Polymer living system

FIG. 16 Time evolution of the average length for a living polymer system [59] after a quench from an initial Tq = 1.0 to three final temperatures = 0.35 (circles),... 

C. M. Marques, M. E. Cates. Nonlinear thermodynamic relaxation in living polymer systems. J Phys II (France) 7 489-492, 1991. 

New kinds of living polymer systems result from the reactions of transition metals with cyclic, strained olefins 16). These polymerizations proceed through the intermediacy of metal carbenes and are exemplified by the polymerization of norbomene initiated by bis(cyclopentadienyl)-titane-cyclobutane described recently by Grubbs17>. 

Only a few quantitative data are available on copolymerization of methacrylates. Direct determination of the cross-propagation constants is readily achieved in living polymer systems whenever the absorption spectra of the two propagating species are different. Unfortunately, this is not the case in the methacrylate series. A new approach to this problem was developed by Muller 43). A mixture of two monomers is copolymerized, the reaction is interrupted at various times, and the concentrations of the residual monomers are determined as functions of time. The pertinent differential equations include 4 constants ku, k12, k21, and k22. Since kn and k22 were independently determined, the remaining cross-propagation constants are obtained by computer fitting the experimental conversion curves to the calculated ones. 

C. In the living polymer system, the termination of growing ends as well as the initiation can be controlled by switching the current. 

For example, the formation of living polymers allows the preparation of block polymers by sequential addition of monomers. It also permits the introduction of functional groups on the ends of each chain. From kinetic considerations of live polymer systems, it follows that, in a batch reaction, a fast initiation step relative to the propagation step will result in a very narrow molecular-weight distribution. It also follows that the molecular weight will be directly proportional to the mole ratio of initiator to monomer. 

Keii, T., Doi, Y. and Soga, K., Living Polymer Systems - Ziegler-Natta polymerisation , in Encyclopedia of Polymer Science and Engineering, Wiley-Interscience, John Wiley Sons, New York, 1989, Vol. 18, pp. 437 145. 

Marques, C.M., Turner, M.S., and Cates, M.E. "End-evaporation kinetics in living-polymer systems".. Chem. Phys. 99, 7260-7266 (1993). 

The study of the spectra of living polymer systems is valuable from a more practical point of view and indicates that the term has some limitations. At room temperature all the polymer-lithium compounds in hydrocarbon solvents show spectra which are stable for considerable time intervals. At elevated temperatures spectral changes occur at least for polystyryllithium, which indicate that isomerization reactions are occurring 4). Most of them display instability in solvents containing appreciable amounts of more polar constituents such as tetrahydrofuran. This effect was first noticed for poly-sty rylsodium 11) and has been attributed to the elimination of sodium hydride, followed by a subsequent reaction to form the more stable substituted allyl anion 21). 

Such "living" polymer systems, however, are not limited to polymerizations in solvating media, such as ethers. Thus, the lithium-catalyzed polymerizations, which can lead to the synthesis of cis-1,4-pol yisoprene, also demonstrate the virtual absence of... 

Finally, two more examples of living polymer systems should be mentioned. Polymerization of diazomethane initiated by boron trifluoride gives a stable compound485... 

Molecular Weight Distribution in Equilibrated Living Polymer Systems... 

The detailed treatment of trioxepane polymerization serves to illustrate the complexities that could be encountered in living polymer systems and to outline the approaches helpful in unraveling the resulting problems. 

The living chains can be terminated when desired by adding suitably reactive materials, such as water, alcohol, or ammonia. The unique features of living polymer systems described above provide fascinating possibilities of polymer syntheses, which include making monodisperse polymers (by controlled addition of monomer), structures with specific end groups (by chain termination with ap-... 

One of the first living polymer systems studied was the polymerization of styrene initiated by sodium naphthalene. The initiator is formed by adding sodium to a solution of naphthalene in an inert solvent, tetrahydrofuran. 

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