Polymerization by transition metal

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

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 polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

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. 

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. 

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). 

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. 

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). 

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... 

Coordination Polymerization by Transition-Metal Alkyls and Hydrides. 

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]. 

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). ... 

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. ... 

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