8-Propiolactones cationic polymerization

Slomkowski S, Szymanski R, Hofman A (1985) Formation of the intermediate cyclic six-membered oxonium ion in the cationic polymerization of P-propiolactone initiated with CHsCO SbFs. Makromol Chem 186 2283-2290... 

The propagating species in the cationic polymerization can be examined from the copolymerization behavior (21). Cyclic ethers such as tetrahydrofuran (THF) or 3,3-bischloromethyloxetane (BCMO), and cyclic esters such as 0-propiolactone (/3-PL) or -caprolactone (c-CL) are classified as oxonium ion type monomers. Copolymerizations between these monomers are observed easily as in the case of BCMO-THF (12, 13), BCMO-/3-PL (14, 15), BCMO-c-CL (16), and THF- -CL (21). 

Even carboxylate ions can serve as active centres of / -propiolactone polymerization [316]. Cationic polymerization is characterized by the formation of an oxonium transition salt generated by the reaction of an active centre with an exo- or endocyclic oxygen atom. The reaction mode depends on the kind of initiator and monomer [317]... 

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. 

Irradiation of a donor monomer-electron acceptor charge transfer complex, e.g. N-vinylcarbazole-sodium chloroaurate (2), -nitrobenzene (2) or -p-chloroanil (3) and P-propiolactone-uranyl nitrate (U), results in the initiation of cationic polymerization. 

In the cationic polymerization of p-propiolactone ( 3PL), initiated with dimethyl-iodonium hexafluoroantimonate (a source of CH cations), the initiator induces rapid and quantitative formation of the active centers (by transfer of the methyl cation). The structure of the end-groups has been determined, the terminal one by cation trapping with triphenyl phosphine. These end-groups indicate O-alkyl ringopening 16) (route a in Equ. 9.1.) ... 

In the cationic polymerization of P-trichloromethyl-P-propiolactone retention (lie) dominates 21). 

This polymer has also been obtained synthetically via cationic polymerization of D( + )(3-methyl-p-propiolactone 19). The racemic monomer polymerizes predominantly to the isotactic polymer. 

Epoxides readily undergo anionic copolymerization with lactones and cyclic anhydrides because the propagating centers are similar—alkoxide and carboxylate [Aida et al., 1985 Cherdron and Ohse, 1966 Inoue and Aida, 1989 Luston and Vass, 1984]. Most of the polymerizations show alternating behavior, with the formation of polyester, but the mechanism for alternation is unclear. There are few reports of cationic copolymerizations of lactones and cyclic ethers other than the copolymerizations of [5-propiolactone with tetrahydrofuran and... 

Lactones are easily polymerized by both cationic and anionic initiators, and in fact they (particularly /3-propiolactone) are known to polymerize explosively and unpredictably without addition of any initiators. Inhibitors, such as trialkylborates, have been used to prevent undesired polymerization (73GEP2255194). 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

Solvation by monomer itself was postulated by two of us in the polymerization of B-propiolactone. This is highly polar and powerfully solvating compound. It could be assumed that due to its dipolar feature 6-propiolacto-ne could solvate not only cation but also the growing anion. 

Assumptions concerning a possible zwitterionic mechanism were made by a number of authors for different cases of anionic and cationic polymerization68-73. This mechanism was well substantiated for the anionic polymerization of diethyl vinyl-malonate under the action of triphenylphosphine68, and 0-propiolactone under the action of trimethylamine69 and betaine70. ... 

Optically active 3-substituted 3-propiolactones were generally polymerized with cationic or coordination initiators such as AlEt, AlEt /H O,... 

Racemic and optically active a- and 3-substituted poly(3-propiolactones) can be synthesized by the current methods of pol5rmerization. However, the synthesis of a-substituted 3-propiolactones usually involves the ring closure of 3-substituted acids and they are often polymerized with anionic initiators. In contrast, the synthesis of 3-substituted 3-propiolactones is often made by the reaction of ketenes with carbonyl compounds (in the presence of a chiral base to obtain optical purity) and their polymerization proceeds with a cationic or coordinated initiator. 

In the anionic polymerization there are three monomers only that have been studied in more detail, namely ethylene oxide, propylene oxide (methyloxirane), and p-propiolactone (propano-3-lactone). In the anionic active species, like alcoho-late anions, the negative charge (in contrast to onium cations) is localized almost exclusively on one atom. Therefore, dissociation constants are much lower and the differences in reactivity of ions and ion pairs are much more pronounced (Table 9). 

In Lodz, p-propiolactone was initiated with crowned sodium acetate, and in Paris, with the same initiator but with the cryptated cation. Both systems behave nicely as living ones and were the first instances (published at the same time) of living polymerization of cyclic esters (lactones) where two kinds of active species could be detected. 

A recent example of the initiation method involves the living anionic polymerization of y-propiolactone initiated by potassium methacrylate in which the potassium cation is complexed with dibenzo-18-crown-6 (DB18C6) (equation 14). Because the growing chain end is a car-boxylate anion, no side reactions occur at the initiator s double bond. 

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