Transition Metal Coordination in Polymeric Complexes

II. METHODOLOGY OF TRANSITION METAL COORDINATION IN POLYMERIC COMPLEXES... 

Methodology of Transition Metal Coordination in Polymeric Complexes 5... 

We wish also to point out that the fact that in the above schemes only the transition metal, the last polymerized unit, and the coordinated monomer are represented does not mean that the catalysts prepared from A1(C2H5)3 and a titanium alkoxyde or from A1(C2H5)2C1 and a Co compound are pure organo-metallic compounds of Ti or Co, respectively. These catalysts are presumably complexes containing A1 and the transition metal, the latter probably being part of a cation, the A1 of an anion. In the above schemes we have represented only the transition metal for the sake of simplicity. 

There are a number of different types of polymers that contain metal carbonyl complexes attached to polymer backbones and side chains. In the 1970s, Pittman and coworkers reported the synthesis and polymerization of a wide range of vinyl monomers containing transition-metal-coordinated cyclopentadienyl rings. Polymer 21 is an example of a copolymer containing chromium and manganese complexes in the side chain. Another class... 

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. 

In another process for the synthesis of PPS, as well as other poly(arylene sulfide)s and poly(arylene oxide)s, a pentamethylcyclopentadienylmthenium(I) TT-complex is used to activate -dichlorobenzene toward displacement by a variety of nucleophilic comonomers (92). Important facets of this approach, which allow the polymerization to proceed under mild conditions, are the tremendous activation afforded by the TT-coordinated transition-metal group and the improved solubiUty of the resultant organometaUic derivative of PPS. Decomplexation of the organometaUic derivative polymers may, however, be compHcated by precipitation of the polymer after partial decomplexation. 

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. 

At the present time the concept of catalytic (or ionic-coordination ) polymerization has been developed by investigating polymerization processes in the presence of transition metal compounds. The catalytic polymerization may be defined as a process in which the catalyst takes part in the formation of the transition complexes of elementary acts during the propagation reaction. 

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

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. 

While these spectroscopic and redox properties alone would be sufficient for direct use of transition metal complexes in solution-phase ECDs, polymeric systems based on coordination complex monomer units, which have potential use in all-solid-state systems, have also been investigated. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

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