Living ROMP initiators

The group of Nomura has explored the end-functionalization of molybdenum carbene initiated ROMP polymers extensively. Terminal monool [81, 82] or diols [83] were prepared via ROMP and used to synthesize different copolymer architectures. Pyridine and bi-pyridine ligands were successfully introduced to the polymer chain end in order to complex to ruthenium carbenes. Polymeric recyclable hydrogen transfer reduction catalysts were prepared in this manner [84, 85]. Notestein et al. [86] used a polymeric mono-aldehyde to functionally terminate a living ROMP initiated with a molybdenum catalyst to prepare diblock copolymers during the end-capping step. 

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

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. 

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. 

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 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 insight derived from the investigation of ill-defmed ruthenium ROMP initiators was successfully applied to the development of Ru(II) alkylidenes 8 and 9 [15-18], In contrast to the classical complexes, these well-defined alkylidenes initiated ROMP quickley and quantitatively, reacted readily with acyclic alkenes, and could be used to initiate living polymerizations in organic solvents. 

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. 

The carbonyl-olefination reaction, eqn. (2), is like a Wittig reaction in which the Mt—C bond takes the place of a P=C bond, and applies to the first three entries in Table 4.1. It is important both as a means of effecting clean termination of living ROMP reactions when initiated with Ti, Mo, or W complexes, and as a convenient means of converting C=0 groups into C=CH2 or C=CHR groups in organic sjmthesis. 

Another type of bJock-copolymer was obtained by conversion of living ROMP into controlled atom-transfer radical polymerization (ATRP). The approach involves the termination of a living ROMP polymer with a />bromomethylbenzaldehyde. This prepolymer may be Initiated using CuBr/2,2-dipyridyl and used for bJock-copolymer formation with styrene and methyl acrylate (Scheme 48). Finally, the preparation of 7-oxabicyclo[2.2.1]hept-2-ene-5,6-dimetha-nol-derived ROMP polymers and their use for the preparation of 7V-alkyl-3-aza-8-oxabicyclo[3.2.1]octane-... 

A, A -dimethylpiperidinium)]" "Cl . These complexes initiate the ROMP of functionalized 7-oxanorbornenes in water and methanol as well as aqueous emulsions [88]. More recently the same authors made the remarkable discovery that in the first case the activity is greatly enhanced by the addition of an equivalent of DCl, the role of which is to remove one of the two phosphine ligands, so generating the more active monophosphine complex 15 eqn. (22). This rapidly brings about the living ROMP of monomers 16 or 17 at 45°C (My/M = 1.3), eqn. (23), and block copolymers can also be made from the two monomers [89]. 

The first method of transformation of living ROMP into controlled/ living ATRP was reported by Matyjaszewski for the preparation of diblock copolymers. Thus, macroinitiators were prepared by ROMP of norbomene (Scheme 5) or dicyclopentadiene and subsequent Wittig-like reactions with /7-(bromomethyl)benzaldehyde. In these cases, the ROMP of norbomene was terminated with a terminating reagent that could also function as an ATRP initiator. Thus, these compounds were used as efficient macroinitiators for homogeneous controlled/ living ATRP to prepare block copolymers with styrene and methyl acrylate (Scheme 6) [21]. 

EICURE 20.11 Titanacyclobutane compounds that initiate living ROMP (Cp = cyclopentadienyl). 

---