Polymerization of oxirane

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. 

This explanation turned out to be incorrect for this system, although it might apply to some future one. Indeed, recent studies of Teyssie8 b) revealed that in the polymerization of oxiranes initiated in non-polar solvents by Al—Zn oxyalkoxides the slower methyl oxirane is more strongly coordinated with the catalytic center than the faster epichlorhydrine. It is possible that the behaviour postulated by Korotkov might be observed in such systems. 

Organotin compounds with the general formulae RSnX3, R2SnX2, R3SnX and RSnOOH (R = alkyl or aryl and X = Cl, Br or J) are efficient catalysts for the polymerization of oxiranes 73). 

Polymerization of oxiranes with succinic anhydride proceeded in the presence of PPL at 60 °C or 80 °C [63]. The ring-opening of the oxirane might proceed by a carboxylic acid catalyst, which is formed by the reaction of succinic anhydride with serine residue of the lipase catalyst. 

Scheme 26 Photoinduced cationic polymerization of oxiranes using 1-pyrenyltriarylbismutho-nium salts [97]...

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]. 

Polymerization is frequently observed as a side reaction in acid-catalyzed reactions, and under appropriate conditions many oxetanes can be quantitatively transformed into high molecular weight linear polyethers with useful properties. Polymerization is very general for oxetanes and is closely related to Lewis acid-catalyzed polymerization of oxiranes, THF and oxepane, but oxetanes generally polymerize much more rapidly than THF and oxepane and at a rate similar to oxiranes (72MI51300). 

According to Entelis the formation of cyclic oligomers in the polymerization of oxiranes initiated by borontrifluoride would be an intramolecular reaction between the chain ends of the zwitter ion produced in such polymerizations ... 

Polyethylene oxide)macromonomers have thus been synthesized by two-step processes. Poly(oxyethylene)mo nomethyl ethers with widely varying molecular weights are commercially available. They are obtained by anionic polymerization of oxirane initiated by monofunctional alkoxides261 such as potassium 2-methoxyethanol. 

Attempts to synthesize the same type of compounds by anionic polymerization of oxiranes in the presence of transfer agents snch as HEMA failed because anionic polymerization requires temperatures at which thermally initiated polymerization of the methacrylic double bond occurs... 

So far, not many systems are known to meet the conditions of the growth mechanism with activated monomer. Polymerizations of oxirane, chlo-romethyloxirane [145, 146] and of lactams [147] belong to this class. The existence of an electroneutral propagating macromolecule and of an activated monomer results in reduced probability of back-biting reactions and in easier preparation of macromers [145] by means of exchange reactions. 

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]. 

The formation of aggregates with low to zero polymerization activity is quite general in ionic polymerizations. This statement can be further documented by the observation of centre aggregation during anionic polymerization of oxirane [100-103]... 

Thus, cationic polymerization of oxiranes is of little synthetic value, if the preparation of linear polymers is attempted. The high tendency for cyclization may be employed, however, for preparation of macrocyclic polyethers (crown ethers). Polymerization of ethylene oxide in the presence of suitable cations (e.g., Na+, K+, Rb +, Cs + ) leads to crown ethers of a given ring size in relatively high yields, due to the template effect [105], Thus, with Rb+ or Cs+ cations, cyclic fraction contained exclusively 18-crown-6. 

In this sequence of reactions, it is the monomer that forms oxonium ion [thus Activated Monomer (AM) mechanism] and the growing chain end is neutral. As shown in the series of papers [107-115], if the conditions are created, when AM mechanism predominates, by keeping the low instantaneous ratio of [monomer]/[HO-] (slow addition of monomer to reaction mixture), back-biting is effectively eliminated. Linear polymers, free of cyclic fraction are obtained under these conditions. The mechanism and kinetics of AM polymerization of oxiranes is discussed in detail in recent monograph [6]. 

Although there are some limitations on the molecular weights of the linear polymers which may be obtained by this method, AM polymerization offers an attractive, synthetic route for preparation of functional, medium molecular weight polymers by cationic polymerization of oxiranes. As the process involves the extension of the chain of hydroxyl group containing compound used to initiate the polymerization (initiator), the method is especially well suited for preparation of oligodiols (low molecular weight diols as initiators) and macromonomers (for example, hydroxy-ethyl acrylate as initiator) ... 

Preparation of epichlorohydrin oligodiols [114] and epichlorohydrin or propylene oxide macromonomers [115] has been described. AM polymerization of oxiranes containing nitromethyl [116] or N-vinyl carbazoyl substituents [117] has also been studied. Photoinitiated AM polymerization of 2,3-epoxypropylphenylether has been reported [118],... 

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]. 

In another series of papers, published by Yamashita a.o. and describing macrocyclization in the cationic polymerization of oxiranes, much more complicated mixture of cyclic products was observed. Besides typical macrocyclic polyethers ° cyclic oligomers having acetal structure were found. These products can be fom d, according to Yam hita, as a result of back-biting to the rearranged growing species ... 

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. 

Polymerization of oxiranes with succinic anhydride proceeded in the presence of PPL catalyst.207 Under appropriate conditions, Mw reached 1 x 104. During the polymerization, the enzymatically formed acid group from the anhydride may open the oxirane ring to give a glycol, which is then reacted with the anhydride or acid by lipase catalysis, yielding polyesters. 

It is widely recognized that Et Zn-H O system is one of the most active catalysts for the stereospecific polymerization of oxiranes. A variety of chemical species are formed in the following way rapid formation of ethylzinc hydroxide, its aggregation, and elimination of ethane to form zinc oxide structure. The maximum catalyst activity was achieved when the mole ratio of zinc to water was one to one, where the predominant formation of a species, Et(ZnO) H, (III), was observed. If we use less amount of water, another species, Et(ZnO) ZnEt, (IV), was also produced concurrently. Contrary to the anionic nature of the former species (III), the latter species (IV) exhibited a cationic nature. For instance, more than 957o of ring cleavage of methyloxirane takes place at O-CH bond with species (III), while the cleavage at 0-CH bond also takes place concurrently with species (IV). 

Preparation The simplest method to obtain a stable polynuclear"system, containing the metal atoms experimentally recognized to give active systems for the high M.W. polymerization of oxiranes, is a thermal condensation process between metal alkoxldes and carboxylates,... 

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