Molecular fractions, polymeric lithium

Molecular fractions of polymeric lithium cyclopentadlenyl derivatives... 

FIGURE 16. Molecular fractions of polymeric lithium cyclopentadienyl derivatives in the solid-state. The cations of the lithocene anions 47 and 48 (Ph4P+ and [(12-crown-4)2Li], respectively) have been omitted for clarity. With 55 the cation is [(THF)4Li]+. AU three are solvent-separated ion pairs. 

Polymers - The PS, PDMS, polyhexylisocyanate (PHIC), and polylso-prene (PI) samples had been extensively characterized to determine molecular weights, molecular sizes, and thermodynamic parameters (5, 6, 7 ). The samples were anionlcally polymerized using butyl lithium as the initiator. The pertinent data are shown in Table L Polylsobutylene/PIB polymers were obtained by fractionation of commercial polymers and their molecular weights were measured (8). 

A catalyst system with a zinc complex that also induces the chain-transfer reaction has been developed by Jerome et al When the polymerization of PO is conducted by using a mixture of zinc/aluminum bimetallic complex (Bu0)2A10Zn0Al(0Bu)2 and phenoxyethanol ([PO]/[Zn]/[phenoxyethanol] = 1000/1/20), the conversion reaches 97% to give PPO that contains low and high molecular weight fractions. An additive of lithium chloride or... 

COLEMAN and Fox (18) have pointed out that the non Bernoullian sequence distribution observed in some of these systems can be formed without the hypothesis of penultimate effects. All that is required is that two or more types of active species be present which do not rapidly interconvert. Each can add monomer at its own rate and with its own characteristic regulating effect. No penultimate effect is necessary but the sequence distribution will be non-Bernoullian. This type of mechanism is particularly attractive in the explanation of stereoblock polymer formation in the lithium alkyl systems in toluene with small amounts of ether present. The presence of at least two species of active centres has been inferred from an examination of polymer fractions obtained from butyllithium initiated polymerizations (19) in toluene. The change in molecular weight distribution with time suggests the presence of two... 

Only one kinetic study exists on initiation of methacrylate polymerization by a sodium compound. The initiator was the disodium oligomer ( tetramer ) of a-methylstyrene and polymerization was investigated at 25°C in toluene in presence of 0.05—0.2 mole fraction of tetrahydrofuran [181]. An internal first order disappearance of monomer was observed, the first order coefficient being directly proportional to active chain and tetrahydrofuran concentrations. The rate coefficients evaluated, e.g. fep = 3.1—13 X 10 1 mole sec at various tetrahydrofuran concentrations, are much lower than those for lithium initiators. They were, however, evaluated using a methyl iodide titration technique to estimate the active chain concentration. In view of the reactivity of tritiated acetic acid with many short chains which are clearly not active in chain propagation, there must be suspicion of similar behaviour with methyl iodide. If this happens, the active chain concentration would be over-estimated and the derived fep value would be too low. Unfortunately no molecular weights of the precipitable polymer were determined, so that it is impossible to check on active chain concentration using this alternative method. 

Initiation of anionic polymerization of styrene, dienes and their derivatives by alkyl lithium in hydrocarbon solvents was extensively studied by Ziegler163) and thereafter by many other workers. Since the rates of initiation are often comparable to those of propagation, both processes occur simultaneously and then, while the monomer is quantitatively polymerized, an appreciable fraction of the initiator remains unutilized in the system. Hence, it is advantageous to use fast alkyl lithiums as initiators, especially when a polymer of a narrow molecular weight distribution is the desired product. 

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