Ruthenium living polymerization

The complex is also active in ring-opening metathesis polymerization of 1,5-cyclooctadiene (COD), where the ruthenium—carbene bond is now the initiating point. Therefore, a mixture of MMA and COD undergoes a dual or tandem living polymerization of both monomers to generate block copolymers of COD and MMA, which can be converted into ethylene-block-MMA copolymers on subsequent hydrogenation, also catalyzed by the complex. 

A haloalkane with mixed halogens (1-7) led to living polymerization of methacrylates, acrylates, and acrylamides when coupled with ruthenium and nickel complexes.133 135 159 160 The weak C—Br bond is preferentially activated, while multifunctional initiation is possible. However, CCl3Br is the initiator of choice if obtaining narrow MWDs is desired without paying attention to monomer structures. 

The use of arenesulfonyl halides was also investigated for the ruthenium-catalyzed polymerization of MMA.180 Living polymers are indeed attained, where the a-end group (/, 1.0) and the MWDs are controlled (MJMn = 1.2—1.5), whereas the Mn values were higher than the calculated values due to low initiation efficiency (f,.n 0.4). 

Grubbs and co-workers reported the ring-opening metathesis polymerization (ROMP) of norbornene derivatives in water using Ru(H20)6(ts)2 as the catalyst [127, 128]. More recently, these authors have described the first example of a homogeneous living polymerization in water using a water-soluble ruthenium carbene [Eq. (24)] [129]. 

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. 

With the work by Grubbs et al. [27] and Herrmann et al. [28], the use of ruthenium carbene complexes as homogeneous catalysts for the ROMP (Ring-Opening Metathesis Polymerization) of olefins was estabhshed (see Section 2.4.4.3). The development of catalysts that can catalyze hving polymerization in water was an important goal to achieve, especially for applications in biomedicine. In this context, two water-soluble ruthenium carbene complexes (3 and 4) have been reported that act as initiators for the living polymerization of water-soluble monomers in a quick and quantitative manner [29]. 

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. 

In particular, the "living character [36,130] of ruthenium-catalyzed polymerizations [131-134] and the high tolerance of the catalytic system toward different functional monomers made the ROMP approach very attractive. In fact, the active ruthenium sites could be used for derivatization after rod formation was complete. 

Sawamoto et al. have revealed that the ruthenium complex induces the living radical polymerization of MMA [30,273-277]. For example, RuCl2(PPh)3 provided poly(MMA) with Mw/Mn 1.1 and the block copolymers. This system has a unique characteristic in that it is valid not only for MMA and other methacrylates, but also for acrylates and St derivatives. 

Much research has already been devoted in the past couple of years to (i) the immobilization of ATRP active metal catalysts on various supports to allow for catalyst separation and reycycling and (ii) ATRP experiments in pure water as the solvent of choice [62]. A strategy to combine these two demands with an amphiphilic block polymer has recently been presented. Two types of polymeric macroligands where the ligand was covalently linked to the amphiphilic poly(2-oxazo-line)s were prepared. In the case of ruthenium, the triphenylphosphine-functiona-lized poly(2-oxazoline)s described in section 6.2.3.2 were used, whereas in the case of copper as metal, 2,2 -bipyridine functionalized block copolymers were prepared via living cationic polymerization [63] of 2-methyl-2-oxazoline and a bipyridine-functionalized monomer as shown in Scheme 6.8. 

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. 

Kato M, Kamigaito M, Sawamoto M et al. (1995) Polymerization of methyl methacrylate with the carbon-tetrachloride dichlorotris(triphenylphosphine)-ruthenium(ll) methyla-luminum bis(2,6-di-tert-butylphenoxide) initiating system - possibility of living radical polymerization. Macromolecules 28 1721-1723... 

Due to the fact that the living initiator is almost quantitatively located at the surface of the microglobides, the efficiency of metal removal from the monolith after polymerization is high. Investigations revealed that the remaining ruthenium concentration after capping with ethyl vinyl ether is below 10 xg/g, corresponding to a metal removal of more than 99.8%. 

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