Narrow-polydispersity polymer

Schriemer D, Li L (1996) Detection of high molecular weight narrow polydisperse polymers up to 1.5 million daltons by MALDI mass spectrometry. Anal Chem 68 2721-2725... 

The discovery that Ti complex (4) was an effective catalyst for living ROMP chemistry resulted in the synthesis of several new types of polymers that were inaccessible with conventional catalysts. Narrow polydispersity polymer and di- and triblock copolymers were synthesized soon after living ROMP was discovered. Because polymer chains that are formed in a living ROMP reaction are terminated with metal carbene groups, fimctionalization of the chain ends is possible. Reaction of the carbene with an aldehyde occurs in a Wittiglike fashion, making a metal oxo complex that is inert to finther metathesis chemistry and terminates the chain with the alkylidene group of the aldehyde. 

NMRP is not a true living polymerization but it has some attributes of a living polymerization, e.g. rather narrow polydispersity polymers can be produced, polymer molecular weight increases linearly with monomer conversion, and sequential addition of monomers leads to block copolymers. However, no one has yet produced truly monodisperse polymer using NMRP. Therefore, there are likely side reactions going on during chain growth that lead to adventitious termination. 

Chelating diamide titanium complexes of the type [RN(CH2)3NR]TiMe2 (R = 2,6- Pr2CeH3, 2,6-Me2CeH3) have been found to catalyze the living aspecific polymerization of a-olefins at ambient temperature, when activated with FAB, thereby producing narrow polydispersity polymers (MJMn = 1.05—1.09). ° To explain the fact that activities are suppressed when the polymerizations are performed in the presence of toluene, a cationic alkyl arene complex has been proposed, although no spectroscopic evidence or isolation has been provided in the paper. 

Zhu, H., Yalcin, T., and Li, L., Analysis of the Accuracy of I3eteriiiming Average Molecular Weights of Narrow Polydispersity Polymers by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry, /. Am. Soc. Mass Spectrom., 9, 275, 1998. 

Zhu H, Yalcin T, and Liang L (1998) Analysis of the accuracy of determining average molecular weights of narrow polydispersity polymers by matrix-assisted laser desorption ionization time-of-flight mass sepctrometry. Journal of the American Society for Mass Spectrometry 9 275-281. 

While MALDl-MS is widely used for polymer characterization, it does have certain limitations. The first limitation is that not all narrow-polydispersity polymers can be ionized by MALDl. For example, polyethylene, perfluoropolymers, and polycationic polymers are difficult to analyze by MALDl-MS. Direct laser desorption ionization with the assistance of metal powder can be used to ionize low-molecular mass polyethylene (<5000 Da) [112], but high-mass polyethylene is currently not amenable to MALDl analysis. 

The current MALDI techniques used for polymer analysis have several limitations. For example, they are unable to analyze some narrow-polydispersity polymers, while there is a need to develop sample-preparation protocols for the analysis of important polymers such as polyethylene, perfluoropolymer, and polycationic materials. Although the analysis of polystyrenes with molecular masses of up to 1.5 million using a conventional MALDI-TOF instrument has been demonstrated, it remains to be seen how the technique can be appUed to other high-mass polymers (>500000 Da). 

While the living free-radical polymerization of styrene from an alkoxyamine can lead to narrow polydispersity polymers, these having the nitroxyl radical at the terminals can be expected to behave as polymeric counter radicals for propagating polymeric radicals. The irse of such... 

Hydrogenated polybutadiene makes polyethylene. In this case, a narrow polydispersity polymer not available otherwise. 

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