Sequence distribution, microstructural analysis

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

With the development of polymer structural characterizations using spectroscopy, there has been a considerable effort directed to measurements of tacticity, sequence distributions and number average sequence lengths (59 65). Two methods have been traditionally used for microstructure analysis from polymer solutions. Vibrational spectroscopy (infrared) and Nuclear Magnetic Resonance (NMR). Neither of these techniques is absolute. The assignment of absorption bands requires the use of model compounds or standards of known structure. 

Since the discovery of olefin polymerization using the Ziegler-Natta eatalyst, polyolefin has become one of the most important polymers produeed industrially. In particular, polyethylene, polypropylene and ethylene-propylene copolymers have been widely used as commercial products. High resolution solution NMR has become the most powerful analytieal method used to investigate the microstructures of these polymers. It is well known that the tacticity and comonomer sequence distribution are important factors for determining the mechanical properties of these copolymers. Furthermore, information on polymer microstructures from the analysis of solution NMR has added to an understanding of the mechanism of polymerization. 

The factors determining the stereospecificity in the polymerization of a-olefins have not been singled out with certainty. We tackled this problem from three different points of view, i.e. analysis of the sequence distribution in ethylene-propylene copolymers, microstructural analysis of partially stereoregular propylene polymers, microstructural analysis of ethylene-propylene copolymers. 

Laser Raman spectroscopy has been proposed as a useful technique for probing the microstructure of copolymers. Good correlations were found between the concentrations of some isolated, dyad, triad and tetrad comonomer sequences in vinyl chloride/vinylidene chloride copolymers and certain scattering intensities [99]. The positions and intensities of particular absorption bands have also been correlated with chain microstructure in an infrared study of ethylene/vinyl chloride copolymers, previously characterised by C-NMR analysis [100]. More recently, FTIR spectra have been analysed for monad, dyad and triad monomer sequence-distribution dependencies in random styrene/acrylonitrile copolymers [101]. Changes in peak intensities from normalised spectra were correlated with microstructure probabilities assignments were given if there existed a linear relationship between peak intensity and the number fraction of a microstructure. 

Kinetic studies of the solution (benzene) and bulk polymerization of methyl methacrylate with MA have been run at 60°C and 70°C, using AIBN initiator. The microstructure of the purified copolymers was determined by H-NMR and IR spectroscopy. Analysis of the comonomer pair sequence distribution for the solution-prepared copolymers supported a copolymerization mechanism involving participation of an association species between the two monomers. A terminal model or the classical Mayo-Lewis concept more adequately explained the results of bulk copolymerization, where the comonomer sequence distribution was more random. Theoretically, the concentration of associating species should be greatest in bulk, which was... 

IR spectra have been shown to be very useful for examining the compositional microstructure of copolymers [19-22]. However, whilst the measurement of the properties of the monomer constituents in a copolymer using characteristic IR absorption bands is comparatively easy (and has been used widely for various copolymer systems) by comparison with the NMR technique, the analysis of the sequence distribution of monomer units which comprise a copolymer is often difficult and complex [18]. Therefore, there have only been a small number of reports of the use of IR spectroscopy for sequence microstructure analysis for a limited number of copolymer systems [23-25]. 

High-resolution nuclear magnetic resonance spectroscopy, especially 13C NMR, is a powerful tool for analysis of copolymer microstructure [Bailey and Henrichs, 1978 Bovey, 1972 Cheng, 1995, 1997a Randall, 1977, 1989 Randall and Ruff, 1988], The predicted sequence length distributions have been verihed in a number of comonomer systems. Copolymer microstructure also gives an alternate method for evaluation of monomer reactivity ratios [Randall, 1977]. The method follows that described in Sec. 8-16 for stereochemical microstructure. For example, for the terminal model, the mathematical equations from Sec. 8-16a-2 apply except that Pmm, Pmr, Pm and Prr are replaced by p, pi2, p2j, and p22. 

The first method (i.e., measurement of 1,2 vinyl/c/s 1,4 and trans 1,4) necessitates two independent measurements ( H/ C-NMR) the second method for cis l,4ltrans 1,4 ( C-NMR) is severely hampered by questionable assignments [19,20] and different compositionally induced diads for the methylene carbons and traids for the methane carbons [18]. T1 and nuclear Overhauser effects (NOE) effects are not necessarily equal for these different signals. With the above mentioned assignments, Elgert and co-workers [21] outlined a simple method for measuring the isomeric distribution along the polybutadiene chains. Sequence analysis can, therefore, be used to quantify the microstructure of polybutadiene. 

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