Steric Control in Polymerizations of Oxiranes

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.

In all polymerizations of oxiranes by cationic, anionic, and coordinated anionic mechanisms, the ring-opening is generally accompanied by an inversion of the configuration at the carbon where the cleavage takes place. A linear transition state mechanism involving dissociated nucleophilic species has been proposed [15]. Yet, there are some known instances of ring-opening reactions of epoxies that are stereochemically retentive. For instance, ring opening of 2,3-epoxybutane with AlCl₃ results in formation of 3-chloro-2-butanol, where the cis and trans epoxides are converted to the erythro and threo-chlorohydrins. Inoue and coworkers [19] found, however, that polymerizations of cis and trans 2,3-epoxybutanes take place with inversion of configuration when aluminum 5,10,15,20- tetraphenylphosphonium and zinc 5,10,15,20-tetraphenyl-21-methylporphine catalysts are used. To explain the inversion, Inoue and coworkers proposed a linear transition state mechanism that involves a simultaneous participation of two porphyrin molecules [19]. One porphyrin molecule accommodates a coordinative activation of the epoxide and the other one serves as a nucleophile to attack the coordinated epoxide from the back side.

Potassium hydroxide or alkoxide polymerizes racemic propylene oxide with better than 95% regioselectivity of cleavage at the bond between oxygen and the carbon substituted by two hydrogens. The product, however, is atactic. Both (R) and (S) propylene oxides react at the same rate. This shows that the initiator is unable to distinguish between the two enantiomers of propylene oxide. When t-butyl ethylene oxide is polymerized by KOH it yields a crystalline product. This product is different in its melting point, X-ray diffraction pattern, and solution-NMR spectra from the typical isotactic polymers. It contains alternating isotactic and syndiotactic sequences [31]. It was suggested [34] that this may be a result of the configuration of the incoming monomer being opposite to that of the penultimate unit. Chelation of the paired cation (K) with the last and the next to the last oxygen is visualized. Geometry of such a chelate is dictated by the requirement that the penultimate t-butyl group be in an equatorial conformation. This makes it reasonable to postulate that the necessary preference for the incoming monomer is to be opposite to that of the penultimate unit [31]:

When phenyl glycidyl ethers are polymerized under the same conditions, the steric arrangement is all isotactic rather than isotactic-syndiotactic [31]. Price explained that on the basis of the oxygen in C₆H₆-O-CH₂ seeking to coordinate potassium ions in the transition state [31]. In the case of t-butylethylene oxide, on the other hand, the tertiary butyl group tends to be as far as possible away from the potassium ion [34]. This is supported by the observation that p-methoxy and p-methyl groups on phenyl glycidyl ether increase the crystalline portion of the polymer, while the p-chloro substituent decreases it [31]. Most stereoselective coordination catalysts polymerize propylene oxide to yield polymers that contain high ratios of isotactic to syndiotactic sequences. Large portions of amorphous materials, however, are also present in the same materials. These amorphous portions contain head to head units that are imperfections in the structures [29, 30]. For every head to head placement, one (R) monomer is converted to an (S) unit in the polymer [23]. This shows that at the coordination sites abnormal ring openings occur at the secondary carbon with an inversion of the configuration and results in head to head placements [23, 31]. Also, erythro and threo isomers units are present. The isotactic portion consists almost exclusively of the erythro isomer while other amorphous fraction contains 40-45% erythro and 55-60% threo [31] All the above information is indirect evidence that a typical catalyst, such as (C₂H)₂Zn-H2O contains isotactic and amorphous sites. The isotactic sites are very selective and coordinate either with (R) or with (S) monomers. The amorphous sites, on the other hand, coordinate equally well with both (R) and (S) monomers. In addition, there is little preference for attack on either the primary or the secondary carbons during the ring-opening reactions [23]. According to a Tsuruta mechanism [36] the first step in propylene oxide polymerization, with catalysts like zinc alcoholates, is the coordination of the ether oxygen onto a zinc atom. The second step is a nucleophilic attack at the oxirane ring by the alkoxy ion. Almost all the bond cleavage takes place at the CH₂-O bond. This results in retention of the steric configuration of the carbon atom at the C-H group. The next oxirane molecule repeats the process, coordinates with the same zinc atom and then undergoes the ring-opening reaction to form a dimer. Repetition of this process many times yields a high molecular weight polymer [36]:

The catalyst can also be ZnR₂-CH₃-OH. Special catalyst complexes, like [Zn (OCH₃)₂-(C₂H₅OCH₃)₆], form through carefully control of reaction conditions by adding 16 moles of methyl alcohol to 14 moles of diethylzinc in heptane under an argon atmosphere. X-ray analysis shows that two different structures [36]. One of them is a centrosym- metric complex of two enantiomorphic distorted cubes that share a corner Zn atom. The two would be equivalent if they were not distorted. Another structure, also centrosymmetric, consists of two enantiomorphic distorted structures that resemble "chairs without legs," where the surfaces share a common seat. Both types of complexes are active initiators for polymerization of propylene oxide. Each has two enantiomorphic sites for polymerization. Based on that knowledge, NMR spectra and GPC curves, Tsuruta suggested the following mechanism of a monomer coordinating with the catalyst [36] (see Fig. 5.2). The bonds at the central zinc atom are loosened and coordination takes place with methyl-oxirane molecule at the central atom. Cleavage at the O-CH₂ bond of the oxirane takes place by a concerted mechanism. If the bond loosening takes place at the d cube and the nucleophilic attack takes place at one of the methoxy groups on that cube then chirality around the central zinc will favor L monomer over the D monomer. This is the origin of the I* catalyst site. If the bond loosening takes place in the I cube the catalyst site will have d* chirality. Because the probability of bond loosening in the d cube is exactly the same as in I cube, an equal number of I* and d* sites should be expected to form. These two cubes become a source of d* and I* chiral nature [35].

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مواضيع ذات صلة

Polymerization by Coordination Mechanism

Cationic Polymerization

Kinetics of Ring-Opening Polymerization

Chemistry of Ring-Opening Polymerizations

Configurational Statistics and the Propagation Mechanism in Chain-Growth Polymerization

Group Transfer Polymerization

Copolymerizations by Ionic Mechanism

Polymerization of Ketones and Isocyanates

Polymerizations of Di Aldehydes

Polymerization of Unsaturated Aldehydes

Anionic Polymerization of Aldehydes

Polymerization of Aldehydes