Anionic polymerization, coordinated lactones

A very broad range of initiators and catalysts are reported in the scientific literature to polymerize lactones. The polymerization mechanisms can be roughly divided into five categories, i.e., anionic polymerization, coordination polymerization, cationic polymerization, organocatalytic polymerization, and enzymatic polymerization. 

The general subject of lactone polymerization has been reviewed (7, 19). Polymerization of e-caprolactone can be effected by at least four different mechanisms categorized as anionic, cationic, coordination, and radical. Each method has unique attributes, providing... 

Since then, the process has been extended to a wide variety of lactones of different size and to several lipases, as recently reviewed [93-96]. Interestingly, large-membered lactones, which are very difficult to polymerize by usual anionic and coordination polymerizations due to the low ring strain, are successfully polymerized by enzymes. Among the different lipases available, that fi om Candida antarctica (lipase CA, CALB or Novozym 435) is the most widely used due to its high activity. An alcohol can purposely be added to the reaction medium to initiate the polymerization instead of water. The polymerization can be carried out in bulk, in organic solvents, in water, and in ionic liquids. Interestingly, Kobayashi and coworkers reported in 2001 the ROP of lactones by lipase CA in supercritical CO2... 

A variety of anionic initiators, both ionic and covalent, have been used to polymerize lactones [Duda and Penczek, 2001 Jedlinski, 2002 Jerome and Teyssie, 1989 Penczek and Duda, 1993]. Much of the more recent activity involves the use of anionic covalent (coordination) initiators such as alkylmetal alkoxides and metal alkoxides such as R2A OR and Al(OR)3, metal carboxylates such as tin(II) 2-ethylhexanoate, metalloporpyrins (VI), and aluminox-anes such as oligomeric [A1(CH3)0] [Biela et al., 2002 Duda et al., 1990 Endo et al., 1987a,b Gross et al., 1988 Kricheldorf et al., 1990 Penczek et al., 2000a,b Sugimotoa and Inoue, 1999]. 

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. 

Compared with 49, 2,5-dioxabicyclo[2.2.2]octan-3-one (54) prepared from sodium 3,4-dihydro-2//-pyran-2-carboxylate has a much low polymerization reactivity [54] Lewis acids such as antimony pentachloride, phosphorus pentafluoride, and boron trifluoride etherate were not effective at all to initiate the polymerization of 54. Trifluoromethanesulfonic acid induced the polymerization of 54, but the yield and molecular weight of the polymer were low. Bicyclic lactone 54 was allowed to polymerize with anionic and coordination initiators such as butyl-lithium, lithiumbenzophenone ketyl, and tetraisopropyl titanate. However, the... 

Some heterocycles have both nucleophilic and electrophilic atoms in their molecule. Thus they can be opened and polymerized by the anionic, cationic or coordination mechanisms. Examples are lactams, lactones, and cyclic siloxanes. Investigations of the mechanism of lactam propagation are complicated by the occurence of side reactions. In principle, the mechanism described in Chap. 3 by the schemes (55)—(57) and (71) is accepted. Anionic polymerization of cyclic esters consists, in most cases (see Chap. 4, Sect. 2.2) of repeated reversible attacks on the carbonyl carbon by the anion 0]-. From e-caprolactone, polyester chains grow according to [315]... 

Monomers listed above polymerize by the cationic mechanism. For some groups of monomers (lactones, carbonates) anionic or coordinate mechanism also operates and, from a synthetic point of view, this is the preferred method of converting cyclic esters into linear polyesters. The cationic polymerization of lactones, glycolide and it substituted analog, lactide, as well as spiroorthoesters and bicyclic orthoesters has been studied in some detail. 

Lactones, i.e. esters of hydroxyacids and their dimers, like glycolide and lactide, are two major groups of cyclic esters used in polymerization. These compounds are used on the large scale and polymerized mostly by anionic or coordinative mechanisms. Four, six-, and seven-membered lactones polymerize by both cationic and anionic mechanisms. Polyesters prepared in this way are however only a small fraction of the polyesters prepared by polycondensation. 

Coordination-insertion, anionic, cationic, and nucleophilic polymerization are the most frequently reported controlled ring-opening polymerization (ROP) of cyclic monomers in the literature [37, 38]. The coordination-insertion and nucleophilic polymerization are undoubtedly the most efficient and general methods reported so far for the ROP of lactones, with cationic and anionic polymerization being much less investigated. While coordination-insertion polymerization uses metal-alkoxides and related complexes as catalysts, the organocatalytic nucleophilic polymerization is a metal-free approach to ROP. 

Polycaprolactone (PCL) is obtained by ring-opening polymerization of the six-membered lactone, e-caprolactone (Figure 30.4f), which yields a semicrystalline polymer with a melting point of 59°C-64°C and a glass transition temperature of 60°C with great organic solvent solubility. Anionic, cationic, coordination, or radical polymerization routes are all applicable for synthesis. " " " ... 

Lactones polymerize by three different mechanisms, namely, cationic, anionic, and coordinated... 

