Dextran homopolymer

A similar approach has been also used for the synthesis of covalently linked DHBCs. Polysaccharide based DHBCs were prepared by end-to-end eoupling of two readily available biocompatible water-soluble homopolymers [35]. The synthesis was a two step reaction where a) a terminal aldehyde group of a dextran homopolymer was oxidized and b) a monoamine end functionalized PEG reaeted with the oxidized dextran, via a lactone aminolysis reaction. Interestingly, the obtained polymer could be chemically modified, in a controlled way, in order to produce neutral-cationic or neutral-anionic DHBCs. 

Dextran is a homopolymer of glucose with predominantly a-(l -> 6) linkages (50-97%) [12], Figure 1 shows a part of the dextran main chain with branching points in the 2, 3 and 4 positions. The degree and nature of branching units is dependent on the dextran-producing bacterial strain [6]. 

Homopolysaccharides are synthesised by relatively few spedfic enzymes and are not constructed from subunits. The commerdally important homopolymer dextran is synthesised extracellularly by the enzyme dextrar sucrase. In Leuconostoc mesenteroides die enzyme is induced by the substrate sucrose. This is cleaved to release free fructose and link die glucose to die redudng end of the acceptor dextran chain, which is bound to the enz)mae. The product from diis bacterium is composed almost exdusively of... 

There were also attempts to calibrate the SEC columns with help of broad molar mass dispersity poplymers but this is less lehable. The most common and well credible SEC cahbration standards are linear polystyrenes, PS, which are prepared by the anionic polymerizatioa As indicated in section 11.7, according to lUPAC, the molar mass values determined by means of SEC based on PS calibration standards are to be designated polystyrene equivalent molar masses . Other common SEC calibrants are poly(methyl methaciylate)s, which are important for eluents that do not dissolve polystyrenes, such as hexafluoroisopropanol, further poly(ethylene oxide)s, poly(vinyl acetate)s, polyolefins, dextrans, pullulans, some proteins and few others. The situation is much more complicated with complex polymers such as copolymers. For example, block copolymers often contain their parent homopolymers (see sections 11.8.3, 11.8.6 and 11.9). The latter are hardly detectable by SEC, which is often apphed for copolymer characterization by the suppliers (compare Figure 16). Therefore, it is hardly appropriate to consider them standards. Molecules of statistical copolymers of the same both molar mass and overall chemical composition may well differ in their blockiness and therefore their coils may assume distinct size in solution. In the case of complex polymers and complex polymer systems, the researchers often seek support in other characterization methods such as nuclear magnetic resonance, matrix assisted desorption ionization mass spectrometry and like. 

Due to the relative ease of control, temperature is one of the most widely used external stimuli for the synthesis of stimulus-responsive bmshes. In this case, thermoresponsive polymer bmshes from poly(N-isopropylacrylamide) (PNIPAM) are the most intensively studied responsive bmshes that display a lower critical solution temperature (LOST) in a suitable solvent. Below the critical point, the polymer chains interact preferentially with the solvent and adopt a swollen, extended conformation. Above the critical point, the polymer chains collapse as they become more solvophobic. Jayachandran et reported the synthesis of PNIPAM homopolymer and block copolymer brushes on the surface of latex particles by aqueous ATRP. Urey demonstrated that PNIPAM brushes were sensitive to temperature and salt concentration. Zhu et synthesized Au-NPs stabilized with thiol-terminated PNIPAM via the grafting to approach. These thermosensitive Au-NPs exhibit a sharp, reversible, dear opaque transition in solution between 25 and 30 °C. Shan et al. prepared PNIPAM-coated Au-NPs using both grafting to and graft from approaches. Lv et al. prepared dual-sensitive polymer by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from trithiocarbonate groups linked to dextran and sucdnoylation of dextran after polymerization. Such dextran-based dual-sensitive polymer is employed to endow Au-NPs with stability and pH and temperature sensitivity. 

Another classification of polymers is based on the number of different types of monomers and their distribution along the polymer chain. Thus, polymers are classified as homopolymers, alternating copolymers, random copolymers, block copolymers, and heteropolymers. They are schematically represented in Figure 12.2. Homopolymers are made up of one type of monomeric unit. Well-known examples of synthetic homopolymers are poly(styrene), poly(vinyl alcohol), poly(vinyl chloride), poly (ethylene), poly (ethylene oxide), and so on. Various natural polysaccharides, such as amylose, cellulose, dextran, chitin, and others, belong to this class as well. Copolymers contain two (or a few more) types of monomers that may be... 

The synthesis of exocellular polysaccharides by lactic acid bacteria is a very widespread character. L. mesenteroides and Streptococcus mutans produce glucose homopolymers such as dextran and glucan fructose homopolymers (levans) and heteropolymers are also synthesized. Dextran of L. mesenteroides is the best known, as much for its different structures and its biosynthesis as for its various applications. 

Dextran is a homopolymer of 1,6-linked a-D-glucopyranose monomers (Fig. 5-2 D). The individual polymer chains do not associate on the molecular level to give an ordered three-dimensional structure. It therefore lacks the structural rigidity required for a chromatographic support. To improve the mechanical strength the individual polymer chains are cross-linked, for example, using epichlorohydrin in the case of... 

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