Anionic ring-opening mechanism

For example, see Slomkowski, S. and Duda, A. Anionic ring-opening polymerization, In Ring-Opening Polymerization, Mechanisms, Catalysis, Structure, and Utility, Brunelle, D. J. (Ed.), Hanser-Verlag, New York, pp. 87-128, 1993. 

Slomkowski, S. and A. Duda, Anionic Ring-Opening Polymerization, Chap. 3 in Ring-Opening Polymerization. Mechanism, Catalysis, Structure, Utility, D. J. Brunelle, eds., Hanser, Munich, 1993. 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

Abstract. This paper reviews ring-opening polymerization of lactones and lactides with different types of initiators and catalysts as well as their use in the synthesis of macromolecules with advanced architecture. The purpose of this paper is to review the latest developments within the coordination-insertion mechanism, and to describe the mechanisms and typical kinetic features. Cationic and anionic ring-opening polymerizations are mentioned only briefly. 

Theoretical studies using the results of molecular dynamics simulation of A-methylazetidin-2-one in aqueous solution predicted a stepwise mechanism for the hydrolysis <1998JA2146>. In the alkaline hydrolysis, the first reaction step involved the formation of a tetrahedral intermediate, which required a desolvation of the hydroxyl anion, which is difficult to simulate by calculations. Afterwards, the reaction proceeded through either a concerted or stepwise mechanism for ring opening and proton transfer. 

Sormani PM, McGrath JE (1985) Kinetics and mechanisms of the anionic ring opening polymerization of cyclosiloxanes in the presence of bis(l,3-aminopropyl tetramethyl-disiloxane) In McGrath JE (ed) Ring opening polymerization kinetics mechanisms and synthesis. ACS Symposium Series No 286... 

The mechanism illustrated in equations 13-15 was proposed by Grubb and Osthoff (3 ) for the anionic ring opening polymerization. There is, however, considerable question as to the nature of the ion pair formed in equations 13 and 14. 

The subjects Include fundamental and applied research on the polymerization of cyclic ethers, slloxanes, N-carboxy anhydrides, lactones, heterocycllcs, azlrldlnes, phosphorous containing monomers, cycloalkenes, and acetals. Block copolymers are also discussed where one of the constituents is a ring opening monomer. Important new discussions of catalysis via not only the traditional anionic, cationic and coordination methods, but related UV Initiated reactions and novel free radical mechanisms for ring opening polymerization are also Included. 

Much of the recent activities in anionic ring-opening polymerization involve the use of anionic coordination initiators. For these initiators, the metal coordinates with the carbonyl (C=0) oxygen of the monomer, which is followed by cleavage of the acyl-oxygen (CO-0) bond of the monomer and insertion into the metal-oxygen (M-O) bond of the initiator. The experimental evidence for this coordination-insertion mechanism comes from end group analysis of the polymer formed. 

Figure 21.1 Possible mechanisms for anionic ring-opening polymerisations of non-substituted lactones showing both acyl-oxygen scission (a) as well as alkyl-oxygen scission (b).

Among these reactions, the Cu(l)-catalyzed azide-alkyne cycloaddition (CuAAC) is the most widely used. This reaction has been implemented for the preparation of segmented block copolymers from polymerizable monomers by different mechanisms. For example, Opsteen and van Hest [22] successfully prepared poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA) and PEO-b-PSt by using azide and alkyne end-functionalized homopolymers as the click reaction components (Scheme 11.2). Here, PEO, PSt, and PMMA homopolymers were obtained via living anionic ring-opening polymerization (AROP), atom transfer radical polymerization (ATRP), and postmodification reactions. Several research groups have demonstrated the combination of different polymerization techniques via CuAAC click chemistry, in the synthesis of poly(e-caprolactone)-b-poly(vinyl alcohol) (PCL-b-PVA)... 

Several reaction mechanisms were offered to explain the mechanism of anionic ring-opening polymerizations of lactams. One mechanism is based on nucleophilic attacks by the lactam anions at the cyclic carbonyl groups of N-acylated lactams. This leads to formations of intermediate symmetrical mesomeric anions that rearrange with openings of the rings [133, 134] ... 

Fig. 2.12 Mechanism of anionic ring-opening polymerization of small cyclic (a) and laiger cyclic...

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