Polymerization of lactones

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

Various reaction schemes were proposed to explain the cationic mechanism. They tend to resemble the schemes suggested for polymerizations of cyclic ethers. The initiation step involves an equilibrium that is followed by a ring-opening reaction  

The propagation consists of many repetitions of the above step  

Because the caiboiwl oxygen is the most basic of the oxygens in the lactone molecule, a reverse reaction is possible.  

Conductivity measurements during polarizations of j -propiolactone with antimony pentafluoride etherate or p-toluenesulfonic acid show that ion triplets form during the reaction. These are  

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. 

Cationic polymerizations of lactones has been carried out with the help of alkylating agents, acylating agents, Lewis acids, and protonic acids. Various reaction schemes were proposed to explain the 

The polymerization of propiolactone in methylene chloride with an antimony pentachloride-dietherate catalyst was investigated [88]. The results show that the concentration of the active centers is dependent upon catalyst concentration and upon the initial concentration of the monomer. They also support the concept that opening of the lactone rings includes initial formation of an oxonium ions [88]  

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We conclude this section by citing some examples of ring-opening polymerizations. Table 5.9 lists several examples of ring-opening polymerizations. In addition to the reactions listed, we recall the polymerizations of lactones and lactams exemplified by equations in Table 5.3 and 5.4, respectively. 

Lipases have also been used as initiators for the polymerization of lactones such as /3-bu tyro lac tone, <5-valerolactone, e-caprolactone, and macrolides.341,352-357 In this case, the key step is the reaction of lactone with die serine residue at the catalytically active site to form an acyl-enzyme hydroxy-terminated activated intermediate. This intermediate then reacts with the terminal hydroxyl group of a n-mer chain to produce an (n + i)-mer.325,355,358,359 Enzymatic lactone polymerization follows a conventional Michaelis-Menten enzymatic kinetics353 and presents a controlled character, without termination and chain transfer,355 although more or less controlled factors, such as water content of the enzyme, may affect polymerization rate and the nature of endgroups.360... 

The living polymerization of lactones, oxiranes, and thiiranes became also possible by improved preparation of the Al—Zn oxyalkoxides. These catalysts were first studied by Tsuruta 6) and by Vanderberg 7), and later by Teyssie 8 b). 

Enzyme activity for the polymerization of lactones was improved by the immobilization on Celite [93]. Immobilized lipase PF adsorbed on a Celite showed much higher catalytic activity than that before the immobilization. The catalytic activity was further enhanced by the addition of a sugar or poly(ethylene glycol) in the immobilization. Surfactant-coated lipase efficiently polymerized the ring-opening polymerization of lactones in organic solvents [94]. 

The enzymatic polymerization of lactones is explained by considering the following reactions as the principal reaction course (Fig. 9) [83,85,95,96]. The key step is the reaction of the lactone with lipase involving the ring-opening of the lactone to give the acyl-enzyme intermediate (enzyme-activated monomer,... 

An alcohol could initiate the ring-opening polymerization of lactones by lipase catalyst ( initiator method ). In the lipase CA-catalyzed polymerization of DDL using 2-hydroxyethyl methacrylate as initiator, the methacryloyl group was quantitatively introduced at the polymer terminal, yielding the methacryl-type polyester macromonomer [98]. This methodology was expanded to synthesis of co-alkenyl- and alkynyl-type macromonomers by using 5-hexen-l-ol and 5-hexyn-l-ol as initiator. 

The enzymatic polymerization of lactones could be initiated at the hydroxy group of the polymer, which expanded to enzymatic synthesis of graft copolymers. The polymerization of c-CL using thermophilic lipase as catalyst in the presence of hydroxyethyl cellulose (HEC) film produced HEC-gra/f-poly( -CL) with degree of substitution from 0.10 to 0.32 [102]. 

Table 2 Ring-opening polymerization of lactones initiated by lanthanide complexes (CL = e-caprolactone LA = lactide TMC = trimethylene carbonate). 

Living polymerization of lactones has been successfully performed by catalysis of rare earth metal complexes to obtain Mw/Mn of 1.07-1.08 [8]. The lowest polydispersity index attained so far with the AlEt3/H20 system is 1.13. 

Table 7. Living polymerization of lactones with organolanthanide complexes...

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

Aluminum Alkoxides Mediated Ring Opening Polymerization of Lactones and Lactides... 

Aluminum isopropoxide has proved to be a very effective initiator for the polymerization of lactones pPL, 6VL, eCL, DXO, and pBL and dilactones D,L- and L-LA, and GA. It is worth noting that cycHc carbonates, e.g., 2,2-dimethyltri-methylene carbonate (DTC) and cycHc anhydrides such as adipic anhydride (AA) have also been polymerized by aluminum trialkoxides with the unique possibility to control the molecular parameters of the polycarbonate (PDTC) and polyanhydrides (PAA). This is illustrated in Scheme 2 and the preferred reaction conditions are given in Table 3. 

Lofgren A, Albertsson AC, Dubois P, Jd6me R (1995) Recent advances in ring opening polymerization of lactones and related compounds. JMS Rev Macromol Chem Phys... 

Main Mechanisms of Polymerization of Lactones by Ring Opening. 179... 

Aliphatic polyesters occupy a key position in the field of polymer science because they exhibit the remarkable properties of biodegradability and biocompatibihty, which opens up a wide range of applications as environmentally friendly thermoplastics and biomaterials. Three different mechanisms of polymerization can be implemented to synthesize aliphatic polyesters (1) the ring-opening polymerization (ROP) of cyclic ketene acetals, (2) the step-growth polymerization of lactones, and (3) the ROP of lactones (Fig. 1). 

The polymerization of lactones is initiated by nucleophilic metal alkoxides. It is worth noting that bulky alkoxides are not nucleophilic enough and react as bases. For example, potassium ferf-butoxide deprotonates (3-propionolactone and sCL into new anionic species, which are anionic initiators for the polymerization of lactones [8] (Fig. 4). As a rule, carboxylic salts are less nucleophilic than the corresponding alkoxides and are, in general, not efficient initiators for the polymerization of lactones. Nevertheless, (3-lactones are exceptions because their polymerization can be initiated by carboxylic salts [8]. 

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