Polymer epimerization reactions

This paper illustrates its application to problems encountered in studies on polymer modification reactions, polymer epimerization reactions, p-cresol-formaldehyde condensation reactions, and monomer-promoted polyaddition reactions. A 23-statement CSMP program was written, for example, to duplicate the predictor-corrector program of Bauer for simulating irreversible polymer modification reactions. 

The Suter-Flory model was successfully used to interpret the results of the epimerization reaction carried out on propylene oligomers (204) and on polypropylene itself (106, 205). In both cases a slight prevalence of the r dyad over the m (52/48) is observed. The epimerized polypropylene has a microstmcmre almost coincident with a Bernoulli distribution and represents the polymer sample closest to an ideal atactic polymer so far obtained. 

The atactic polymer obtained with MAO can be rationalized by an intramolecular epimerization reaction of the growing polypropylene chain at the last inserted monomeric unit [50-57], which at low monomer concentration was found to be faster than the stereoregular insertion of propylene (Scheme 2a). 

Investigations of the bis(benzamidinate) dichloride or dialkyl complexes of Group 4 metals show that these complexes, obtained as a racemic mixture of c/s-octahedral compounds with C2 symmetry, are active catalysts for the polymerization of a-olefins when activated with MAO or perfluoroborane cocatalysts [29-41]. As was demonstrated above, polymerization of propylene with these complexes at atmospheric pressure results in the formation of an oily atactic product, instead of the expected isotactic polymer. The isotactic polypropylene (mmmm>95%, m.p.=153 °C) is formed when the polymerization is carried out at high concentration of olefin (in liquid propylene), which allows faster insertion of the monomer and almost completely suppresses the epimerization reaction. 

To corroborate that the epimerization reaction is responsible for the stereoerrors in the polypropylene chain, we treated 1-octene with complex 14 activated with MAO. The reaction resulted in the quantitative formation of frans-2-octene. In addition, the reaction of allylbenzene with the system 11/MAO at room temperature produced 100% conversion of the former to trans-methylstyrene. The isomerization results indicate that during the polymerization, the metal center in 14 is also able to migrate through the growing polymer chain, inducing branching and additional environments for the methyl ligands (Scheme 2) [124]. 

By adding only about ten additional statements to this program, a program for simulating reversible polymer modification and epimerization reactions was derived. Use of CSMP to generate plotted output and to evaluate kinetic parameters are also illustrated. 

A kinetic model has been proposed based on microstructural analysis, including both chain-epimerization and site-epimerization reactions in both C2- and C.-symmctric metallocenes, and rationalizing the observed pseudo-second-order kinetics of propylene polymerization promoted by C2-symmetric metallocene catalysts. This point has been extended to co-polymers.298 A thorough study of propylene polymerization with the Me2C(Cp)(9-Flu)ZrCl2 system in the presence of a large series of different counterions that rationalized the correlation between the nature of ion pair and the microstructure of the resulting PPs has been performed.104... 

Monte Carlo Simulations. The epimerization reaction was simulated using the Monte Carlo program we described earlier(17). A 5000 element array was allocated to store information about the configurations of monomer units at various positions in a 5000 unit polymer chain. The positions were indexed in such a way that the polymer could be considered cyclic. This was done to avoid end group effects. The configurations (R or S) at individual sites were indicated by 0 or 1 values. The polymer chain was made isotactic by giving all elements of the array initial values of 0. 

It is interesting that the reaction epimerizes 75% of the M residues, but not more. No solvent deuterium incorporation into M residues during the epimerization reaction could be detected, which indicates that the proton abstraction that is the necessary first chemical step in the reaction is irreversible. However, the enzyme does not epimerize all of the M residues in the substrate. The equilibrium constant for epimerizations is usually close to one for reactions that involve simple substrates, so one might expect the epimerase reaction to reach equilibrium when the M content of the alginate is equal to the G content. The fact that the reaction occurs on a polymeric substrate made up of chiral monomers complicates considerations of the energetics of the reaction somewhat. Nonetheless, it is difficult to reconcile the apparent irreversibility of proton abstraction with the 1 3 M G ratio in the product, unless one proposes that the conformation of the polymer becomes such that it can no longer bind to the enzyme. 

