkably, the acids do not react with the ruthenium alkylidene bond. 

The synthesis of block copolymers via ROMP of conventional monomers has been the focus in the last 10 years. This has included making block copolymers via sequential polymerization of different ROMP monomers as well as those made from a combination of ROMP with other polymerization techniques [74, 141] A metathesis approach has been reported involving ROMP combined with the polymerization of 1,6-heptadiynes by molybdenum or ruthenium initiators [142,143], or with monomers allowing enyne metathesis polymerization [144]. Choi et al. [145] prepared block copolymers of NBE derivatives with a 6-heptadiyne derivative leading to an in situ crosslinking of the conjugated segment and in turn to nanoparticle formation. Moreover, a combination of ROMP and insertion polymerization of ethylene has also been reported [146]. 

In most instances, ROMP is utilized to arrange monomer blocks sequentially into distinct amphiphilic domains in order to create a polymer capable of self-assembly. In this way, one can vary the hydrophobic tail length and hydrophilic head size in order to create a wide range of morphologies. Shunmugam and coworkers [84] have shown that homopolymers comprised of only one type of... 

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