Chemical heterogeneity characterization, synthetic

Chemical heterogeneity in synthetic polymers offers a challenge to the analytical chemist to devise sensitive techniques for the characterization of these chemical distributions. It is well known that many synthetic copolymers consist of a collection of polymer chains that differ in their individual compositions. This distribution of repeat-unit composition from chain to chain can influence the physical properties of synthetic polymers significeuitly. Consequently, a thorough characterization of a copolymer sample would include a description of the average composition eUid its compositional distribution. 

Characterization of polymer mixtures is also of interest due to the wide use of polymer blend systems. Mixtures of homopolymers are relatively a simple form of chemical heterogeneity compared to copolymers. Even in this case, precise characterization is often non-trivial since many of polymer blend systems contain various additives in addition to polymer resins. In this section, recent progress on the characterization of synthetic polymers having chemical heterogeneity is reviewed. For the sake of convenience, the content is divided into mixtures, block copolymers, random copolymers, and functionality distribution. 

To obtain an unambiguous characterization of a particular material, it is often essential to fractionate a material (1-3). Synthetic polymers are rarely homogeneous chemical species, but have multivariate distributions in molecular weight, chemical composition, chain architecture, and functionality (4). For a precise characterization of a synthetic polymer, all the distributions need to be determined, which is a difficult, if not virtually impossible, task. Traditionally, fractionation has allowed separation of pol5miers on the basis of molecular mass or chemical composition (2). With proper techniques it is often possible to separate and characterize complex homo- and copolymer species on the basis of chemical heterogeneity and molar mass. 

Thanks to all the enhancements developed in the past decades, confocal Raman spectroscopy is now a technique of choice for the characterization of synthetics membranes [17-19] and chemical heterogeneous systems [20]. 

We have emphasized here that the dynamic aspects of NMR studies are crucially important for structurally or dynamically heterogeneous systems such as synthetic or natural hydrogels, protein fibrils and membrane proteins. This is in order to characterize their unique chemical, physical and biological properties in terms of a variety of fluctuation frequencies, including high (> 108 Hz) or intermediate (104-105 Hz) frequency fluctuations. It turns out that the presence of the high-frequency motions, which are readily evaluated by comparative CPMAS and DDMAS studies, is... 

The structural complexity of synthetic polymers can be described using the concept of molecular heterogeneity (see Fig. 1) meaning the different aspects of molar mass distribution (MMD), distribution in chemical composition (CCD), functionality type distribution (FTD) and molecular architecture distribution (MAD). They can be superimposed one on another, i.e. bifunctional molecules can be linear or branched, linear molecules can be mono- or bifunctional, copolymers can be block or graft copolymers, etc. In order to characterize complex polymers it is necessary to know the molar mass distribution within each type of heterogeneity. 

A well-defined monodisperse penta(L-alanine)- -butylamide H-[Ala]5-NHBu was synthesized by an activated ester method " and other natural abundant polypeptides, [Ala]n-5, [Leu]n-1 and [Leu]n-2, were synthesized by the N-carboxy a-amino-acid anhydride (NCA) method.Fully N-labelled homopolypeptides, [Ala ]n (99 at.% of N purity MASSTRACE, Inc.) and [Leu ]n (99 at.% of N purity MASSTRACE, Inc.), which show characteristic differences in conformation such as the a-helix and /3-sheet forms, were prepared by the heterogeneous polymerization of the corresponding NCAs in acetonitrile with -butylamine as an initiator. Conformational characterization of these samples was made on the basis of the conformation-dependent C and chemical shifts determined from the CP-MAS NMR method and from the characteristic bands in the IR and far-IR spectra. Figs. 38 and 39 show the 75.5 MHz C and 30.4 MHz N CP-MAS NMR spectra respectively of these fully N-labelled (99 at.% purity of N) homopolypeptides adopting the a-helical and /3-sheet forms (A) [Ala ]n-2 (a-helix), (B) [Ala ]n-1 (/3-sheet), (C) [Leu ]n-2 (a-helix), (D) [Leu ]n-1 (/3-sheet) in the solid state. Synthetic conditions and conformational characteristics of these samples are summarized... 

Heterogeneous stationary-phase surfaces, on the other hand, have several inherent problems associate with their synthesis and characterization. While the need for careful synthetic and characterization steps has been addressed in this article, ftirther research is necessary in this area, especially in the area of determining structure rather than stoichiometry of such phases. In addition, surface heterogeneity enormously complicates the process of studying the mechanistic properties imparted by such phases. However, both this complexity and variability have presented a powerful tool for separating large molecules that possess a simil chemical nature. 

Contrary to the usual organic compounds, polymers are far from being homogeneuos maferials (i.e., polymer chains do not possess the same molar mass and chemical structure). As matter of fact, many synthetic polymers are heterogeneous in several respects. Homopolymers may exhibit both molar-mass distribution (MMD) and end-groups (EG) distribution. Copolymers may also show chemical composition distribution (CCD) and functionality distribution (FTD) in addition to the MMD. Therefore, different kinds of heterogeneity need to be investigated in order to proceed to the structural and molecular characterization of polymeric materials. 

---