Methyl methacrylate Ruthenium complexes

Ruthenium(II)-NHC systems ean be used for atom transfer radical polymerization (ATRP). Generally, similar results as for the analogous phosphine complexes are obtained. For the ATRP of styrene and methyl methacrylate (MMA) [(NHC)2peBr2] was found to rival copper(I)-based systems and to yield poly (MMA) with low polydispersities. Polymerizations based on olefin metathesis that are catalyzed by ruthenium-NHC complexes are discussed separately vide supra). 

The ruthenium indenylidene Schiff base complexes XXVIIIa and XXVIIId, synthesized by Verpoort, were evaluated in atom-transfer radical polymerization of methyl methacrylate. The polymerization was initiated by ethyl 2-bromo-2-methyl-... 

Table 2. Polymerisation of Methyl Methacrylate Initiated by Ethyl 2-Bromo-2-methylpropionate and Catalysed by Ruthenium Complexes 4, 6, and 7...

Polymerization of methyl methacrylate (MMA) and styrene in bulk via ruthenacarboranes and carbon tetrachloride was investigated first, ft was found that proposed ruthenium complexes were able to efficiently catalyze polymerization of MMA in conjunction with carbon tetrachloride as an initiator. Moreover, as follows from the data obtained (Table I), high conversion of the monomer is achieved in a number of cases. The structure of ruthenium carborane complexes closo or exo-nido) has a substantial effect on the kinetic parameters of MMA and styrene polymerization, as well as on the molecular-weight characteristics of synthesized polymers. 

The ATP process developed by Sawamoto and coworkers [226], uses an initiating system consisting of carbon tetrachloride, dichlorotri(triphenyl-phosphine)-ruthenium (II) and methylaluminum bis(2,6-di-fe/"f-butylphenoxide) to polymerize methyl methacrylate [226]. The polymerization involves reversible and homolytic cleavages of carbon-halogen terminal groups assisted by transition metal complexes [226]. 

The ruthenium(II) complexes interact with CCI4 and are oxidized in the process to become Ru(III) and radicals CCl3 that add to molecules of methyl methacrylate. The polymerization proceeds via repetitive additions of methyl methacrylate molecules to the radical species that are repeatedly generated from the covalent species with carbon-halogen terminal groups [226]. Suwamoto also reported [226] that addition of a halogen donor, PhsC-Cl aids the shift of the equilibrium balance to dormant species. The reaction of polymerization can be illustrated as follows ... 

TABLE 2. ATRP of methyl methacrylate catalysed by various ruthenium alkoxycarbene complexes and initiated by ethyl 2-bromo-2-methylpropanoate at 85 °C. ... 

The fate of the benzylidene fragment in these ruthenium complexes is another matter of debate [20a, 25]. The thermal stability of benzylidene complexes 4-6 was tested at 85 °C, under conditions mimicking polymerisation of methyl methacrylate. As monitored by H NMR, complete disappearance of the benzylidene fragment of the mixed phosphine/Af-heterocyclic carbene complex 5 (R = Cy, R = (5j-CHMePh) was observed within 20 min, whereas the Grubbs complex, RuCl2(=CHPh)(PCy3)2, showed only 55 % decomposition, and the bis-A/ -heterocyclic carbene ruthenium complex 6 (R = Cy) 88 % decomposition over the same time interval (Figure 6). 

Particularly noteworthy, in the ATRP of methyl methacrylate (Figure 6), the observed thermolytic half-lives of the mixed ruthenium benzylidene complex 5 (R = Cy, R = (5)-CHMePh fvi = 5 min), the bis-A-heterocyclic carbene complex 6 (R =Cy t = 1,5 min), and RuCl2(=CHPh)(PCy3)2 4 (f% = 16.5 min) approximate the relative reactivity of the complexes with (meth)acrylates. This observation could be indicative that a metathesis reaction takes place between those ruthenium benzylidene complexes and methyl methacrylate. At this stage, however, one can object that the so far reported inertness of RuCl2(=CHPh)(PCy3)2 towards methacrylates [35] is inconsistent with its decomposition over the course of ATRP through a metathetical pathway. 

In conclusion, the disappearance of the benzylidene fragment during the ATRP of methyl methacrylate could be explained by the reaction of the ruthenium benzylidene with the monomer, giving rise to highly unstable ruthenium ester-carbene complexes, and it is possible that these species then quickly decompose. In addition, the absence of [Ru=CH2] is also most probably indicative of the decomposition of these ruthenium carbene species, since [Ru=CH2] are presumed to be the propagating species in RCM and related ruthenium methylidene derivatives have a quite long lifetime in olefin metathesis. Until now, the exact nature of the inorganic decomposition products is not known. 

ATRP is analogous to atom transfer radical addition reactions which are well known in the field of organic chemistry as Kharasch addition reactions [83]. These methods often utilize a transition metal complex based on copper, iron, ruthenium, and nickel to abstract a halogen and produce a carbon-based radical [84, 85]. Since the first reports in 1995 of living radical polymerizations based on copper(I) for styrene and methyl methacrylate [86] and ruthenium(II) for methyl methacrylate [87], this technique has become widely utilized in polymer science. 

Ando T, Kato M, Kamigaito M et al (1996) Living radical polymerization of methyl methacrylate with ruthenium complex formation of polymers with controlled molecular weights and very narrow distributions. Macromolecules 29 1070-1072... 

Demonceau et al. [61,62] investigated the radical polymerization of methyl methacrylate with palladium(II) complexes 50-53 (Figure 22.21). As mentioned above, ruthenium complexes 54-56 are active in the Kharasch reaction, in which poly(methyl methacrylate) was obtained as the by-product. 

Hetorogenized Catalysts.—Reaction of [Ru(NH3)60H] + with a Faujasite-type zeolite gives a supported Ru complex, which effects hydroformylation of ethylene the catalytic species may be ruthenium clusters that are trapped in the zeolite cages. The effect of reaction conditions upon the selectivity of the hydroformylation of methyl methacrylate with [RhH(CO)(PPh3)3] or its polymer-anchored analogue has been investigated and hydroformylation of hex-l-ene and cyclo-octa-1,5-diene has been carried out with cobalt, rhodium, and platinum-tin complexes anchored to an ion-exchange resin via quaternary amino-phosphines. ... 

Breul AM, Schafer J, Friebe C, Schliitter F, Paulus RM, Festag G, Hager MD, Winter A, Dietzek B, Popp J, Schubert US (2012) Synthesis and characterization of poly(methyl methacrylate) backbone polymers containing side-chain pendant ruthenium(II) bis-terpyridine complexes with an elongated ctmjugated system. Macromol Chem Phys 213 808-819... 

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