2,3-Epoxybutane polymerization

If, during ring opening polymerization a tertiary carbon is involved in the reaction, a configurational inversion may occur in which case a change in the stereochemistry of the monomer unit with respect to the monomer will be observed. Thus, the cf5-2,3-epoxybutane gives rise to a polymer with threo monomer units, 39, whereas the trans isomer is transformed into erythro or meso units, 40 (Scheme 8) (81, 82). 

This phenomenon of chirality degradation is carried to the extreme in the polymerization of (-F)-rrinactive polymer One of the two equivalent asymmetric atoms inverts its configuration during polymerization giving rise to a monomer unit with eiythro or meso strac-ture. The isotactic polymer, 40, so formed is clearly achiral (280). 

The opposite case is also worthy of consideration. cis-2,3>Epoxybutane is a meso compound but the two halves of the molecule, and particularly the two O—CH(CH3) bonds, are not equivalent but enantiotopic. Ring opening polymerization occurring selectively on one of the bonds converts the R, S) monomer into a succession of monomer units (R, / )—(/ , R)— and so on, or —(5, S)—(S, S)— and so on. A chiral initiator can effect an enantiotopic differentiation (281) and thus give rise to an optically active polymer with an excess of (R, R) or (S, S) units (81, 82). 

The polymerization of optically pure propylene oxide by FeCl3 -derived initiators yields an optically active polymer. E. J. Vandenberg [J. Polym. Sci., A-l(7), 525 (1969)] attempted to gain insight into the mechanism by polymerizing the 2,3-epoxybutanes. The optically active trans-2,3-epoxybutane was polymerized to an optically inactive crystalline polymer. Evaluate this result and discuss its implications on the propagation mechanism for propylene oxide polymerization. 

Some mechanism aspects of epoxide polymerization. Stereochemical structure of the crystalline polyethers from the 2,3-epoxybutanes. J. Polymer Sci.B2,1085 (1964). 

Note Moderately polar solvent, ethereal odor soluble in water and most organic solvents flammable moderately toxic incompatible with strong oxidizers can form potentially explosive peroxides upon long standing in air see the relevant tables in the chapter on laboratory safety commercially, it is often stabilized against peroxidation with 0.5 to 1.0% (mass/mass) p-cresol, 05 to 1.0% (mass/mass) hydroquinone, or 0.01% (mass/mass) 4,4 -thiobis(6-ferf-butyl-m-cresol) can polymerize in the presence of cationic initiators such as Lewis acids or strong proton acids. Synonyms THF, tet-ramethylene oxide, diethylene oxide, 1,4-epoxybutane, oxolane, oxacyclopentane. 

The clear cut case of enantiomorphic selection has been reported by Vander-berg ° ) in polymerization of trans-2,3-epoxybutanes ... 

In contrast to the 1,3-epoxides where a whole variety of substituted oxetanes have been polymerized, there is only one 1,4-epoxide of major importance. This monomer is commonly called tetrahydrofuran, but it has the systematic name 1,4-epoxybutane or tetramethylene oxide. The polymer derived from it is as often called polytetramethylene oxide as poly tetrahydrofuran. We have chosen to use the names tetrahydrofuran (THF) and polytetrahydrofuran (PTHF). An alternate abbreviation for THF that is found in some recent literature is H4furan . 

Finally, the stereospecificity of the phosphorane-promoted cyclodehydration is adequately demonstrated in the reaction of d,Z-2,3-butanediol (9) with DTPP in CD2CI2 (35 C, 10 h). The stepwise nature of the cyclodehydration process gives exclusively d5-2,3-epoxybutane (10 >99%) by NMR analysis [6 12.9 (CH3) and 52.4 (CHO)] (76). This latter result is consonant with the previous findings of Denney etal. 24) where a mixture containing 88% d,/- and 12% zne5o-4,5-dimethyl-2,2,2-triethoxy-1,3,2X5-dioxaphospholanes (11) gave a mixture of 85% cis- and 15% /rans-2,3-butene oxides (10), respectively, during tiiermolysis (117°C, 42 h) (equations 2 and 3). Polymeric Dioxaphospholanes. 

When this polymerization reaction was studied for monomers such as 2,3-epoxypropanal (above) or 2,3-epoxybutanal, however, an unusual Initiation step was found to take place by the Tishchenko-Claisen reaction, which is a well-known disproportionation reaction of aldehydes to esters, and an intermediate product, the dlepoxyester was identified. This intermediate product on polymerization formed, up to about 20% conversion, linear polyethers with both oxlrane and ester side groups by an epoxy ring-opening reaction, as follows ... 

EPOXYBUTANE (106-88-7) Forms explosive mixture with air (flash point -7°F/ -22°C). Unless inhibited, violent polymerization can be caused by elevated temperatures, sunlight, acids, aluminum chlorides, bases, iron, tin, potassium, sodium, sodium hydroxide, or certain salts. Reacts violently with oxidizers, alcohols. Reacts with hydroxides, metal chlorides, oxides. Flow or agitation of substance may generate electrostatic charges due to low conductivity. Storage tanks and other equipment should be absolutely dry and free from air, acetylene, ammonia, hydrogen sulfide, rust, and other contaminants. 

