Polymerization, cationic 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. 

Studies on the cationic polymerization of cyclic ethers, cyclic formals, lactones and other heterocyclic compounds have proliferated so greatly in the last few years that a detailed review of the evidence concerning participation of oxonium and analogous ions in these reactions cannot be given here. Suffice it to say that there is firm evidence for a few, and circumstantial evidence for many such systems, that the reactive species are indeed ions and there appears to be no evidence to the contrary. A few systems will be discussed in sub-sections 3.2 and 4.4. 

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]. 

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. 

Cationic ROP of lactones has been known for a long time but is not very popular due to its poor control of the molecular parameters. In 1984, Penczek and coworkers reported the cationic polymerization of sCL and p-propionolactone... 

The activation of lactones by Bronsted acids was discussed in the section 2.3.3 dealing with cationic polymerization. An alternative relies on the use of nucleophilic species for the activation of lactones (Fig. 21). 

Shibasiki Y, Sanada H, Yokoi M, Sanda F, Endo T (2000) Activated monomer cationic polymerization of lactones and the application to well-defined block copolymer synthesis with seven-membered cyclic carbonate. Macromolecules 33 4316-4320... 

Lactones (cyclic esters) undergo anionic and cationic polymerization to form polyesters (Eq. 7-75) [Duda and Penczek, 2001 Johns et al., 1984 Kubisa, 1996 Lundberg and Cox, 1969 Young et al., 1977]. The reactivity of different lactones generally follows the pattern for other cyclic monomers, except that the 5-membered lactone (y-butyrolactone) does... 

Cationic polymerization of lactones is achieved with the range of initiators used for cyclic ethers (Sec. 7-2b) [Hofman et al., 1987a,b Kricheldorf and Sumbel, 1988a,b Kricheldorf et al., 1986, 1987a,b Kubisa, 1996 Penczek and Slomkowski, 1989a,b]. Initiation was formerly thought to involve attack of a positive species on the endocyclic oxygen to form an... 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

Cationic Polymerization. Cationic polymerization is initiated by the transfer of a cation from the catalyst to the monomer. It allows a wider choice of monomers with double bonds, including carbonyls, cyclic ethers, and lactones. The ion may be within a carbonium or an oxonium ion. Friedel-Crafts halides, like AlCls or A CoHsJCL, are strong Lewis acids and initiate the polymerization directly. Weak Lewis acids need a... 

Evidence on association in cationic polymerizations is less frequent. Nevertheless it does exist, and the occurrence of the effect is beyond any doubt. Belenkaya et al. described ion aggregates in cationic lactone polymerization... 

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. 

As discussed already for cationic polymerization of oxiranes, cycliza-tion can be eliminated if polymerization is performed under the conditions at which the activated monomer mechanism operates. This approach was used for cationic polymerization of e-caprolactone and other higher lactones [191]. Thus, in the polymerization of e-caprolactone in the presence of ethylene glycol (EG) and (C2Hs)30 +, PF6- catalyst, linear increase of molecular weight with conversion was observed up to M 3000 and polymers with DP = [M]o/[EG]0 and relatively narrow molecular weight distribution (MJM 1.3) were obtained. No cyclic oligomers were detected in reaction products. Similar results were obtained for polymerization of 5-valerolactone and j8-butyrolactone. Kinetic studies of the AM polymerization of lactones have been reported [192]. 

Cationic polymerization of cyclic carbonates, e.g., ethylene carbonate resembles cationic polymerization of lactones. The postulated polymerization scheme is shown by Eq. (153) (for initiation with CF3SO3CH3) [201,202] ... 

Recently, a series of works have been published on the cationic po merization of lactones (e.g. p-propiolactone and e-caprolactone ) and various ionic ecies have been reported together with elaborate kinetic treatments and some electrochemical measurements. In our opnion the chemical structure of the growing species in the cationic polymerization of lactones has not yet firmly been established (see also Ref. 190) and, therefore, a more detailed discussion of the% interesting and important systems must wait until these structures are known. 

Copolymerization of N-alkylazlrldlnes with g-proplolactone. g-proplolactone (PL) reacts with tertiary amines to form the corresponding zwltterlon (7 ). If an N-substltuted azlrldlne Is used, a zwltterlon containing an azlrldlnlum Ion and a carboxylate Ion Is formed. This zwltterlon can Initiate a cationic polymerization of the azlrldlne or an anionic polymerization of the lactone or undergo a coupling reaction with another (monomeric or polymeric) zwltterlon. 

A novel well-defined macromonomer of epoxy end-functionalized poly(V -capro-lactone) (PCL) was synthesized and its reactivity in photoinitiated cationic polymerization was examined [28]. PCL macromonomer as the comonomer allowed a rather simple incorporation of PCL side chains into poly(cyclohexene oxide) (PCHO) backbone. This way PCHO-g-PCL copolymer with random sequences of the structure shown in Scheme 13.16 is formed. 

Cationic polymerization of lactones can be initiated by a wide variety of protonic and Lewis acids, and by oxonium and carbenium salts. 

The cationic polymerization of lactones is not yet sufficiently well understood. The structure of the active centers is not known with certainty and the direction of the opening of the lactone ring (O-alkyl (a) vs. O-acyl (b)) has been established only recently for a few representative monomers 16,17) ... 

The ring opening polymerization of lactones, lactides, and glycolides to polyesters can be carried out by both anionic and cationic means using a variety of initiators similar to those used for cyclic ethers. However, cationic polymerization is... 

In the same groundbreaking paper [21], CriveUo and coworkers also demonstrated that the anion plays no role in determining the photosensitivity of the iodonium salt and the photolysis rates of diaryliodonium salts having the same cations but different non-nucleophilic counterions (BF4, PFs, AsFs", or SbFe ) are identical. Likewise, the cation structure has little effect on the photodecomposition of diaryliodonium salts. The utility of iodonium salts as photoinitiators has been demonstrated in several cationic polymerizations using olefins, epoxides, cycUe ethers, lactones and cyclic sulfides as the monomers [21],... 

Describe cationic polymerization of lactones, showing the initiation and propagation processes. 

On the other hand, syntheses of block copolymers based on cyclic esters (lactones) rather than cyclic ethers by a combination of controlled polymerization and AM polymerization techniques have been reported several times. The first example of cationic polymerization of cyclic esters such as CL in the presence of hydroxyl compounds and (Et)sO PFg" was... 

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... 

Bourissou et al. reported recently controlled cationic polymerization of lactones using a combination of triflic acid with a protonic reagent as the initiators [90]. The reaction was carried out in CH2CI2. Results indicated that the process is controlled is a linear relationship between the molecular... 

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