Coordination-insertion chain

Generalized coordination-insertion chain growth mechanism of lactide to PLA R = growing polymer chain. 

Figure 2.10 Coordination-insertion chain growth reaction scheme of lactide to PLA using tin octoate R, growth of polymer chain (Henton et al., 2005).

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... 

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]. 

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. 

Depending on the nature of the active center, chain-growth reactions are subdivided into radicalic, ionic (anionic, cationic), or transition-metal mediated (coordinative, insertion) polymerizations. Accordingly, they can be induced by different initiators or catalysts. Whether a monomer polymerizes via any of these chain-growth reactions - radical, ionic, coordinative - depends on its con-... 

The reactive intermediates used in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes. Of the three common metal catalyzed polymerizations - coordination-insertion, ring-opening metathesis and diene polymerization - the last appears to possess the greatest tolerance toward protic solvents. The polymerization of butadiene in polar solvents was first reported in 1961 using Rh salts [18]. It was discovered that these polymerizations could be performed in aqueous solution with an added emulsifier (sodium dodecyl sulfate, for example). 

When a transition metal alkyl or a metal hydride reacts with olefin molecules to undergo successive insertions, chain growth of a polymer attached to the transition metal takes place. If -hydrogen elimination occurs from the polymer chain, a transition metal hydride coordinated with the olefin derived from the polymer chain will be produced. By displacement of the coordinated olefin from the transition metal by the other monomer olefin, the polymer with an unsaturated terminal bond is liberated with generation of a transition metal hydride coordinated with the olefin. New chain growth will follow from the hydride, with the net result of control of the molecular weight without termination of the polymerization process. The process is in fact a chain transfer process. 

The proposed polymerization pathway differs fundamentally from the coordination-insertion mechanism involving metal complexes, see Fig. 3.7 [5, 38]. Indeed, the nucleophilic catalyst only activates the monomer toward ring opening, whereas the metal complex activates the monomer, initiates the polymerization, and remains bound to the growing chain. The polymerization mechanism of a superbase or thiourea-amine catalyzed ROP will be discussed in more detail below. 

ROP of lactides is very well suited for end-chain functionalization of PLA. In the anionic initiation pathway, a functional anionic initiator (e.g. a functional metal alkoxide) can be used to start the polymerization reaction. Even better suited is the coordination-insertion pathway using an or-ganometalhc compound and a functional alcohol coinitiator that is easier to obtain than the corresponding anionic initiating agent. The anionic initiator and the alcohol coinitiator. 

E-caprolactone (Scheme 4.2), Sn(Oct)2-promoted ROP of LA proceeds via a coordination-insertion mechanism after the in situ formation of Sn-alkoxide moieties at the chain extremity as active centres. These active centres are derived from a rapid exchange reaction of protic compounds (ROH) with tin alkoxides. 

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