Methyloxirane, polymerization

For the initiation of caprolactam polymerization, Grignard reagents [173] have been used as a source of lactamate salt. Tsuchiya and Tsuruta have described an interesting case of methyloxirane polymerization initiated with... 

It is widely accepted that the formation of isotactic polypropylene is brought about by the chiral structure, d or 1, of the titanium species in the catalyst system. The concept of the steric control in propylene polymerization is the same as that in the methyloxirane polymerizations which have been discussed in the foregoing sections. An active center having d - or 1 -chirality is formed along the crystal surface of 6 -titanium trichloride. 

Propylene oxide [75-56-9] (methyloxirane, 1,2-epoxypropane) is a significant organic chemical used primarily as a reaction intermediate for production of polyether polyols, propylene glycol, alkanolamines (qv), glycol ethers, and many other useful products (see Glycols). Propylene oxide was first prepared in 1861 by Oser and first polymerized by Levene and Walti in 1927 (1). Propylene oxide is manufactured by two basic processes the traditional chlorohydrin process (see Chlorohydrins) and the hydroperoxide process, where either / fZ-butanol (see Butyl alcohols) or styrene (qv) is a co-product. Research continues in an effort to develop a direct oxidation process to be used commercially. 

Polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific. For example, the polymerization of a chiral monomer, e.g., R)-propylene oxide ((i )-methyloxirane), with retention of configuration is not considered to be a stereospecific reaction however, selective polymerization, with retention, of one of the enantiomers present in a mixture of R)- and (S)-propylene oxide molecules is so classified. 

This intermediate reacts with another molecule of BF3OEt2 forming a neutral molecule and Et03 +BF. Komratov et al. [158] discovered macro-zwitterions, apparently analogous to the type shown above, on the polymerization of THF through BF3 in the presence of methyloxirane. Depending on the chain length, temperature and polarity of the medium [157], both ends will either be free or will form an ion pair the latter may be of the contact or solvent-separated type. At 293 K, most active centres exist in the form of cyclic zwitterions [158]. 

Sokolskii et al. [233] noticed that ZN coordination centres spontaneously change to cationic centres during polymerization. In our laboratory we have attempted to obtain quantitative data on a similar reaction by means of models [232], Tsuchyia and Tsuruta [234] pointed out the possibility of choice between anionic and cationic polymerization of methyloxirane with diethylzinc —HzO (see Chap. 3, Sect. 2.1). 

The ratio of back- and end-biting reactions depends on the reaction conditions, and may differ considerably in polymerizations of various monomers. In cationic polymerization, methyloxirane produces mainly a mixture of cyclic tetramers [335] and chloromethyloxirane yields both dimers and tetramers [336]. Under similar conditions, 1,3-dioxolane or 1,3-dioxepane yield a number of cyclic derivatives [337], the distribution of cyclics being in agreement with the Jacobson-Stockmayer theory [338],... 

This review article is concerned with chemical behavior of organo-lithium, -aluminum and -zinc compounds in initiation reactions of diolefins, polar vinyls and oxirane compounds. Discussions are given with respect to the following five topics 1) lithium alkylamide as initiator for polymerizations of isoprene and 1,4-divinylbenzene 2) initiation of N-carboxy-a-aminoacid anhydride(NCA) by a primary amino group 3) activated aluminum alkyl and zinc alkyl 4) initiation of stereospecific polymerization of methyloxirane and 5) comparison of stereospecific polymerization of methyloxirane with Ziegler-Natta polymerization. A comprehensive interpretation is proposed for chemistry of reactivity and/or stereospecificity of organometallic compounds in ionic polymerizations. 

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). 

When racemic methyloxirane is polymerized with zinc dimethoxide, D-and L-monomers are separately incorporated into growing chains to form an isotactic polymer consisting of poly(D-methyl-oxirane) and poly(L-methyloxirane). This stereoselective polymerization can be satisfactorily explained in terms of the enantio-morphic catalyst sites model (1 ). The d -sites accept D-methyl-oxirane in preference to the L-monomer, resulting in the formation of -DDDD- isotactic sequences. The same situation is valid for the l -catalyst sites. 

Comparison of Stereospecific Polymerization of Methyloxirane with Ziegler-Natta Polymerization... 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

The pol5uner chain contains the less reactive (nucleophilic) hydroxyl end group. The driving force for this reaction comes from the protonated monomer, and no cyclic oligomers are formed. Similar results have been obtained from the polymerization of methyloxirane (290) and chloromethyloxirane in the presence of ethylene glycol (291). Photo- and azo-functional telechelics were also prepared by the so-called activated monomer poljnnerization (292,293). 

Kageyama, H. Miki, K. Tanaka, N. Kasai, N. Ishimori, M. Heki, T. Tsuruta, T. Molecular structure of [Zn(0CH2CH20Me)2(EtZn0CH2CH20Me)g]. An enantiomorphic catalyst for the stereoselective polymerization of methyloxirane. Mo romoZ. Chem., Rapid Commun. 1982, 3, 947-951. 

Methyloxirane behaves absolutely different way from t-butyl-oxirane, no change at all in the optical purity of the monomer phase being observed in the course of l ,S-copolymerization under similar reaction conditions (Fig. 3). It was also confirmed that the main chain of poly(methyloxirane) formed possesses randomly distributed R- and 5-monomeric units with the same R/S ratio as that in the monomer phase regardless of the conversion of polymerization. These results indicate that no stereoselection takes place in the 5,5-copolymerization of methyloxirane with KOR initiator... 

Recently Ishimori and al ( showed that Zn(0Me)2.(EtZn0Me)g have a centrosymmetric structure formed of two enantiomorphic disturbed cubes. This complex had no tiCactivity at room temperature, but polymerized methyloxirane at 80 . A process of dissociation at 80 could explain such a reactivity. 

Enantiomorphic sites concept The stereospecific polymerization of racemic methyloxirane produces generally a polymer with some cristallinity which can be fractionated by selective solubility (using acetone for example) in a crystalline fraction and an amorphous one (an intermediate "semi-crystalline fraction can be also isolated). 

Table 1. Different products obtained in the polymerization of methyloxirane using various bimetallic y-oxoalkoxides as initiators.

Impressive progress has been made in stereospecific polymerization of cyclic ethers and cyclic esters since the first edition of CPS was published. A novel ROP phenomenon called stereocomplexation, which is especially important when it involves homochiral macromolecules, poly(R)- and poly(S)- polylactides, is analyzed in detail (chapter devoted to cyclic esters). A breakthrough in the stereoregulation of methyloxirane (propylene oxide) polymerization is the subject of a chapter on stereospecific polymerization of oxiranes. 

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). 

Polymerization of (5)-2-methyloxirane catalyzed by a Lewis acid in ether yields an optically inactive polymer. 

Both anionic and cationic polymerizations can be used to synthesize telechelics from oxiranes. Anionic polymerization of oxirane yields a,co-dihydroxy(PEO). Anionic polymerization of methyloxirane occurs predominantly by attack at the least sterically hindered carbon, yielding a... 

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