Coordination insertion

As the transition metal atom is usually part of the crystal lattice of the insoluble catalyst component, the growing chain must migrate to its original position after each monomer insertion. According to De Bruin, the monomer is coordinated to the transition metal atom, from which it is redirected into the metal—carbon bond of the neighbouring transition metal atom [301] 

The growing alkyl migrates between the neighbouring Ti atoms at the crystal surface. Rodriguez and Van Looy consider the complexed alkylaluminium compound as an integral part of the active centre even though growth only takes place at the transition metal—carbon bond [302], 

Ivin et al. doubt the general possibility of alkene insertion into the transition metal—carbon bond [303]. But insertion into the metal—H bond is regarded as established. They noted the similarity of disproportionation [303] and ZN catalysts, and of the respective reactions. They postulated the following mechanism of homogeneous and heterogeneous alkene polymerization. 

A similar mechanism can also be written for alkene oligomerizations on Al centres in the idealized, transition form, because the Al d orbitals can hardly contribute to other bonds in the organoaluminium compounds 

The stereospecific activity of the catalysts is given by the possibilities of the space arrangement of the aletane transition form in scheme (162). According to Ivin et al., insertions into the simple metal—carbon bond are an exception rather than a rule [303]. 

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This scheme is remarkably close to the coordination insertion mechanism believed to operate in the metal alkoxide-catalyzed ring-opening polymerization of cyclic esters (see Section 2.3.6). It shares many features with the mechanism proposed above for the metal alkoxide-catalyzed direct polyesterification (Scheme 2.18), including the difficulty of defining reaction orders. 

Given the success of the Grubbs-type NHC-Ru catalysts in metathesis polymerisation (Chapter 3), it is somewhat surprising that more research has not been done on mid-transition metal carbene complexes for coordination-insertion polymerisation. At this stage however, there are only a few reported attempts with the metals Co, Fe and Ir. 

More success has been had with Ir complexes incorporating permelhylcyclopentadiene and NHC ligands. Complexes 18-20 (Fig. 4.7) were evalnated for norbomene polymerisation following activation with MAO [22]. Complex 19 was the most active, giving a TOF of 12 220 h over 10 min, followed by 18 (TOF = 3 220 h" ), while 20 was inactive, indicating that a hemilabile pendant group seems essential. Analysis (NMR) of the polymers formed with 18 and 19 shows that polymerisation proceeds via an addition (coordination-insertion) mechanism. 

Mecerreyes, A, Dubois, P and Jerdme, R. Novel Macromolecular Architectures Based on Aliphatic Polyesters Relevance of the Coordination-Insertion Ring-Opening Polymerization. VoL 147, pp. 1 -60. 

Coordination of Ni(0) to the alkyne gives a n complex, which can be written in its Ni(II) resonance form. Coordination and insertion of another alkyne forms the new C6-C7 bond and gives a nickelacyclopenta-diene. Maleimide may react with the metallacycle by coordination, insertion, and reductive elimination to give a cyclohexadiene. Alternatively, [4+2] cycloaddition to the metallacycle followed by retro [4+1] cycloaddtion to expel Ni(0) gives the same cyclohexadiene. The cyclohexadiene can undergo Diels-Alder reaction with another equivalent of maleimide to give the observed product. 

M. A. Paul, M.Alexandre, P. Degee, C. Calberg, R. Jerome, P. Dubois, Exfoliated polylactide/clay nanocomposites by In-situ coordination-insertion polimerization, Macromol. Rapid. Commun., vol. 24, pp. 561-566, 2003. 

Fig. 1. Melting temperature (—and enthalpy (—, AH) of poly(8CL-co-6VL) random copolymers at different compositions, as synthesized by coordination-insertion ROP initiated with Al(OzPr)3 in toluene at 0 °C. is the molar fraction of eCL in the copolyester. (Tjjj and AH were determined by DSC at a heating rate of 10 °C/min)...

Significant advances in the understanding of the coordination-insertion ROP mechanism have been made owing to the kinetic studies by Duda and Penczek. 