Following the epimerization step, three different O-sulfation reactions take place, starting with 2-0 sulfation of IdoA residues and followed by 6-0 sulfation and 3-0 sulfation of GlcN units. While several isoforms with different substrate specificities have been found both for the 6-0- and the 3-0-sulfotransferases (see the main text), only one mammalian 2-0-sulfotransferase has so far been identified (45). This enzyme species appears to catalyze also 2-O-sulfation of GlcA units, a less common reaction. The precise relation of this latter step to the other polymer-modification reactions is unclear. 

Epimerization at C-5 of hexuronic acids is a reaction that proceeds both on the polymer and on the sugar nucleotide level. In addition to the three pairs of parent acids, namely, the u-glucolL-ido-, D manno/L-gulo-, and D-ga-lacto/L-altro-he uron c acids, the 2-amino-2-deoxy acids belonging to the last two and the 2,3-diamino-2,3-dideoxy acids belonging to the middle pair have been found. 

It is important to note that the foregoing, biosynthetic-polymer modification is usually incomplete. In fact, only a fraction of the heparin precursor undergoes all of the transformations shown in Scheme 1. However, as the product of each enzymic reaction constitutes the specific substrate for the succeeding enzyme, the biosynthesis of heparin is not a random process. Thus, sulfation occurs preferentially in those regions of the chain where the amino sugar residues have been N-deacetylated and N-sulfated, and where D-glucuronic has been epimerized to L-iduronic acid.20... 

A polymer-bound Burgess reagent has also been developed. Aside from the mUd, neutral cyclization conditions, this reagent also offers the advantage of a clean reaction with little epimerization and an easy work-up. Examples are listed in Table 8.9. 

When sugars are treated with aqueous ammonia for a short time at low temperature in the absence of a catalyst, the reaction is arrested before heterocyclic compounds can be formed in appreciable proportion, and the products are mainly epimerization products of the sugars, probably formed by way of their 2,3-enediols. These epimerization products are summarized in Table I which shows the reactions of D-glucose, D-fructose, lactose, maltose, and melibiose with aqueous ammonia for a short time at low temperature. A dark-colored, high polymer is also formed in some instances (the browning reaction). In the ammoniacal solution, the monosaccharides are epimerized the disaccharides are epimerized and, in addition, may be hydrolyzed to monosaccharides that can also be epimerized hence, the variety of products obtained may be considerable. 

P up to 90 (by vapor-phase osmometry). The structures of the polymers, as indicated by their optical rotation, were strongly influenced by changes in the reaction conditions (especially by the temperature and polarity of the solvent). Lower temperatures (—78°) gave rise to higher yields and greater Pn- No epimerization occurred during polymerization, other than that at the anomeric carbon atom. 

Loss of stereospecificity, however, has also been reported in the addition of amines. The use of homogeneous Pd° catalysts in the addition of dimethylamine to a cyclohexenyl acetate led to substantial stereochemical scrambling (equation 186). Employment of polymer-bound Pd° catalysts, however, gave complete stereospecificity via ligand addition.398 The epimerization noted in this reaction is apparently due to acetate attack at the metal center, which is prohibited by steric congestion of the metal in the polymer matrix (equation 187).398... 

Abstract The synthesis and X-ray structure of various octahedral zirconium complexes and their catalytic properties in the polymerization of a-olefins are described. Benzamidinate, amido, allylic, and phosphinoamide moieties comprise the study ligations. For the benzamidinate complexes, a comparison study between homogeneous and heterogeneous complexes is presented. For the phosphinoamide complex, we show that the dynamic symmetry change of the complex from C2 to C2v allows the formation of elastomeric polymers. By controlling the reaction conditions of the polymerization process, highly stereoregular, elastomeric, or atactic polypropylenes can be produced. The formation of the elastomeric polymers was found to be the result of the epimerization of the last inserted monomer to the polymer chain. 

The polymerization of propylene using complex 14 activated by MAO (Al Zr ratio=500, solvent toluene, 25 °C) yielded 80 g polymer-mol Zrl-hrl with a molecular weight Mw= 115,000 and polydispersity=2.4 [119]. The reaction was carried out in liquid propylene to avoid, as much as possible, the epimerization of the last inserted monomer unit and to allow rational design of the elastomeric polymer. The formation of elastomeric polypropylene is consistent with the proposed equilibrium between ds-octahedral cationic complexes with C2 symmetry inducing the formation of the isotactic domain, and tetrahedral complexes with C2v symmetry responsible for the formation of the atactic domain (Scheme 7). The narrow polydispersity of the polypropylene obtained supports the polymerization mechanism in which the single-site catalyst is responsible for the formation of the elastomeric polymer. 

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