Polymerization molybdenum alkylidene complexes

Figure 3 Chiral molybdenum alkylidene complexes used in asymmetric ring closing metathesis polymerization...

Cis/Trans Selectivity in Polymerizations Catalyzed by Imido Molybdenum Alkylidene Complexes... 

Cycloolefin macromonomers have been recently used in ring-opening metathesis polymerization reactions to manufacture block and graft copolymers of novel macromolecular architectures [84]. For this purpose, a- and co-norbornenyl-polybutadiene macromonomers, a-NBPB (R = CH2) and co-NBPB (R = COO), were reacted in the presence of molybdenum alkylidene complex, Mo(NAr)(CHtBu)(OtBu)2, to form polynor-bornene-polybutadiene diblock copolymers (117), with comb-like structure [85] [Eq. (49)]. 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

The proposed idea that metal alkyhdene complexes are be able to catalyze olefin metathesis was confirmed in 1980 [8] and consolidated in 1986 by Schrock with the development of the first well-characterized, highly active, neutral tungsten (Cl, Fig. 3) [9] and molybdenum (C2) [10] alkylidene complexes. These complexes were able to catalyze both the metathesis of different olefins and the ROMP of functionalized norbomene to polynorbomene with low polydispersities [11]. Moreover, these catalysts were used by Wagener and coworkers to perform the first quantitative ADMET polymerization [12] and copolymerization [13] of 1,5-hexadiene and 1,9-decadiene. However, the low stability of these catalysts in... 

Further synthetic utility can be introduced by changing the metal in the above alkylidene complexes from tungsten to molybdenum [52]. Molybdenum which is less oxophilic than tungsten will tolerate monomers containing mildly reactive functionalities such as esters [52a] and nitriles [52b] without appreciable catalyst deactivation during the polymerization reaction. For example, the polymerization of e rfo,endo-5,6-dicarbomethoxynorbornene (DCNBE) with Mo(CHtBu)(NAr) (OtBu)2 (Ar = 2,6-diisopropylphenyI) (35a) has been reported to give the cone-... 

Alkyne Polymerization by Molybdenum-imido-alkoxy-alkylidene Complexes... 

The molybdenum catalyst 2 has been used extensively for ADMET polymerization. This complex is easier to handle than the tungsten analog and is more tolerant of functionality. This complex has allowed the synthesis of polymers containing esters, carbonates, ethers, sulfides, aromatic amines, boronates, dichlorosilanes, siloxanes, acetals, and conjugated carbon-carbon double bonds [38-45]. Aldehydes, ketones, and protic functionahty are not tolerated. The molybdenum alkylidene will react with aldehydes and ketones, but not esters, in a Wittig fashion [64]. 

Alkyne metathesis is catalysed by alkylidene complexes of tungsten and molybdenum, but not by all alkylidene complexes of these metals. Complexes which do not catalyse metathesis catalyse polymerization of acetylenes to give polypropynes. 

Molybdenum imido alkylidene complexes also have been employed for a variety of other catalytic reactions of interest to the organic or polymer chemist, among them selective cross-couplings of olefins, [99] polymerization of terminal alkynes, [100-102] step-growth polymerization of dienes, [103,104] and cyclopolymerization of 1,6-heptadiynes. [105-107]... 

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