Silyl dextran

Table 16 shows the influence of reaction conditions and of activation reagents such as TMSC1, hexamethylphosphoric triamide (HMPA), M-methyl-imidazole (NMI), saccharin and tetrabutylammonium fluoride (TBAF) on the DS of silyl dextran. For complete silylation, a mixture of 1.8 mol HMDS and 0.2 mol chlorotrimethylsilane (TMSC1) per mol OH groups have to be used. The more reactive TMSC1 reacts in a first step with hydroxyl groups to trimethylsilyl dextran and HC1. Subsequently, the HC1 cleaves HMDS in TMSC1 and NH3 [218]. Complete silylation of dextran could also be carried... 

The degree of silylation can be determined by H NMR in CDCI3 or in DMSO-de- hi the case of slightly silylated dextran, the analysis of the ll NMR spectra in D2O and DMSO-de give similar results. A precise assignment of the NMR signals was achieved by 2D H - H - and H - 13C techniques (COSY 45° and HMQC, Fig. 61). 

Fig. 61 -NMR spectra of silylated dextrans in DMSO-d6 with different DSsiiyi (adapted from [217])...

Fig. 63 Controlled synthesis of poly(e-caprolactone) grafted onto dextran via partially silylated dextran...

The synthesis of these block copolymers is based on the reductive amination of dextran using a bromoisobutyrate containing primary amine, followed by silylation of the oligosaccharide hydroxyls before polymerization of styrene by ATRP. ° Three diblock copolymers were thus prepared with a DP of the PS block ranging from 5 to 775, tlat is from 7% to 92% w/w, using the same dextran-based precursor (Mn = 6600 g/mol, PDI -1.5 corresponding to a DP = 40). Finally, these (silylated dextran)-Z)-PS block copolymers were readily... 

Table 1. Molecular characteristics of silylated dextran-b-PS copolymers.

Poly(lactide-g-dextran) copolymers were synthesized by a three-step process partial silylation of the dextran hydroxyl groups, ROP of D,L-lactide initiated by the remaining hydroxyl groups of dextran, followed by silyl ether deprotection under mild conditions. The emulsifying properties of these glycopolymers depend on the PLA/dextran ratio [104]. PLA-g-dextran and PLA-g-silylated dextran adopt a core-shell conformation in various solvents [105]. Studies on encapsulation and release behavior of bovine serum albumin from PLA-g-dextran revealed a higher loading than in PLLA microspheres [106]. 

Silylation reactions on polysaccharides with chlorosilanes and silazanes were attempted more than 50 years ago resulting in hydrophobic silyl ethers with both increased thermal stability and solubility in organic solvents [376]. The silylation reaction for the protection of hydroxyl groups in mono- and polysaccharides exhibits many advantages, e.g. fast silylation, solubility of silylether in organic solvents suitable for subsequent derivatisation, stability of the resulting silylether under basic conditions but easy deprotection of the silyl moieties by acid hydrolysis or nucleophilic agents like fluoride and cyanide ions [377]. The partial and complete silylation of dextran was studied in detail by Ydens and Nouvel [215-217]. 

The partial silylation was carried out with 1,1,1,3,3,3,-hexamethyldisilazane (HMDS) in DMSO at 50 °C (Fig. 60). The hydroxyl groups react with HMDS resulting in trimethylsilyl dextran and (CH3)3SiNH2, which further reacts with hydroxyl groups to silyl ether and NH3. The silylation reaction starts ho-... 

Synthesis of bromoisobutyramide-ended dextran 1, silylated bromoisobutyramide-ended dextran 2. These compounds were synthesized as described in reference 10. 

As a polysaccharide, the structure of HA is somewhat similar to that of the previously-mentioned cellulose, chitin, and dextran yet the methods reported in the literature were not successfiil in sUylating HA. Thus, the James group at Colorado State University developed a new process for the silylation of HA [80] to improve its solubiUty in organic solvents and its compatibility with UHMWPE. This is the key step to solvent introduction of HA into the UHMWPE porous preforms. 

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