Polymerization by transition metal catalysts

Our literature search on alkene polymerization by transition metal catalysts yielded nearly 20,000 publications since 1993 even after the exclusion of the patents or patent applications, indicating a tremendous interest in this field. Three mutually interconnected trends were identified from the search. 

Two mechanisms have been proposed for acetylene and substituted acetylene polymerization by transition metal catalysts one is the metal-alkyl mechanism and the other is the metal-carbene mechanism. In general, it has been proposed that the polymerization of acetylenes by Ziegler-Natta catalysts proceeds by the metal-alkyl mechanism, while the metal-carbene mechanism has been accepted for the polymerization of substituted acetylenes by metathesis catalysts whose main components are halides or complexes of group 5 and 6 transition metals. The latter will be discussed in Section III. 

The coordination polymerization by transition-metal catalysts, such as Ziegler—Natta catalysts or metallocenes, is the most versatile method for preparing Unear polyolefins under mild and controlled conditions. Unfortunately, attempts at direct incorporation of functional monomers during polymerization run into the problem of catalyst poisoning caused by the interaction of organic functimialities with the catalyst center [129]. 

Ketones a-Olefins bearing keto functionalities show also only weak interactions with aluminum compounds resulting in insuffident proteaion for the successful polymerization by transition metal catalysts. Additionally, undesired side reactions, for example, the keto-enol tautomerization of 2,2-dimethyl-11-dodecen-3-one in combination with MAO were reported. ... 

Some of the vinyl monomers polymerized by transition metal benzyl compounds are listed in Table IX. In this table R represents the rate of polymerization in moles per liter per second M sec-1), [M]0 the initial monomer concentration in moles per liter (M) and [C]0 the initial concentration of catalyst in the same units. The ratio i2/[M]0[C]0 gives a measure of the reactivity of the system which is approximately independent of the concentration of catalyst and monomer. It will be observed that the substitution in the benzyl group is able to affect the polymerization rate significantly, but the groups that increase the polymerization rate toward ethylene have the opposite effect where styrene is concerned. It would also appear that titanium complexes are more active than zirconium. The results with styrene and p-bromostyrene suggests that substituents in the monomer, which increase the electronegative character of the double bond, reduces the polymerization rate. The order of reactivity of various olefinically unsaturated compounds is approximately as follows ... 

The discovery of the polymerization processes promoted by transition metal catalysts and the foundation of macromolecular stereochemistry represents a major breakthrough in chemistry in the second half of this century. Since the first discoveries by Ziegler in Miilheim and Natta in Milan there has been an enormous development of fundamental and applied research that still continues... 

Alkene metathesis, a remarkable reaction catalyzed by transition metal catalysts, can be traced back to Ziegler-Natta chemistry as its origin [11], In 1964, Natta et al. reported a new type polymerization of cyclopentene using Mo- or W-based catalyst, without knowing the mechanism. This was the first example of ring-opening metathesis polymerization (ROMP eq. 1.9) [12],... 

A comprehensive review on the whole field of polymerization of conjugated dienes by transition-metal catalysts was compiled by Porri and Giar-russo in 1989 [50]. 

A detailed review on the multiple aspects of the reaction mechanisms of diene polymerization with transition-metal catalysts was published by Porri, Giarrusso and Ricci in 1991 [486]. 

There exists a large number of MAF catalysts and only their general features can be briefly mentioned here. When the cocatalyst (alkylmetal) is omitted in a typical ZN system, the remaining transition metal salt is rarely active. Some transition metals, especially Ti, V, Cr, Co, Ni, Zr, Nb, Mo, W, Pd, Rh, and Ru, are however, active, often after special treatment. Generally alkenes are more readily polymerized by transition metals from the left-hand side of the periodic table and dienes by metals from the right-hand side. 

Figure 1.11 Rates of polymerization with transition metal catalysts may be quite different because of accessibility of active centers, presence of poisons, mechanism of activation, etc. Kinetic profile must be accommodated by process conditions (see Chapter 7).

Polymerization by Transition-Metal Complex Catalysts. Mlly M12, and M13 have been polymerized by Et3Al/TiCl4 catalysts between 50° and 80 °C in n-hexane, the reaction times ranging from a few hours to several days. The polymers obtained have the same structure as those obtained by cationic polymerization. By analogy with mechanisms proposed in the literature (38, 39), the structure shown in Equation 24 may be proposed for the active center. 

Polymerization by Ziegler-Natta Catalysts. Polymerization by transition-metal complex catalysts gives oligomers of the same structures as those obtained cationically, with distinctly higher conversion degrees (Equation 29). 

J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, New York, 1979. A. Yamamoto and T. Yamamoto, Coordination polymerization by transition-metal alkyls and hydrides, Macromol. Revs. 13, 161 (1978). 

Despite the intensive investigation of the stereospecific polymerization of 1,3-dienes by transition metal catalysts in the past 25 years, the detailed mechanism governing stereselectivity is still not well understood. Various proposals on the diene polymerization mechanism have been 14-17... 

The mechanism of the ring-opening polymerization effected by transition metal catalysts has been investigated and an insertion of the transition metal between the Cp-Si bond is believed to be the key initiation step. This is followed by successive insertion reactions leading to the formation of the pol5mer [7]. 

During early investigations into the polymerizations of cycloalkenes by transition metal catalysts such as those used in Ziegler-Natta polymerizations, polymers of unexpected structures that contained the same number of double bonds as originally present in the monomers were formed. This process is illustrated by the polymerization of norbomene. 

Olefin polymerization by transition metal complexes such as those in the catalyst systems of Ziegler and Natta is remarkably stereospecific. A mixture of an alkylaluminum halide and TiCl4 polymerizes ethylene at low pressure to crystalline linear polyethylene 184) with a relatively high density (0.96) and melting point (132° C). These properties contrast sharply... 

Although living a-olefin polymerizations with transition metal catalysts are still quite rare despite the modem efficient examples recently developed (for a recent review, see [69]), some E—N copolymerizations have been shown to be quasi-living or living. The molecular mass of E-N copolymers by catalyst I-l, at temperatures between 30 and 50°C and high norbomene feed fi actions, increases with time for up to 1 h. The polydispersity can be as narrow as 1.1 at [N]/[E] = 28 [49], indicating that E—N copolymerizations are quasi-living under these conditions. 

Table I. Polymerization of Silacyclobutanes by Transition Metal Catalysts ...

The cis-1,4 content of polymers obtained by transition metal catalysts is very high The physical properties of this particular polymer are improved compared with those of polymers obtained by alkali metal catalysts. However, further improvement of physical properties by modification cannot be expected because transfer reactions frequently occur in the polymerization reaction by transition metal catalysts. 

Alkenes.— A text on Ziegler-Natta catalysts has been published and polymerization by transition-metal hydrides, alkyls, and allyl compounds heis been reviewed. A reaction model for Ziegler-Natta polymerization has also been formulated. The conventional mechanism for alkene dimerization, oligomerization, and polymerization has, however, been questioned because there are no unambiguous examples of metal-alkyl-alkene compounds which unda go alkene insertion into the metal-alkyl bond. Also, catalysts which effect Ziegler-Natta polymerization are often active for alkene metathesis reactions and so a similiar mechanism for both has been proposed (Schrane 2). ... 

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