Oxiranes. ring-opening polymerization

The oxirane ring-opening reaction requires the presence of a basic catalyst. An acidic catalyst also works, but the polymerization of the oxirane limits its usehilness. In the case of 2-mercaptoethanol (eq. 8), the product has been found to be autocatalytic, ie, the product is a catalyst for the reaction. 

The use of an unsaturated anionic initiator—such as potassium p-vinyl benzoxide—is possible for the ring opening polymerization of oxirane [43]. Although initiation is generally heterogenous, the polymers exhibit the molecular weight expected and a low polydispersity. In this case, the styrene type unsaturation at chain end cannot get involved in the process, as the propagating sites are oxanions. 

Five-coordinate aluminum alkyls are useful as oxirane-polymerization catalysts. Controlled polymerization of lactones102 and lactides103 has been achieved with Schiff base aluminum alkyl complexes. Ketiminate-based five-coordinate aluminum alkyl (OCMeCHCMeNAr)AlEt2 were found to be active catalyst for the ring-opening polymerization of -caprolactone.88 Salen aluminum alkyls have also been found to be active catalysts for the preparation of ethylene carbonate from sc C02 and ethylene oxide.1 4 Their catalytic activity is markedly enhanced in the presence of a Lewis base or a quaternary salt. 

Some cationic ring-opening polymerizations take place without termination and are reversible. Oxirane and oxetane polymerizations are seldom reversible, but polymerizations of larger-sized rings such as tetrahydrofuran are often reversible. The description of reversible ROP is presented below [Afshar-Taromi et al., 1978 Beste and Hall, 1964 Kobayashi et al., 1974 Szwarc, 1979]. It is also applicable to other reversible polymerizations such as those of alkene and carbonyl monomers. The propagation-depropagation equilibrium can be expressed by... 

Most polymerizations of cyclic monomers are ionic processes. Coordination catalysts are effective only for some heterocycles (oxirane and its derivatives, lactones). Ziegler-Natta catalysts can only be used for cycloalkene polymerization by metathesis heterocycles act as a catalytic poison. Smooth radical polymerization of hydrocarbon monomers with ring strain is unsuccessful [304], The deep-rooted faith that ring strain represents a major contribution to the driving force in ring opening (polymerization) has to be revised [305, 306]. 

Polymerization of oxirane (and of its derivatives) by the mechanism of activated monomer is so far exclusively cationic and can be represented by schemes (27) and (28) of Chap. 4. In contrast to the ring-opening polymerization of lactams, both the classical and the activated monomer mechanisms are operating in this case. Conditions can be found where one or the other mechanism predominates [339]. 

Photoinitiated epoxy crosslinking is generally based (with one exception on cationic ring-opening polymerization of the oxirane group, yielding polyether structures. 

Ring-opening polymerization of methylenecyclopropane in the presence of 4-type precatalyst proceeds to yield cxo-methylene products in low yield [87] 4-type precatalysts do not polymerize oxiranes. 

The ring-opening polymerizations of heterocyclic compounds are important, not least because of the number of commercial polymers produced in this way. The best-known examples include Nylon 6, which is produced from e-caprolactam as shown in Scheme 16 poly(ethylene oxide) produced by ring-opening polymerization of ethylene oxide (or oxirane), a route to which is described in Protocol 14 (Scheme 17), and poly(dimethylsiloxane) which is formed from a cyclic tetramer produced on hydrolysis of dimethyl-silyldichloride in a way similar to that described in Protocol 15. 

In recent studies of styrene oxide polymerization reactions we found the phenyl substituent to have a significant Influence on the course of the polymerization process, too. In our particular case, however, the influence is due not only to steric factors, but also to the inductive effects of the phenyl ring, which Influences directly the course of the oxirane ring-opening reaction. 

According to these experimental results, the proposed reaction mechanism for the formation of poly(styrene oxide) with a regular chain structure by anionic polymerization involves the oxirane ring opening exclusively at the 3 position. However, two kinds of active centers, A and B in the reactions above, occur in the initiation step. The active center A, formed by a-ring opening, adds to a monomer molecule in the next step, but in the second step the oxirane ring is opened at the 3 position. 

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) ... 

Soeda, Y., Okamoto, T., Toshima, K., and Matsumura, S. (2002) Enzymatic ring-opening polymerization of oxiranes and dicarboxylic anhydrides. Macromol. Biosci., 2 (9), 429-436. 

J.4.2.3 Heterocyclic Monomers A variety of heterocyclic monomers can be polymerized by anionic ring-opening polymerizations. The types of anionically polymerizable heterocyclic monomers include oxiranes (epoxides), thiacyclopropanes, thiacyclobutanes, lactones, lactides, lactams, anhydrides, carbonates, and silicones... 

The ring-opening polymerization of oxiranes, thiiranes, and thietanes can be initiated by both cationic and anionic methods, but thrae are some heterocycUc compounds such as lactones and lactams that are more suited to the anionie technique. 

Aluminum porphyrins initiate controlled ring-opening polymerizations of oxiranes [67-69] ]3-lactones [70-72], 5-valerolactone [74], -caprolactone [74] and D-lactide [75], as well as controlled addition polymerizations of methacrylates [76] and methacrylonitrile [77] (Eq. 15). As shown in Eq. (16), propagation occurs by a coordinative anionic mechanism... 

Describe the mechanism and write the chemical reactions of ring-opening polymerizations of oxiranes with potassium hydroxide. In polymerization of propylene oxide with KOH, what type of tacticity polymer forms Explain. 

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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