Pendent chain

Chemically Controlled. These systems are classified together because of the hydrolysis or enzymatic cleavage of a chemical bond that allows dehvery of the dmg. There are two main types of systems, ie, pendent chain systems and bioerodible systems. 

In the pendent chain systems, the dmg is chemically bound to a polymer backbone and is released by hydrolytic or enzymatic cleavage of the chemical bond. The dmg may be attached directiy to the polymer or may be linked via a spacer group. The spacer group may be used to affect the rate of dmg release and the hydrophilicity of the system. These systems allow very high dmg loadings (over 80 wt %) (89) which decrease the cost of the polymeric materials used ia the systems. These systems have beea examiaed by many iavestigators (111,112). 

In order to identify tyrosine derivatives that would lead to polymers that are processible, mechanically strong, and also biocompatible, we initiated a detailed investigation of the structure-property relationships in polyiminocarbonates and polycarbonates. Since the amino and carboxylic acid groups of tyrosine dipeptide (the N and C termini) provide convenient attachment points, selected pendent chains can be used to modify the overall properties of the polymers. This is an important structural feature of tyrosine dipcptide derived polymers. 

The corresponding polyiminocarbonates (Figure 5) were prepared first, using recently developed polymerization procedures (5). Poly(Dat-Tym iminocarbonate), the polymer carrying no pendent chains at all, was an insoluble material. Thermal processing techniques could not be used due to the low thermal stability of the polymer in the molten state. Thus poly(Dat-Tyr iminocarbonate) was a virtually non-processible material without practical applications. 

Figure 4. Three naturally occurring tyrosine derivatives (desaminotyrosine, tyrosine, and tyramine) were used to prepare four different monomeric diphenols that carry no pendent chains (Dat-Tym), only a benzyloxycarbonyl (Z) group, only a hexyl ester group, or both types of pendent chains (Z-Tyr-Tyr-Hex).

Solvent-cast films of tyrosine-derived polyiminocarbonates and polycarbonates were virtually indistinguishable in appearance and exhibited similar morphological features. When examined by X-ray diffraction, using an automated Scintag Pad V diffractometer all tested films were found to be completely amorphous. The lack of ordered domains in solvent-cast films seemed to be a general feature of tyrosine-derived polyiminocarbonates and polycarbonates, irrespective of the pendent chain configuration of the monomeric dipeptide. 

Whereas the tensile strength was not a sensitive function of the monomer structure, the tensile modulus (Young s Modulus) was clearly related to the monomer structure. This is expected since the tensile modulus is a measure of the polymer s resistance to deformation and is related to the "stiffness" of a polymeric material. The highest tensile modulus (22,000 kg/cm2,2.2 GPa) was measured for poly(BPA iminocarbonate). Replacement of BPA by Dat-Tyr-Hex reduced the tensile modulus significantly. This observation can possibly be attributed to the presence of the long hexyl ester pendent chain in Dat-Tyr-Hex. Generally, the polyiminocarbonates were somewhat "stiffer" than the corresponding polycarbonates. Thus, the tensile moduli of poly(Dat-Tyr-Hex iminocarbonate) and poly(Dat-Tyr-Hex carbonate) were 16,300 kg/cm2 (1.6 GPa) and 13,900 kg/cm2 (1.3 GPa) respectively. 

Parallel to the development of the new theoretical approaches considerable experimental work was done on model networks especially synthesized, to show the effects of pendent chains, loops, distribution of chain length, functionality of crosslinks, etc. on properties (5-21). In some instances, the properties turned out... 

Second, some organisms are able to incorporate longer pendent chains yielding another class of PHA medium chain length PHA, poly(HAMCL). Poly (HAmcl) is specifically accumulated by fluorescent pseudomonads. When aliphatic hydrocarbons like n-alkane, n-alkanoate, or n-alkanol serve as feedstocks for Pseudomonas oleovorans the resulting PHA is a random copolymer... 

Fig. 13 General chemical structure of the Brocchini-Kohn Library of polyarylates (R, Y = points of structural variability in the pendent chain and the backbone) [3]...

Effect of Uncoordinated Tertiary Nitrogens and Type of Pendent Chain... 

The redox properties of the hexaaza monocyclic complexes 1-7 are similar to those of the cyclam. This indicates that the electrochemical properties of the complexes are only slightly affected by the presence of uncoordinated tertiary nitrogen atoms, the length of the pendent chain, and the type of the functional group appended at the bridgehead nitrogen 16, 58). 

Particularly noteworthy are the tyrosine-derived polycarbonates (27), a family of polymers based on alkyl esters of desaminotyrosyl-tyrosine. The lead polymer in this family is poly[desaminotyrosyl-tyrosine ethyl ester (DTE) carbonate], a polymer derived from desaminotyrosyl-tyrosine ethyl ester. Other polymers in this series of tyrosine-derived polycarbonates are poly[desaminotyrosyl-tyrosine butyl ester (DTB) carbonate], poly[desaminotyrosyl-tyrosine hexyl ester (DTH) carbonate], and poly [desaminotyrosyl-tyrosine octyl ester (DTO) carbonate], where the letters B, H, and O indicate the presence of butyl, hexyl, or octyl ester pendent chains, respectively. 

About four dimethylsiloxane pendent chain units next to the grafting site were immobilised due to chain anchoring to the silica surface. 

Jefford and Wang prepared both (- )-524 and (- )-525 from L-aspartic acid (571) by the route shown in Scheme 75 (456,457). The pendent chains were introduced by displacing iodide from intermediate 572 with the appropriate dialkylcuprates, after which the sulfonamide and ester groups were hydrolyzed to give the (/ )-P-aminoacid hydrobromide salts 573. Standard pyrrole formation with... 

Tomlison R, Heller J, Duncan R, Brocchini S Pendent chain functionalised polyacetals that display pH-dependent degradation A platform for the development of novel polymer therapeutics, Macromolecules 2002, 35, 473 80. 

Karmalkar, R.N. Kulkarni, M.G. Mashelkar, R.A. Pendent chain linked delivery systems II. Facile hydrolysis through molecular imprinting effects. J. Control. Release 1997, 43, 235-243. 

Diffusion-controlled systems, as (a) Reservoir (membrane systems) or (b) Matrix (monolithic systems) (2) Chemically controlled systems that can either be (a) Bioerodible and biodegradable systems or (b) Pendent chain systems ... 

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