Transition-metal coordination mechanisms polymeric complexes

In general, the compounds of the Group 4 metals, such as halides and alkoxides, are well known as Lewis acids to catalyze two-electron electrophilic reactions, and their metallocenes coupled with alkylation and/or reduction agents were effective catalysts for the coordination polymerization of olefins. For the transition metal-catalyzed radical polymerization, their alkoxides, such as Ti(Oi-Pr)4, have also been employed as an additive for a better control of the products. Contrary to the common belief that the Group 4 metals rarely undergo a one-electron redox reaction under mild conditions, there have been some reports on the controlled radical polymerization catalyzed or mediated by titanium complexes, although the conflict in the mechanism between the (reverse) ATRP and OMRP is also the case with the Group 4 metal complexes. 

Not only the highly Lewis acidic early transition metal-based polymerization catalysts suffer from poisoning by coordination of functional groups. Even in late transition metal-based complexes, the possible o-coordination in certain functional groups has a negative impact on polymerization reactions. The prominent example here is the still ongoing search for active acrylonitrile (AN) copolymerization catalysts. This reaction can serve as an ideal example to illustrate the challenges in late transition metal-catalyzed insertion polymerizations with polar functionalized comonomers. The metal-mediated copolymerization of AN has numerous appearances in literature however, in most cases, the reaction mechanism seems to be of ionic or radical nature. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

This chapter does not intend to provide a complete collection of newly synthesized organometallic or coordination complexes for alkene polymerization, but rather aims to review a cross-section of transition metal catalysts from the viewpoint of polymers and polymerization reactions. We focus particularly on polymers that are difficult or virtually impossible to prepare using conventional catalysts. In this light, we narrow our attention to well-defined molecular catalysts, including a study of progress in the understanding of active species, reactive intermediates, and reaction mechanisms that are indispensable for the synthesis of such polymers. 

The discussion of Mnetic work will be here preceded by a summarized description of the chemical nature of the polymerization, to which we have attributed a mechanism of anionic coordinated type. Such a definition of the reaction mechanism depends upon the fact that the catalyst is a complex in which, generally, a transition metal acts as a coordinating agent and that a carbon atom, which belongs to the extremity of a growing polymeric chain, is coordinated to such a complex and, in the activated state, it possesses a negative charge. 

The mechanism of coordination polymerization of 1,3-butadiene and, in general, that of conjugated dienes follows the same pathway discussed for alkene polymerization that is, monomer insertion into the transition metal-carbon bond of the growing polymer chain occurs. One important difference, however, was recognized very early.47,378,379 In the polymerization of dienes the growing chain end is tt-allyl complexed to the transition metal ... 

Stereospecific emulsion polymerization of butadiene has been achieved in the presence of soluble transition metal salts 350, 351). Polymer microstructure was controlled by varying the transition metal ion and its ligands. Although the initiation mechanism has not been determined, it is most likely to be of the coordinated radical type with steric control arising from the transition metal-diene complexes. 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

Coordination chemistry has become a powerful tool for the control and the living nature of radical polymerization [79,80]. Various examples show that the role of initiator and counter radical can be played by organometallic species with an even number of electrons. Besides aluminum complexes used by Matyjaszewski, several other transition metals, metallocenes, and organolan-thanides with various ligands have been studied in controlled radical polymerization [79-97]. In some cases, a controlled polymerization was achieved [81,83-85,87,90-94,97]. However, the mechanism of the polymerization is not always known and it may happen that heterolytic cleavage of the active bond... 

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

Although there are two metals in the Ziegler-Natta catalyst, the weight of current evidence indicates that polymerization takes place at the transition metal-carbon bond. The mechanism is illustrated here with reference to polymerization by TiCl /Al(CH2CH3)3 catalyst complex. The normal geometry for Ti atoms is octahedral, and the catalyst site, as shown in 9-21, is believed to be a coordinately unsaturated Ti bonded to four Cl s [which in turn are bridged to two other Ti s] and to an alkyl group, derived from the aluminum alkyl component. 

The active species in MAO cocatalyzed metallocene polymerization is likely a coordinatively unsaturated cationic complex, as in 9-24 (for a zirconium-based metallocene), where substituents on the metal atoms have been omitted for clarity. The Zr—O—A1 bond withdraws electron density from the Zr atom. The mechanism for ethylene polymerization is proposed to be as follows, where the monomer forms a t-complex with the transition metal [ 14] ... 

For substitution of monodentate 77-hydrocarbon ligands (ethylene, acetylene) a priori both mechanisms are possible. In this case an ability to change the coordination number in the transition state will be decisive. It is probable that square-planar complexes react by an associative mechanism with an increase in coordination number in the transition state. For the octahedral complexes, intermediates with lower coordination number are preferable (D-type mechanism). There is as yet no evidence for a transition state involving a-bonded ethylene or acetylene. However, both molecules are capable of inserting into transition metal-carbon u-bonds 10). It is quite probable that such an insertion mechanism operates in the Ziegler-Natta ethylene polymerization 11). 

Apparently, the reactivity of organometallic compounds in the addition of olefins to Mt—C bonds is determined by the capability of these compounds to coordinate olefins. The formation of intermediate n-complexes ensures further insertion of olefin by a concerted mechanism with a low activation energy. Thus, a high reactivity of active centers, containing a transition metal, comparable to the reactivity of the radical active centers, is achieved. The activation energy of the propagation in olefin polymerization on catalysts containing transition metals (2-6 kcal/mol) does not exceed its value for the radical polymerization (Table 10). 

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