Schrock-type initiators

These initiators have the advantage of being well-defined compounds, and in particular of possessing preformed metal-alkylidenes. Their development, synthesis and properties wiU be outlined here in more detail 

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Gangadhara et al. have linked the cyanobiphenyl mesogen via a dicarbox-imide-group to an oxanorbornene ring system Vl-n, n=2-8 (see Fig. 4). Polymerization was carried out with Schrock type initiator 4. The dicarboximide linkage probably hindered the formation of LC phases even the introduction of relatively long spacers between the polymer backbone and the mesogen did not lead to liquid crystalline monomers or polymers [41]. 

Ungerank et al. investigated the influence of different molybdenum Schrock-type initiators 1, 2 and 3 (see Fig. 2) on the polymerization of ( )- and (-)-exo,e tdo-bis[4 -cyanobiphenyl-4-yl)oxyalkyl]norborn-5-ene-2,3-dicarboxy-lates [( )-IV-n and (-)-IV-n] [47]. The initiator had a strong influence on the Z/E ratio of the double bond in the polymer chain and therefore also on the tacticity of the polymer chain. Using racemic 2,3-disubstituted norbornene derivatives, such as ( )-IV-n, the two adjacent monomers (diads) can in principle form eight different stereoisomers, as depicted in Fig. 5 [48]. 

The introduction of perfluorinated groups generally favors microphase separation due to the immiscibility of fluorocarbons with hydrocarbons [66]. Norbornene derivatives with perfluorinated endgroups in the side chain were prepared by Wewerka et al. [67]. Monomer XII contained a relatively long (CF2)8-chain, separated via a long spacer (11 methylene-groups) from the norbornene, whereas monomer XIII has two relatively short (CH2)2(CF2)4-side chains (Fig. 11). Homopolymers and block copolymers were synthesized with one fluorinated monomer (XII or XIII) and one non-fluorinated non-liquid crystalline monomer (NBDE or COEN) with the Schrock-type initiators 4 and 5, respectively, leading to microphase-separated block copolymers. Table 9 and Table 10 summarize the physico-chemical properties of the homopolymers and block copolymers. 

Polymerizations of XX-n were carried out with Schrock-type initiator 4 in a monomerdnitiator ratio of about 50, to be independent of polymer chain length, leading to well-defined polymers with low PDIs from 1.08-1.29, as shown in Table 12. 

Schrock-type initiators of the type Mo(NAr )(OR )2(CHCMe2R) possess a tetrahedral geometry. The addition of phosphanes such as PMe3 or amines such as quinucHdine to Mo(NAr)(OCMe(CF3)2)2(CH-6Bu) allows, in analogy to W-based systems [37], the observation and isolation of two isomeric adducts [143]. The attack of the Hgand was found to occur preferably at the CNO-face [144] that is, the face defined by the N, carbon, and one O-substituent - a fact which is in accordance... 

While Grubbs-type initiators may only be used for grafting-from experiments, Schrock-type initiators may be used for both methods, both Schrock-type and Grubbs-type initiators were found suitable for that purpose. Scheme 2 gives an overview over the entire reaction sequence. 

A report by van der Schaaf et al. is further worth to be mentioned [Me(CF3)2CO]2 (NPh)W(CH2SiMe3)2 and Cl(NPh)W(CH2SiMe3)3 are transformed into Schrock-type initiators by irridation and were used for the photoinduced ROMP (PROMP) of norbomene and dicyclopentadiene [168]. The main advantage of these thermally very stable PROMP systems is their latency in pure monomers in the absence of light and the easier synthesis [26,27]. 

Figure 1 Selected examples of Grubbs- and Schrock-type initiators.

Figure 8.1 Selected examples of Grubbs- and Schrock-type initiators. X = Cl, CF3COO, R = CfCHj), C(CH3)(CF3)2 ...

Developments in the area of initiators for ROMP have resulted in the creation of a large armory of transition-metal compounds suited to these purposes. In addition, while the mechanistic details of Schrock-type initiators are well established, the mechanistic understanding of ROMP-particularly with ruthenium-based initiators-has experienced impressive progress such that today, highly sophisticated polymeric architectures can be produced that quite recently were barely achievable with other polymerization methods. Yet, further progress in both polymer and materials science may well be expected. 

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