Chain polymerization microstructural analysis

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

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... 

Dimethacrylate monomers were polymerized by free radical chain reactions to yield crosslinked networks which have dental applications. These networks may resemble ones formed by stepwise polymerization reactions, in having a microstructure in which crosslinked particles are embedded in a much more lightly crosslinked matrix. Consistently, polydimethacrylates were found to have very low values of Tg by reference to changes in modulus of elasticity determined by dynamic mechanical analysis. 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

It has been shown, however, that in the copolymerization much more reactive 1,3-dioxolane polymerizes first and is practically completely consumed at still low conversion of 1,3,5-trioxane [139]. Thus, by polymerization alone, it would not be possible to achieve the random distribution of 1,3-dioxolane units along the chain. Both analysis of the microstructure of the chain [140], as well as thermal behavior of the copolymer [141], indicate, however, that nearly random distribution of comonomer units is indeed attained by scrambling process similar to that described earlier for sequential polymerization of 1,3-dioxolane and 1,3-dioxepane. 

Novel data on the composition of active centers of Ziegler-Natta catalysts and on the mechanism of propagation and chain transfer reactions are reviewed. These data are derived from the following trends in the study of the mechanism of catalytic polymerization a) determination of the number of active centers (mainly with the use of radioactive CO as a tag) b) analysis of the microstructure of polymers with the use of C-NMR c) analysis of specific features of highly active supported catalysts d) quantum-chemical calculation of the electronic structure of active centers and their reactions. 

Microstructure of copolymers typically refers to the proportion and the arrangement of the monomer units in the polymeric backbone. In their structure the copolymers may contain the monomeric units arranged randomly, they may alternate regularly, may form large blocks of one type of monomer, or may appear as side chain blocks connected to a polymer main chain (see Section 1.1). This distribution also depends on the relative amounts of each monomer present in the copolymer [7]. Analytical pyrolysis, particularly Py-GC-MS, has been used successfully for the analysis of microstructure of copolymers (see e.g. [8]). Pyrolysis generates small fragments that represent sections of the polymer and can make distinctions between random and block copolymers fairly straightfonward. 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

Metallocenes are far more versatile in controlling polymer stereochemistry compared to Ziegler-Natta catalysts, as extensively demonstrated in the case of PP. Also in 1-butene polymerization, all kinds of chain microstructures can be obtained with different metallocenes. The 13C NMR pentad analysis of polybutene is somewhat less immediate than that of PP, and has been reported for both ZN 886,887 and metallocenes.180 The 13C NMR spectrum of atactic polybutene, with pentad assignments of the C(3) methylene region, is shown in Figure 37. 

Analysis of end-group or abnormal linkage in polymer chains often gives us important information on the mechanism of polymerization (see Section 3). Although the low concentrations of these groups has often made them difficult to detect, the superconducting NMR spectrometer permits greater sensitivity and thus more detailed analysis of microstructures in polymers. 

Prior to the mid-1980 s, catalysts formed using achiral CpaMCb precursors were found to produce only atactic polypropylene (which, incidentally cannot be obtained in the pure form directly from heterogeneous catalysts). In 1984, Ewen reported the use of metallocene-based catalysts for the isospecific polymerization of propylene.38 The polymerization of propylene at -45°C using a Cp2TiPh2 (I,Fig.4) / MAO catalyst system produced a partially isotactic polymer with an mmmm pentad content of 52% (versus 6.25% for a purely atactic polymer). NMR analysis of the polymer revealed the stereochemical errors mmmr and mmrm in the ratio of 1 1, which is indicative of a stereoblock microstructure (Fig.5). Such a structure is consistent with a chain-end control mechanism,39 where the stereocenter of the last inserted monomer unit provides... 

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