Chain length control

A new approach of controlling NCA polymerization was reported by the Cheng group in 2007 [22]. hr a screen of amine initiators for the polymerization of Bn-Glu NCA, they found that hexamethyldisilazane (HMDS) showed remarkable control of polymerizations and led to formation of PBLG with excellent chain length control, with less than 22% deviation from the expected molecular weights, and narrow molecular weight distributions < 1.2) (Scheme 13). The NCA polymer-... 

One of the most widely studied systems for the polymerization of cyclic ethers is the tetra-phenylporphyrinato aluminum system, (TPP)AIX. Most investigations have focused on the chloride complex, (251), which initiates the living ROP of EO, PO, and Et-EO.936 For example, 400 equivalents of EO require 3 hours in CH2C12 at 25 °C to reach 80% conversion. Mn values increase linearly with monomer conversion, with polydispersities typically <1.10, and chain lengths controlled by the initial monomer initiator ratio. 

Upon completion of the synthesis of (+)-ll,ll -dideoxyverticillin A (1), it was recognized that there were only a handful of reports of monomeric epitri- or epitetrathiodiketopiperazine syntheses, nearly all of which were accomplished with lack of sulfide chain length control. In the limited cases where selectivity was achieved, the results were highly substrate dependent or the method lacked substrate scope [56],... 

Chain-Length Control in the Conversion of Syngas over Carbonyl Compounds Anchored into a Zeolite Matrix... 

It is now superflous to point out the renewed interest for the Fischer-Tropsch (F-T) synthesis (j) i. . the conversion of CO+H2 mixtures into a broad range of products including alkanes, alkenes, alcohols. Recent reviews (292.9k ) emphasized the central problem in F-T synthesis1 selectivity or more precisely chain-length control. 

Chain length control, polymer, 20 529-530 Chain-like organic semiconductors,... 

Adams, M. L. and G S. Kwon. 2003. Relative aggregation state and hemolytic activity of amphotericin B encapsulated by poly(ethylene oxide)-block-pdty(hexyl-r,-aspartamide)-acyl conjugate micelles effects of acyl chain length Control. Rel81 23-32. 

Abe I, Oguro S, Utsumi Y, Sano Y, Noguchi H (2005) Engineered biosynthesis of plant polyketides chain length control in an octaketide-producing plant Type III polyketide synthase. J Am Chem Soc 127 12709-12716... 

Morita H, Kondo S, Oguro S, Noguchi H, Sugio S, Abe I, Kohno T (2007) Structural insight into chain-length control and product specificity of pentaketide chromone synthase from Aloe arborescens. Chem Biol 14 359-369... 

A final crucial issue is the structure of the polyketide assembly lines. The aromatic PKSs apparently require association of their various subunits for proper functioning. In the absence of the appropriate PKS multidomain architecture, synthesis is generally inefficient and results in truncated metabolites that are subject to variable amounts of spontaneous chemistry [8,10,26], It remains unclear what minimum complement of activities is required to achieve the natural PKS products formed in vivo. A consensus is growing, however, that chain length control... 

Moore et al. 100 expanded the use of the complimentary protection-deprotection sequence, facilitated by the diethyltriazine and trimethylsilane groups, to construct phenylacetylene oligomers. Up to 16 arylalkyne units were successfully connected (overall yield 67 % 2251 amu). The method, demonstrating a high level of chain length control ,... 

Alkali metal and alkaline earth polyphosphates crystallize as short chains, two to six phosphate groups per chain or very long chains with hundreds to thousands of PO3 per chain. All polyphosphates in the alkali metal and alkaline earth systems are amorphous in the intermediate chain lengths. Control of the short chain length polyphosphates, both crystalline or amorphous, is a function of R, the M2O-P2O5 ratio. The control of the chain lengths of very long crystalline polyphosphates as Maddrell s salt, Kurrol s salt, and calcium phosphate fibers is not well understood. 

Engineered Biosynthesis of Plant Polyketides Chain Length Control in Novel Type III Polyketide Synthases... 

Tang, Y., Tsai, S.C. Khosla, C. Polyketide chain length control by chain length factor. 

On the contraiy, when the same cation exchanger was used, but after drying to constant weight, the siloxane chain break became more important [93]. The reaction time was significantly reduced, but the chain length control was not possible anymore. However, the glycosidic bonds in cellobiose moieties were not affected by the deprotection catalyst. 

Hsu, H.J., Sen, S., Pearson, R.M., Uddin, S., Krai, P., Hong, S. Poly(ethylene glycol) Corona Chain Length Controls End-Gronp-Dependent Cell Interactions of Dendron Micelles. Macromolecnles 47(19), 6911-6918 (2014). doi 10.1021/... 

Alkali metal alkoxides have also been examined in some detail. The addition of bulky lithium alkoxides to alkyllithium initiators retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised. LiCl has been used to similar effect, allowing the preparation of monodisperse PMMA (Af /Mn = 1.2) at -20 Sterically bulky lithium aluminum alkyls have been used to similar effect, with good chain length control retained even at ambient temperature. However, these approaches have not yet been successfully applied to stereoselective polymerizations. 

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