ABSTRACT:This paper describes the first synthesis of a new class of topological macromolecules which we refer to as "starburst polymers." The fundamental building blocks to this new polymer class are referred to as "dendrimers." These dendrimers differ from classical monomers/ oligomers by their extraordinary symmetry, high branching and maximized (telechelic) terminal functionality density. The dendrimers possess "reactive end groups" which allow (a) controlled moelcular weight building (monodispersity), (b) controlled branching (topology), and (c) versatility in design and modification of the terminal end groups. Dendrimer synthesis is accomplished by a variety of strategies involving "time sequenced propagation" techniques. The resulting dendrimers grow in a geometrically progressive fashion as shown: Chemically bridging these dendrimers leads to the new class of macromolecules-"starburst polymers" (e.g.,

A vibrational analysis has been carried out to analyze the chain conformation of poly(lactic acid) in the crystalline state. In conjunction with a normal-coordinate analysis, the Raman spectrum has yielded data regarding conformational distributions. Raman spectra of different helices were simulated using previously published structures, force constants, and intensity parameters. Some of the chain conformations predicted are inconsistent with the data. Only one of the four 3 1 helical structures predicted by conformational analysis agrees well with experiment. This analysis provides a new understanding regarding the relative probability of a 103 or 31 helix for poly(lactic acid).

Currently, the most studied example of living freeradical polymerization is nitroxyl-mediated styrene polymerization. This chemistry relies upon the extremely facile and selective ability of nitroxyl radicals to trap carbon-centered radicals and the apparent reversible nature of the C-0 bond. Solomon and Rizzardol first reported this chemistry in the mid-1980s but used it to prepare mainly low molecular weight materials. Georges et aL2 reported, in 1993, the use of this chemistry to prepare narrow-polydispersity ( < 1.5) polystyrene. Over the past 2 years several research groups have been studying nitroxyl-mediated styrene polymerization. The work of Georges et al. has focused mainly upon increasing the rate of nitroxyl-mediated styrene polymerization initiated using benzoyl peroxide (BPO). They report that the nitroxyl-mediated styrene polymerization rate is significantly increased if conducted in an acidic e n~i r o n m e n t .~ The mechanism of this effect is not yet understood. Matyjaszewski et aL4 have focused on the theoretical aspects of nitroxylmediated styrene polymerization in the absence of an added initiator. Recently, Hawkel.5 synthesized I (model of the nitroxyl-mediated styrene polymerization chain end; Scheme 1) and demonstrated its use as an initiator. No effort was made to utilize I as a chain-end model to help determine the mechanism of the polymerization. Also, no acid was added to nitroxyl-mediated styrene polymerization initiated using I.The published data of these researchers consistently show the lack of truly narrow polydispersity, and the polydispersity obtained is generally proportional to the molecular weight being produced.Georges et a l . hypothesized that the lack of ability to make truly monodisperse PS using nitroxyl-mediated styrene polymerization was caused by continuous initiation of new PS chains during the p~lymerization.~ To suppress the spontaneous polymerization mechanism, camphorsulfonic acid (CSA) was added. They observed that, rather than decreasing the rate of polymerization and the polydispersity, both i n~r e a s e d .~ Matyjaszewski e t al. have proposed that the mechanism of nitroxyl-mediated styrene polymerization includes degenerative chain transfer (Scheme 2).6 Their evidence in support of this mechanism is the observation that polydispersity narrows as the polymerization temperature is increased. At the higher temperature the rate of degenerative chain transfer increases, causing the polymer chain lengths to average.One of the tools that has been used which has shed significant light on the development of an understand-0024-929719512228-6692$09.00/0 Scheme 2. Degenerative Chain-Transfer Mechanism for Nitroxyl-Mediated Styrene Polymerizationing of polymerization processes is the use of small molecules to model the living chain end.7 In this study we synthesized 2,2,6,6-tetramethyl-l-(l-phenylethoxy)-piperidine8 (TMPEP) and studied both its thermal decomposition and its use as a polymerization initiator. HPLC and IH-NMR were utilized to follow the thermal decompo...

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SYNOPSISA comprehensive I3C-NMR method for the analysis of composition in the most common commercial polyethylene copolymers has been established. The method covers ethene copolymers with propene, butene-1, hexene-1, octene-1, and 4-methyl pentene-1 in the composition range of 1-10 mol %. The chemical shift assignments and TI values of the resonances of the copolymers are presented. Results of precision studies and interlaboratory analyses showed that the molar composition could be determined with a relative precision at 2a of about 6%. This method is being proposed to ASTM as Method X70-8605-2.

Nonsynonymous SNPs in TLR1 are associated with impaired TLR1 function, decreased cytokine responses, and predisposition to candidemia in whites.

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