2. Well-defined ruthenium metathesis catalysts

Up until the mid-1970s, the information about the structure of the active center of olefin metathesis catalysts was not known. Catalytic systems were mixtures of various compounds containing a transition metal. For example, in the late 1960s, Calderon and Goodyear employees published a number of papers about usage of a catalyst consisting of WCl6, AlEtCl2, and ethanol [12, 13]. Cyclic olefins form polymers (copolymers) of a high tacticity in a presence of this catalyst [14]. Catalytic systems prepared from compounds based on transition metals such as vanadium VCl4/Al(Hex)3; V(Ac)3/AlC1(C2H5)2; titanium TiCl4/Al(C2H5)3 and TiCl4/AlCl3/ Al(C2H5)3 [15]; molybdenum MoCl5/Al(C2H5)3 [16] as well as catalysts based on osmium, ruthenium, and iridium chlorides [17] were used in ROMP.

In 1976, through the example of the Fisher catalyst WCPhR(CO)5 (where R]Ph or dOCH3), described by Casey and Fisher [18, 19], Katz was the first who showed the ability of a metalcarbene complex to catalyze the process of metathesis polymerization independently without additional compounds [20]. In the history of metathesis, catalytic complexes with a wellstudied structure were called "well-defined" catalysts. The discovery of "well-defined" catalysts had significantly increased the ability of ROMP to obtain polymers that have unique properties.

The first "well-defined" ruthenium catalyst was synthesized by Grubbs in 1992. The alkylidene source was 3,3-diphenylcyclopropene [21] (Scheme 3).

Unfortunately, this catalyst had a low activity in comparison with already available metathesis catalysts. Replacing triphenylphosphine ligands with tricyclohexylphosphine significantly improved the activity of the catalyst (Figure 1—1 and 2).

Later, in 1995, catalytic complexes known as first-generation Grubbs catalysts (Figure 1—3 and 4) were prepared using phenyl diazomethane. These catalysts not only had equal activity to molybdenum catalysts but also were indifferent to the polar groups in the monomer [22, 23].

In 1999, Grubbs reported the synthesis of second-generation catalysts (Figure 1—5), showing better activity and more stability at air. This catalyst was obtained by replacing tricyclohexylphosphine with an N-heterocyclic carbene ligand [24]. A year later, Hoveyda's group reported on a new type of catalytic system based on the catalysts of Grubbs of the first and second generations (Figure 1—6 and 7). These complexes include a chelating ester ligand [25]. Recently, a new type of ruthenium catalyst has appeared where the N-heterocyclic carbene

Scheme 3. Scheme for the Grubbs I catalyst synthesis.

Figure 1. Main types of ruthenium catalysts.

2. Well-defined ruthenium metathesis catalysts

ruthenium, and iridium chlorides [17] were used in ROMP.

source was 3,3-diphenylcyclopropene [21] (Scheme 3).

improved the activity of the catalyst (Figure 1—1 and 2).

Scheme 3. Scheme for the Grubbs I catalyst synthesis.

properties.

18 Recent Research in Polymerization

Up until the mid-1970s, the information about the structure of the active center of olefin metathesis catalysts was not known. Catalytic systems were mixtures of various compounds containing a transition metal. For example, in the late 1960s, Calderon and Goodyear employees published a number of papers about usage of a catalyst consisting of WCl6, AlEtCl2, and ethanol [12, 13]. Cyclic olefins form polymers (copolymers) of a high tacticity in a presence of this catalyst [14]. Catalytic systems prepared from compounds based on transition metals such as vanadium VCl4/Al(Hex)3; V(Ac)3/AlC1(C2H5)2; titanium TiCl4/Al(C2H5)3 and TiCl4/AlCl3/ Al(C2H5)3 [15]; molybdenum MoCl5/Al(C2H5)3 [16] as well as catalysts based on osmium,

In 1976, through the example of the Fisher catalyst WCPhR(CO)5 (where R]Ph or dOCH3), described by Casey and Fisher [18, 19], Katz was the first who showed the ability of a metalcarbene complex to catalyze the process of metathesis polymerization independently without additional compounds [20]. In the history of metathesis, catalytic complexes with a wellstudied structure were called "well-defined" catalysts. The discovery of "well-defined" catalysts had significantly increased the ability of ROMP to obtain polymers that have unique

The first "well-defined" ruthenium catalyst was synthesized by Grubbs in 1992. The alkylidene

Unfortunately, this catalyst had a low activity in comparison with already available metathesis catalysts. Replacing triphenylphosphine ligands with tricyclohexylphosphine significantly

Later, in 1995, catalytic complexes known as first-generation Grubbs catalysts (Figure 1—3 and 4) were prepared using phenyl diazomethane. These catalysts not only had equal activity to molybdenum catalysts but also were indifferent to the polar groups in the monomer [22, 23]. In 1999, Grubbs reported the synthesis of second-generation catalysts (Figure 1—5), showing better activity and more stability at air. This catalyst was obtained by replacing tricyclohexylphosphine with an N-heterocyclic carbene ligand [24]. A year later, Hoveyda's group reported on a new type of catalytic system based on the catalysts of Grubbs of the first and second generations (Figure 1—6 and 7). These complexes include a chelating ester ligand [25]. Recently, a new type of ruthenium catalyst has appeared where the N-heterocyclic carbene ligand chelates the metal through the Ru-carbon bond (Figure 1—8). Such complexes possess high cis-selectivity in ROMP [26].