Kim [213] studied the effect of the coimter ion on the HKR of epiehlorhydrine, 1,2-epoxybutane, 1,2-epoxyhexane and epoxystyrene. The salen polymer catalyst were synthesized by eopolymerization of salen bearing chloromethyl groups with sodium phenoxide derivatives of hydroquinone, 1,3,5-trihydroxybenzene or l,l,l-tris(p-hydroxyphenyl)ethane in presence of A-methylpyrrolidine and NaH in THF (Sehemes 143 and 144). Co(lI) type polymeric chiral salen ligands were formed by reaction of the corresponding salen ligand with hydrous Co(ll) acetate. To obtained Co(IIl) polymeric chiral... 

The ring-opening copolymerization of propylene oxide and carbon monoxide forms the polyester poly(p-hydroxybutyrate) (PHA, Equation 17.52). The physical and mechanical properties of some PHAs are similar to those of isotactic polypropylene. This polymerization was first reported by Furukawa and co-workers in 1965, and more recent studies have been reported by Osakada, Rieger, and Alper. These polymerizations have been conducted with Co2(CO)j and additives. The combination of Co2(CO)j, a 1,10-phenan-throline derivative, and benzyl bromide afforded polyester with an value of 19.4 kg/ mol and M /M of 1.41. In addition to propylene oxide, 1,2-epoxybutane was successfully copolymerized with CO to yield the corresponding poly(3-hydrox5 entanoate) with an value of 16.7 kg/mol and a M /M of 1.28. The role of benzyl bromide is unclear. Related copolymerizations of aziridines and CO to form polyamides have also been reported. Polymer values as high as 27.5 kg/mol have been reported, and tjqjical M /M values varied from 1.11 to 1.64. 

SCHEME 24.4 Stereoselectivity of polymerization of cis- and fran -2,3-epoxybutane using AIR3/H2O catalysts (R = alkyl). 

In work investigating the mechanism of this system, Vandenberg used AIR3/H2O catalysts to polymerize cis- and tra i-2,3-epoxybutane. From the properties of the resultant polymers and the examination of the diol decomposition products, mechanistic information was obtained. These results are summarized in Scheme... 

Although EO polymerization can be achieved quite rapidly (reaction half-time is about 30 min at room temperature for [EO]/[(TPP)AlCl] =400 in dichloromethane), other epoxides show a lower reactivity. In similar conditions, it requires several hours for POx and 1,2-epoxybutane (BOx), ° whereas for styrene oxide or l,2-epoxy-2-methylpropane conversions do not exceed 15% after 8 days of reaction (Table 7). ° ... 

Indeed the polymerization of 2,3-epoxybutane with aluminum porphyrin revealed a coordinative process with inversion of configuration, which counters the above nondissociative reaction. Aida and Inoue proposed a mechanism involving two porphyrin molecules, one activating the monomer via coordination on the opposite face of the X group while the other proceeds as nucleophilic attack of the activated monomer (see Scheme 30). ° Initiation results from the insertion of the monomer into the Al-X bond leading to an aluminum alkox-ide which becomes the propagating species. Consequently, the synthesized polyether chains possess an X and a hydroxyl group at their extremities. 

Vandenberg, following a particularly penetrating line of research, used the polymerization of cis- and trans-2,3-epoxybutane to distinguish the polymerization mechanisms of oxirane coordination polymerization and then to generalize this basic mechanism to monosubstituted oxiranes (95). The mechanism... 

The polymerization of 2,3-epoxybutane with the same initiator as used with propylene oxide shows that the oxygen/substituted-carbon atom bond can be cleaved and, hence, a mechanism can be logically proposed to account for the head-to-head, tail-to-tail structures identified by Price and Vandenberg. With 2,3-epoxybutane, it was found that amorphous polymer could also be as pure disyndiotactic as the crystalline forms. Amorphous polymer could arise from short sequences of stereoregularity that were too short to form crystallizable segments. These could arise when coordination of monomer temporarily displaced alkoxide, interrupting chain growth, which, when resumed, could be selective for the antipode monomer. 

Some of the complexities of coordination copolymerization of alkylene oxides are highlighted by the work of Vandenberg with 2,3-epoxybutanes (88). Using the Vandenberg Catalyst, it was shown that one could selectively polymerize cis-2,3-epoxybutane from an equal mixture of cis- and trans-isomers and obtain fairly pure cis-polymer. The cis-oxide enters the crystalline polymer fraction about 20 times faster than does the trans-oxide. In the amorphous fraction of the polymer produced, the enrichment of cis-oxide was about tenfold over that of the trans-isomer. 

At low temperatures, about -78°C, in triisobutylaluminum/ water-initiated polymerization, which is presumed to be a cationic initiation, an amorphous (elastomeric) polymer is obtained from cis-2,3-epoxybutane, and a crystalline polymer, melting point 100°C, is obtained from the trans-isomer. In a copolymerization of the two isomers, the cis-oxide enters the copolymer at about twice the rate of the trans-isomer. Further, the low-temperature, cationic poly(trans-2,3-epoxybutane) with a crystalline melting point of 100°C was found to consist of diad units with a mesodiisotactic structure, while the crystalline polymer formed by coordinate polymerization of the cis-monomer, melting point 162°C, had diad units that were racemic diisotactic. These results make apparent the importance of the monomer coordination step in polymer chain growth in coordinate polymerizations. 

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