Polymerization, coordination 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. 

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]. 

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. 

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... 

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]. 

Initially, hydroxyl functionalities coordinate to stannous octoate. In the next step a lactone coordinates. Both the lactone ester and hydroxyl group are now activated for a nucleophilic attack of the latter on the ester. Polymerization has now been initiated and propagation proceeds by repetitions of the reaction sequence. Whether and how lactones are polymerized by stannous octoate in the absence of impurities remains unsolved, and hypothetical. 

PHB is a polymer obtained from many strains of bacteria. Von Korsatko claimed that PHB of various MW can be readily manufactured depending on the extraction method [162]. Grassie et ai and Tanahashi et al. described how PHB can be prepared synthetically [163-164]. Bleoembergen et al. reported the synthesis of HB/HV copolymers by coordination polymerization of )9-lactones [165]. HB/HV copolymers from 0 to 30% HV contents are available as Biopol on the market. 

Bleoembergen, S., Holden, D. A., Bluhm, T. L. (1987) Synthesis of crystalline p-hydroxybutyrate/ jff-hydroxyvalerate copolyesters by coordination polymerization of / -lactones. Macromolecules, 20,3086-9. 

S. Bleoembergen, D.A. Holden, T.L. Bluhm, G.K. Hamer and R.H. Marchessault, "Synthesis of crystalline b-hydroxybutyrate/ b-hydroxyvalerate copolyesters by coordination polymerization of b-lactones". Macromolecules, 20, 3086-3089,1987. 

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. 

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... 

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

NMR studies on the alkoxide initiators confirm that all the lactones polymerize via an acyl— oxygen scission, including /3-PL (which, by contrast, opens at the alkyl—oxygen bond with (251)). Monomer coordination and subsequent ring opening may be observed by 111 NMR spectroscopy. Coordination is also observed with 7-BL and 7-VL, although these adducts are stable to insertion and polymerization does not proceed. 

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]. 

The purpose of this review is to report on the recent developments in the macromolecular engineering of aliphatic polyesters. First, the possibilities offered by the living (co)polymerization of (di)lactones will be reviewed. The second part is devoted to the synthesis of block and graft copolymers, combining the living coordination ROP of (di)lactones with other living/controlled polymerization mechanisms of other cyclic and unsaturated comonomers. Finally, several examples of novel types of materials prepared by this macromolecular engineering will be presented. 

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]. 

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... 

Some initiators used for the coordinative ROP of lactones are known to form aggregates. Aluminum isoporopoxide is a typical example because it is known to exist as a mixture of trimers (A3) and tetramers (A4) (Fig. 11). If the polymerization of eCL is initiated at 0 °C by a mixture of A3 and A4, only A3 is prone to initiate the... 

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]. 

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]. 

Early-on it was discovered that these Salen compounds, and the related six-coordinate cations [6], were useful as catalysts for the polymerization of oxiranes. These applications were anticipated in the efforts of Spassky [7] and in the substantial work of Inoue [8]. Subsequently, applications of these compounds in organic synthesis have been developed [9]. Additional applications include their use in catalytic lactide polymerization [10], lactone oligomerization [11], the phospho-aldol reaction [12], and as an initiator in methyl methacrylate polymerization [13]. 

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