Synthetic polymers chemical heterogeneity

Synthetic polymers are produced by chain polymerization or step growth polymerization. Due to differences in the lifetime of activated species or the size and reactivity of the oligomers which are coupled in each reaction step, synthetic polymers are heterogeneous in molar mass. Copolymers are produced from more than one monomer species. In general, the different monomer species are differently incorporated in the polymer chain which causes distribution in chemical composition. Distributions in molar mass and chemical composition are also to be expected in polymers derived from homopolymers by incomplete chemical modifications, e.g. in partially hydrolyzed poly(vinyl acetate) [1]. 

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. 

Conversion of polymers and biomass to chemical intermediates and monomers by using subcritical and supercritical water as the reaction solvent is probable. Reactions of cellulose in supercritical water are rapid (< 50 ms) and proceed to 100% conversion with no char formation. This shows a remarkable increase in hydrolysis products and lower pyrolysis products when compared with reactions in subcritical water. There is a jump in the reaction rate of cellulose at the critical temperature of water. If the methods used for cellulose are applied to synthetic polymers, such as PET, nylon or others, high liquid yields can be achieved although the reactions require about 10 min for complete conversion. The reason is the heterogeneous nature of the reaction system (Arai, 1998). 

Chemical Stability. Chemical stability is just as important as the physical stability just discussed. In general, chemical deterioration of the polymers is no problem, and they can be stored at room temperature for years. However, the polymeric surfaces are subjected to an extreme variety of chemicals during the accumulation process. Some of these may react with the polymer. For example, reactions of styrene-divinylbenzene polymers and Tenax with the components of air and stack gases have been documented (336, 344, 540). The uptake of residual chlorine from water solutions has also been observed in my laboratory and elsewhere (110, 271, 287). Although the homogeneous nature of synthetic polymers should tend to reduce the number of these reactions relative to those that occur on heterogeneous surfaces of activated carbons, the chemical reaction possibility is real. In the development of methods for specific chemicals, the polymer stability should always be checked. On occasion, these checks may lead to... 

Synthetic polymers are highly complex multicomponent materials. They are composed of macromolecules varying in chain length, chemical composition, and architecture. By definition, complex polymers are heterogeneous in more... 

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. 

Synthetic peptide-based polymers are not new materials homopolymers of polypeptides have been available for many decades and have only seen hmited use as structural materials [5,6]. However, new methods in chemical synthesis have made possible the preparation of increasingly complex polypeptide sequences of controlled molecular weight that display properties far superior to ill-defined homopolypeptides [7]. Furthermore, hybrid copolymers, that combine polypeptide and conventional synthetic polymers, have been prepared and combine the functionality and structure of peptides with the processabihty and economy of polymers [8,9]. These polymers are well suited for applications where polymer assembly and functional domains need to be at length scales ranging from nanometers to microns. These block copolymers are homogeneous on a macroscopic scale, but dissimilarity between the block segments typically results in microphase heterogeneity yield-... 

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. 

It is important to compare the structural features of proteins with those of synthetic polymers or of polysaccharides in order to determine the specific properties of the derivative materials. Contrary to homopolymers or copolymers in which one or two monomers are repeated, proteins are heteropolymers consisting of amino acids. Proteins have a specific amino acid sequence and spatial conformation which determine their chemical reactivity and thus their potential for the formation of linkages that differ with respect to their position, nature and/or energy. This heterogenic structure provides many opportunities for potential crosslinking or chemical grafting—it even facilitates modification of the film-forming properties and end-product properties. 

The problem is further complicated by the fact that synthetic polymers exhibit chemical heterogeneity along the chain. Chemical heterogeneity reflects the distribution of the structural entities in the chain. The nature of the chemical heterogeneity influences the ultimate properties of the polymer as it produces heterogeneity in the molecular mobility of the chains. 

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