Polymerization of lactones can be carried out by three mechanisms, namely, cationic, anionic, and coordinated one. Often, the mechanism by which a specific lactone polymerizes depends upon the size of the ring. 

In spite of the number of investigations that were devoted to the polymerization of lactone the exact mechanism whereby the polymer is formed is still not entirely clear. It is the purpose of this paper to present the various factors that influence the reaction and to describe its complexity when it is performed in the melt in the presence of either anionic or coordination catalysts. 

In spite of the living character of the anionic polymerization of PL, it was revealed that in the case of p-substituted p-lactones or medium-size cyclic esters (LA and CL) this process suffers from various side reactions such as chain transfer to monomer, racemization, segment exchange, or formation of macrocydic esters. " Finally, it has been concluded that in the so-called coordination-insertion (pseudoanionic) polymerization, initiated with covalent metal alkoxides (R MtOR c. where Mt is Zn, Al, Sn, Ti, etc.), these side reactions can be kinetically suppressed or even eliminated. Correlation of the selectivity parameters, defined as the ratio of rate constants of propagation and transfer (fep/fetr). with the reactivity of active species (fep) showed that these polymerizing systems conform to the reactivity-selectivity principle. ... 

ROP of aliphatic cyclic esters is a continuously and dynamically developing research field. Initially, fundamental aspects of polymerization, such as thermodynamics, kinetics, and mechanisms of the elementary reactions, were explored. The best understood systems encompass polymerization of lactones and LAs. Determination of the standard thermodynamics parameters of polymerization for a majority of the most important monomers now allows the estimation of the equilibrium monomer concentration at given polymerization conditions. For a few polymerizing systems, such as anionic polymerization of PL, CL, or coordinated (proceeding on polarized covalent bonds) polymerizations of CL and LAs, the absolute rate constants have been determined. However, in a majority of the polymerizations, only the net reactivities have usually been determined which does not provide direct access to absolute rate constants of propagation. Nonetheless, the ROP of cyclic esters seems to be a convenient model system for studies of mechanism of cyclic monomers, in general. 

Anionic and Coordinated Polymerization The alkoxide active centers in the polymerization of oxiranes, and carboxylate active centers in the polymerization of P-lactones, are not only nucleophilic but also sufEciently basic to abstract protons from the monomer molecule, and this eventually results in an irreversible chain transfer to monomer. 

In the following sections, the polymerization of P-lactones will be discussed with regards to the nature of the active species, whether anionic, cationic, coordination-type or carbene-based. Finally, a brief overview will be provided of the enzymatic ROP of four-membered lactones. 

In contrast to the fact that cyclic acetals can be polymerized only by cationic initiators, lactones undergo polymerization both cationically and anionically, and therefore a wide variety of initiators including coordinated catalysts can be used. In this section, the polymerization of bicyclic lactones is described, although only a limited number of papers on this subject have been published. 

FIGURE 2 Anionic, cationic, and coordination mechanisms of polymerization of e-caprolactone and related lactones. 

The first attempts at ROP have been mainly based on anionic and cationic processes [4,5]. In most cases, polyesters of low molecular weight were recovered and no control on the polymerization course was reported due to the occurrence of side intra- and intermolecular transesterification reactions responsible for a mixture of linear and cyclic molecules. In addition, aliphatic polyesters have been prepared by free radical, active hydrogen, zwitterionic, and coordination polymerization as summarized in Table 2. The mechanistic considerations of the above-mentioned processes are outside the scope of this work and have been extensively discussed in a recent review by some of us [2 ]. In addition, the enzyme-catalyzed ROP of (di)lactones in organic media has recently been reported however, even though this new polymerization procedure appears very promising, no real control of the polyesters chains, or rather oligomers, has been observed so far [6]. 

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]. 

Synthetic routes include anionic, cationic, zwitterionic, and coordination polymerization. A wide range of organometallic compounds has been proven as effective initiators/catalysts for ROP of lactones Lewis acids (e.g., A1C13, BF3, and ZnCl2) [150], alkali metal compounds [160], organozinc compounds [161], tin compounds of which stannous octoate [also referred to as stannous-2-ethylhexanoate or tin(II) octoate] is the most well known [162-164], organo-acid rare earth compounds such as lanthanide complexes [165-168], and aluminum alkoxides [169]. Stannous-2-ethylhexanoate is one of the most extensively used initiators for the coordination polymerization of biomaterials, thanks to the ease of polymerization and because it has been approved by the FDA [170]. 

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. 

Aluminum porphyrins initiate controlled ring-opening polymerizations of oxiranes [67-69] ]3-lactones [70-72], 5-valerolactone [74], -caprolactone [74] and D-lactide [75], as well as controlled addition polymerizations of methacrylates [76] and methacrylonitrile [77] (Eq. 15). As shown in Eq. (16), propagation occurs by a coordinative anionic mechanism... 

In principle, the polymerization of a lactone should follow mechanism(s) similar to the catalyzed reactions of simple esters. The transformations that are observed are a function of the catalyst and can be subdivided into (a) cationic, (b) anionic, and (c) coordination type. A simplified description for the three mechanisms is shown with -caprolactone as an example. 

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