With the idea of extending the scope of the macromolecular engineering of aliphatic polyesters, the coordination-insertion ROP of lactones and dilactones has been combined with other polymerization processes. This section aims at reviewing the new synthetic routes developed during the last few years for building up novel (co)polymer structures based on aliphatic polyesters, at least partially. 

Considerable effort has been carried out by different groups in the preparation of amphiphihc block copolymers based on polyfethylene oxide) PEO and an ahphatic polyester. A common approach relies upon the use of preformed co- hydroxy PEO as macroinitiator precursors [51, 70]. Actually, the anionic ROP of ethylene oxide is readily initiated by alcohol molecules activated by potassium hydroxide in catalytic amounts. The equimolar reaction of the PEO hydroxy end group (s) with triethyl aluminum yields a macroinitiator that, according to the coordination-insertion mechanism previously discussed (see Sect. 2.1), is highly active in the eCL and LA polymerization. This strategy allows one to prepare di- or triblock copolymers depending on the functionality of the PEO macroinitiator (Scheme 13a,b). Diblock copolymers have also been successfully prepared by sequential addition of the cyclic ether (EO) and lactone monomers using tetraphenylporphynato aluminum alkoxides or chloride as the initiator [69]. 

This method exclusively yields macrocyclic polyesters without any competition with linear polymers. Furthermore, the coordination-insertion ROP process can take part in a more global construction set, ultimately leading to the development of new polymeric materials with versatile and original properties. Note that other types of efficient coordination initiators, i.e., rare earth and yttrium alkoxides, are more and more studied in the framework of the controlled ROP of lactones and (di)lactones [126-129]. These polymerizations are usually characterized by very fast kinetics so as one can expect to (co)polymerize monomers known for their poor reactivity with more conventional systems. Those initiators should extend the control that chemists have already got over the structure of aliphatic polyesters and should therefore allow us to reach again new molecular architectures. It is also important to insist on the very promising enzyme-catalyzed ROP of (di)lactones which will more likely pave the way to a new kind of macromolecular control [6,130-132]. 

The coupling of epoxides and carbon dioxide as catalyzed by metal complexes is generally believed to occur via a coordination-insertion mechanism involving either one or two metal centers. The array of reaction processes that are frequently associated with this transformation are indicated below (Schemes 1 ). 

ROP of p-lactones is highly prone to numerous side reactions, such as transester-fication, chain-transfer or multiple hydrogen transfer reactions (proton or hydride). Specifically, the latter often causes unwanted functionalities such as crotonate and results in loss over molecular weight control. Above all, backbiting decreases chain length, yielding macrocyclic structures. All these undesired influences are dependent on the reaction conditions such as applied initiator or catalyst, temperature, solvent, or concentration. The easiest way to suppress these side reactions is the coordination of the reactive group to a Lewis acid in conjunction with mild conditions [71]. p-BL can be polymerized cationically and enzymatically but, due to the mentioned facts, the coordinative insertion mechanism is the most favorable. Whereas cationic and enzymatic mechanisms share common mechanistic characteristics, the latter method offers not only the possibility to influence... 

Later, Kricheldorf and coworkers extended the concept of the aluminum alkox-ide-initiated ROP of lactones to a set of other metal alkoxides such as tin(lV) [23-25], titanium, and zirconium alkoxides. As a rule, the polymerization takes place according to the same coordination-insertion mechanism shown in Fig. 12. 

Lewis acids were also screened for the ROP of lactones [65]. The polymerization takes place according to a cationic mechanism provided that the counterion is not too nucleophilic. Conversely, when Lewis acids with a nucleophilic counterion are used, several examples are reported where the polymerization takes place according to the usual coordination-insertion mechanism (Fig. 12). This coordination-insertion mechanism was indeed reported for the ROP initiated by ZnCl2 [66], TiCU, and AICI3 [67]. 

Raquez J-M, Degee P, Narayan R, Dubois P (2000) Coordination-insertion ring-opening polymerization of l,4-dioxan-2-one and controlled synthesis of diblock copolymers with e-caprolactone. Macromol Rapid Commun 21 1063-1071... 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

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