Carbenes polymerization catalysts type

Carbenes as Ligands for Metal-Based Polymerization Catalysts (Type 3)... 

Due to the water insolubility of these metal carbenes, aqueous polymerizations represent heterogeneous multiphase mixtures. Investigation of ROMP of the hydrophilic monomer 8 or of a hydrophobic norbomene in aqueous emulsion (catalyst precursor 7 b added as methylene chloride solution) or suspension demonstrated that the polymerization can occur in a living fashion. For example, at a monomer to catalyst ratio 8/7b = 100 with 78% yield, poly-8 of Mw/Mn 1.07 vs. polystyrene standards was obtained [68]. Using water-soluble carbene complexes of type 9 and water-soluble monomers 10, living polymerization can be carried out in aqueous solution, without the addition of surfactants or organic co-solvents [69]. 

The utility of the Tebbe type complex in carbonyl olefination is discussed in Chapter 4. The bridged complex may be regarded as a special type of a carbene complex where the Cp2Ti=CH2 unit is masked by interaction with the AlMe2Cl entity. Formation of the Tebbe s complex suggests the occurrence of a-hydrogen elimination in the preparation of the Ziegler-Natta and Kaminsky type olefin polymerization catalysts from titanium chlorides and methylaluminum compounds. 

Type 3 As catalysts for polymerizations, where the carbene moiety itself catalyzes a reaction, is present as a ligand on a catalytically active metal complex, or otherwise comprises an integral part of a polymerization catalyst. 

Figure 31.4 The three distinct methods through which carbenes have been utilized for polymer synthesis. Type 1 Carbenes are essential to the polymerization process Type 2 Carbenes as side-group functionalities or reagents for post-polymerization modification Type 3 Carbenes are used as catalysts or ligands for polymerization catalysts.

H) Emulsion and miniemulsion polymerization Emulsion-type polymerizations for ROMP of norbornene and its derivatives were first reported using hydrates of Ru, Ir, and Os four decades ago. " However, polymerization rates were very low. ROMP using water-soluble ruthenium carbene complexes as catalysts was used to polymerize functionalized 7-oxanorbornenes not only in water and methanol, but also in aqueous emulsions. Gationic water-soluble aliphatic phosphines were used in the synthesis of the ruthenium carbene complexes. The polymer polydispersity was low (1.1-1.3), although few details of the dispersed phase polymerization were reported. 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

To date, the most frequently used ligand for combinatorial approaches to catalyst development have been imine-type ligands. From a synthetic point of view this is logical, since imines are readily accessible from the reaction of aldehydes with primary or secondary amines. Since there are large numbers of aldehydes and amines that are commercially available the synthesis of a variety of imine ligands with different electronic and steric properties is easily achieved. Additionally, catalysts based on imine ligands are useful in a number of different catalytic processes. Libraries of imine ligands have been used in catalysts of the Strecker reaction, the aza-Diels-Alder reaction, diethylzinc addition, epoxidation, carbene insertions, and alkene polymerizations. 

Among the first 18-electron (18e) Fischer-type metal carbene complexes to be used as part of an olefin metathesis catalyst system were W[=C(OMe)Et](CO)5 with BU4NCI (for pent-l-ene)79, and W[=C(OEt)Bu](CO)5 with TiCLt (for cyclopentene)80. These complexes may also be activated thermally, e.g. for the polymerization of alkynes81, or photochemically, e.g. for the ROMP of cycloocta-1,5-diene82. The essential requirement is that a vacancy be created at the metal centre to allow the substrate to enter the coordination sphere. Occasionally the substrate may itself be able to displace one of the CO ligands. 

Olefin metathesis is a unique carbon skeleton redistribution in which unsaturated carbon-carbon bonds are rearranged in the presence of metal carbene complexes. With the advent of efficient catalysts, this reaction has emerged as a powerful tool for the formation of C-C bonds in chemistry [1]. Olefin metathesis can be utilized in five types of reactions ring-closing metathesis (RCM), ring-opening metathesis (ROM), respective ringopening metathesis polymerization (ROMP), cross-metathesis (CM), and acyclic diene metathesis polymerization (ADMET). 

When coupled with living radical systems, living ringopening metathesis polymerization (ROMP) also permits the synthesis of other types of block copolymers (Figure 23) such as B-102 to B-108.67,396,397 A molybdenum carbene or ROMP intermediate is converted into a benzyl bromide-type terminal by quenching the ROMP with /> (b r omo me thy 1) b en z al d e hy d e by a retro-Wittig reaction.396 The macroinitiator thus obtained induced living radical polymerizations of styrene and MA with copper catalysts to afford B-102 to B-105. 

In at least one case, there is an example of ethene polymerization using a Ta-carbene complex in which there is strong evidence for a metathesis-based mechanism. See H. W. Turner and R. R. Schrock, J. Am. Chem. Soc., 1982,104, 2331, a paper that describes oligomerization of up to 35 ethene units in the presence of Ta[=CH(f-Bu)](H)(PMe3)3l2. It seems clear that although the Cossee mechanism is operative when polymerization occurs in the presence of Z-N-type catalysts, some polymerizations may involve metathesis, especially when hydrido-metal carbenes can form readily. 

ROM has been used to prepare phosphine-containing polymer supports (Scheme 20). Norbornyl-substituted monomer 22 was prepared in two steps from d-bromo-iodobenzene. This was then polymerized with diene 23. It was initially envisioned that it would be necessary to convert the phosphine to the borane adduct in order not to poison the metathesis catalyst. Although protection was needed when using the Grubb s type 1 complex as a catalyst, when employing the more active second-generation complex 24, the free phosphine monomer could be used. This has been attributed to the lower affinity of the active form of the catalyst toward coordination of phosphines due to the presence of the electron-rich heterocyclic carbene ligand. 

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