Epimerization of isotactic

A general method has been developed for the estimation of model parameters from experimental observations when the model relating the parameters and input variables to the output responses is a Monte Carlo simulation. The method provides point estimates as well as joint probability regions of the parameters. In comparison to methods based on analytical models, this approach can prove to be more flexible and gives the investigator a more quantitative insight into the effects of parameter values on the model. The parameter estimation technique has been applied to three examples in polymer science, all of which concern sequence distributions in polymer chains. The first is the estimation of binary reactivity ratios for the terminal or Mayo-Lewis copolymerization model from both composition and sequence distribution data. Next a procedure for discriminating between the penultimate and the terminal copolymerization models on the basis of sequence distribution data is described. Finally, the estimation of a parameter required to model the epimerization of isotactic polystyrene is discussed. 

Epimerization of Isotactic Polystyrene.This application considers modelling the epimerization of isotactic polystyrene by Monte... 

Dizinc-monoethoxide complex 58 (Fig. 11) reported by Hillmyer and coworkers [81] displayed rapid polymerization of rac-lactide in CH2CI2 at room temperature with a [LA]o/[l]o ratio of 300 and [LA]q = 1 M greater than 90% conversion to PLA occurred within 30 min. PLA derived from L-LA was isotactic, lactide, signifying an absence of epimerization of stereogenic centers during the polymerization. In a similar study, zinc alkyl complexes (59a, b) and the alkoxo bridged... 

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. 

When methyl iodide is replaced by 2-vinylpyridine in these reactions we have in effect the propagation step of polymerization or oligomerization, and indeed reaction proceeds in the same selective fashion. In the case of lithium counterion >95 % of isotactic trimer is formed by the next addition. Epimerization of the selectivity methylated product using K+OBu " in dimethylsulphoxide at 25 °C for 2 weeks yields a final mixture of approximately 1 2 1 of isotactic, heterotactic, and syndiotactic trimers. Thus the trimerization process must be kinetically controlled. 

Smmmmmm can be measured from the chemical shift of isotactic polystyrene or from that of the largest signal in polymers epimerized to low extents and is 146.40 ppm for the conditions employed in the present study. Accordingly, the chemical shift of any heptad can be calculated by the following general formulas and specific examples. 

Busico et al., on the other hand, came to a conclusion [299] that the stereoregularity of polypropylene produced with C2-symmetric group 4 ansa-metallocene catalysts is a result of the interplay of two competing reactions. These are isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end. That makes this class of homogenous catalysts different from the typical Ziegler-Natta catalyst, because with these catalysts enantioselectivity and stereoselectivity are not necessarily coincidental [96]. 

The stereochemical mechanism responsible for the isoselectivity of Ci-symmetric metallocene catalysts has been a topic of considerable debate. There are two limiting mechanisms possible for the formation of isotactic polypropylene with such Ci-symmetric catalysts having diastereotopic coordination sites. These are the site epimerization mechanism and the alternating mechanism, as shown in Scheme 2.3. 

FIGURE2.12 A statistical analysis of isotactic polypropylene from i-l/MAO cannot conclusively differentiate between the site epimerization model and the alternating model. (Reprinted with permission from Miller, S. A. Bercaw, J. E. OrganometalUcs 2006, 25, 3576-3592. Copyright 2006 American Chemical Society.)... 

A stereochemical analysis has predicted the distribution of isotactic stereoblocks present in isotactic-hemiisotactic polypropylene. For an ideal case (without site epimerization and with perfect enantiofacial selectivity at the more stereoselective site), the derived equations depend only on... 

Site epimerization is estimated to be fast relative to olefin uptake and insertion. Thus, repetitive enchainment of one olefin enantiomer will occur, producing an isotactic block of poly(a-olefin) until a syndiotactic enchainment error occurs. When this happens, olefin insertion will be switched to the opposite side, and another block of isotactic poly(a-olefin) will be produced. Thus, the resulting polymer is predominantly isotactic with isolated r dyads stereoblocks of the two olefin enantiomers surround each isolated r dyad (Figure 4.23). 

SCHEME 8.6 Proposed mechanism of isotactic/atactic stereoblock propylene polymerization with bis(benzamidinate) Zr(IV) complexes. Chain propagation at high monomer concentration would be site-controlled isotactic-selective, whereas at low monomer concentration it would become nonstereoselective owing to intramolecular growing chain epimerization. (Reprinted with permission from Volkis, V. Nelken-baum, E. Lisovskii, A. Hasson, G Semiat, R. Kapon, M. Botonshansky, M. Eishen, Y. Eisen, M. S. J. Am. Chem. Soc. 2003,125, 2179-2194. Copyright 2003 American Chemical Society.)... 

The tyclic architecture was supported by a combination of techniques, including NMR, MALDI-TOF MS, and viscometry of the cyclic polymers in comparison with the linear homologs of the same molar mass (prepared in the presence of alcohol initiators). In addition, thermogravimetric analysis (TGA) suggested that cyclic PLAs were more thermally stable than the respective linear polymers. When IMes-mediated ROP was applied to optically pure l-LA, a crystalline isotactic cyclic PLA was formed, indicating that the polymerization occurred without epimerization of the chiral center. 

Lactide (LA), the cyclic diester of lactic acid, has two stereogenic centers and hence exists as three stereoisomers L-lactide (S,S), D-lactide (R,R), and meso-lactide (R,S). In addition, rac-lactide, a commercially available racemic mixture of the (R,R) and (S,S) forms, is also frequently studied. PLA may exhibit several stereoregular architectures (in addition to the non-stereoregular atactic form), namely isotactic, syndiotactic, and heterotactic (Scheme 15). The purely isotactic form may be readily prepared from the ROP of L-LA (or D-LA), assuming that epimerization does not occur during ring opening. The physical properties, and hence medical uses, of the different stereoisomers of PLA and their copolymers vary widely and the reader is directed to several recent reviews for more information.736 740-743... 

The dependence the polymer microstructure on the propylene concentration in the reactor is shown in Table XVI. By varying the monomer concentration in operation with the supported catalyst, one can obtain polypropylenes with relatively low concentration of rrrr pentads. At low propylene concentrations, it is possible that m insertions are formed by epimerization. The yield of these isolated m diads is significantly increased when the catalyst is supported. Similar effects were shown by catalysts producing isotactic polymer. 

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. 

Three types of elastomeric polypropylene are known (1) a large molecular weight atactic polymer [125], (2) polymers with alternating isotactic and atactic blocks [108,109,126-129], and (3) polymers formed by the dual-site mechanism [110]. The structure of the obtained elastomeric polypropylene was elucidated by comparison of the 13C-NMR spectra with those of an atactic oily polymer and isotactic polypropylene prepared by the zirconium complex 4. On the basis of the NMR data, for all the samples the statistical lengths of the isotactic blocks between two neighboring epimerization stereodefects were calculated [41]. It was found that for the isotactic polymer 90%), the... 

---