Oxirane reactions cationic polymerization

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]. 

The evolution of nitrogen on photolysis of the aryIdiazonium salts appears to have limited the use of these systems to thin film applications such as container coatings and photoresists (23). Other efficient photoinitiators that do not produce highly volatile products have been disclosed (24-27). These systems are based on the photolysis of diaryliodonium and triarylsulfonium salts. Structures I and II, respectively. These salts are highly thermally stable salts that upon irradiation liberate strong Bronsted acids of the HX type (Reactions 43 and 44) that subsequently initiate cationic polymerization of the oxirane rings ... 

Epoxide monomers readily undergo cationic polymerization whereby chain propagation is based on the attack of a carbocation on the negatively polarized oxygen of the oxirane (XL). Chain initiation is either by another cation or by a strong electrophile, such as a Lewis acid (BF3) or a protonic Brpnsted acid. The process is illustrated in the reaction mechanism (Scheme 6.40). ... 

Cationic polymerizations of oxiranes are much less isospecific and regiospecific than are anionic polymerizations. In anionic and coordinated anionic polymerizations, only chiral epoxides, like propylene oxide, yield stereoregular polymers. Both pure enantiomers yield isotactic polymers when the reaction proceeds in a regiospecific manner with the bond cleavage taking place at the primary carbon. 

Penczek and Kubisa developed a new cationic polymerization technique for cyclic monomers in which the chain propagation involves the reaction of a protonated (activated) monomer molecule with a nucleophilic site in the neutral growing macromolecule. This so-called activated monomer (AM) polymerization is depicted mechanistically in Scheme 58. According to this mechanism, when the polymerization of an oxirane (a cyclic ether) is carried out in the presence of... 

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]. 

The polymerization of oxiranes, a reaction of importance for both industry and commerce, has been abundantly described in the literature. Several hundred articles are published in this field annually. The quantity and great variety of themes discussed mean that a survey of this immense literature material exceeds the scope of the present review. Accordingly, we shall merely mention some of the works attempting to clarify the situation regarding the mechanism of polymerization reactions. ° We shall also outline the fundamental types of oxirane polymerizations. These can be classified into three groups with anionic, cationic, " and coordination mechanisms. 

Most cationic ring-opening polymerizations of cyclic ethers involve the formation and propagation of oxonium ion centers. Reaction involves the nucleophilic attack of monomer on the oxonium ion, e.g., for 1,2-epoxides (oxiranes) ... 

Oxiranes (a-oxides, a-epoxides) are known to polymerize by cationic and anionic mechanisms as well as by ionic-coordinative reactions 1 3). Their ability of ring opening by one of these processes depends mainly on the structure and number of substituents. Some examples are given below ... 

Cationic ring-opening polymerization of oxiranes can also be carried out photochemically (photochemical reactions are discussed in Chap. 10). Yagci and coworkers reported polymerizations of cyclohexene oxide with the aid of highly conjugated thiophene derivatives [12]. The reaction is illustrated as follows ... 

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