An in situ study of the t-butyllithium initiated polymerization of butadiene in d-heptane via small angle neutron scattering and H1-NMR

Abstract

We present a combined 1H-NMR and small angle neutron scattering in situ study of the anionic polymerization of butadiene using t-butyllithium as the initiator. Both initiation and propagation phases were explored. This combined approach allows the structural and kinetic characteristics to be accessed and cross compared. The use of the D22 instrument (ILL Grenoble) permits the attainment of Q approximately equal to 2 x 10(-3) A. This, in turn, led to the identification of coexisting large-scale and smaller aggregates during all stages of the polymerization. The smaller aggregates contain most of the reacted monomers. Their structure changes from high functionality wormlike chains at early stages of the reaction to starlike aggregates where the crossover occurs at a degree of polymerization of approximately equal to 40. The initiation event involved these small, high functionality (approximately equal to 120) aggregates that apparently consisted of cross-associated t-butyllithium with the newly formed allylic-lithium head groups. As the initiation event progressed the initiation rate increased while the functionality of these small aggregates decreased and their size increased. Propagation, in the absence of initiation, was found to have a rate constant that was molecular weight dependent. At approximately 11 kg/mol the measured polymerization rate was found to increase while no further structural changes were seen.

Living anionic polymerization of methyl methacrylate controlled by metal-free phosphazene catalyst as observed by small-angle neutron scattering, gel-permeation chromatography and UV–visible spectroscopy

“…This is in contrast to the reports (Stellbrink et al, 1998;Niu et al, 2005;Miyamoto et al, 2006) on conventional living anionic polymerization in nonpolar solvents with alkyllithium compounds as an initiator: in this case, the anionic living ends of the polymers and the lithium cations form aggregates, which are characterized by SANS as narmed star polymers, via electrostatic attraction. Upon termination reaction, the charges in the living ends are lost and the aggregates formed via electrostatic attraction also disappear.…”

“…For in-situ observation of superstructures on the length scales of sub-nanometres to micrometres appearing during chemical reactions, small-angle neutron scattering (SANS) is a powerful tool because of the low energy and high transmittance of neutrons. Recently, the aggregation behaviors of the anionic living ends in nonpolar solvents were analyzed by SANS (Stellbrink et al, 1998;Niu et al, 2005;Miyamoto et al, 2006).…”

Dynamics of Monofunctional Polybutadienyl Lithium Chains Aggregated in Benzene

“…C with a JEOL JNM-AL400 [16][17][18] revealed that living PBLi chains form aggregates in heptane and cyclohexane and the aggregation number f has some distribution: The main component of the aggregates is the tetramer ( f ¼ 4) but a small amount of large aggregates ( f > 100) is also formed. Similar aggregation behavior was expected also in our PBLi/ d-Bz solutions.…”

“…These small aggregates coexist with a minor amount of large aggregates with f > 100, as revealed from light/neutron scattering experiments. [8][9][10][16][17][18] Recently, the neutron scattering and 1 H NMR studies were conducted during the polymerization process of PBLi in heptane to reveal an interesting decrease of f with increasing conversion (from f of the order of 100 to f ¼ $ 4 for the main component of the aggregates). This decrease was intimately related to the NMR-determined initiation/polymerization rates.…”

Kinetics of Anionic Polymerization of Polybutadienyl Lithium in Benzene: An Osmotic Effect on Propagation Process

“…4,9,10 On the other hand, for the propagation process of the anionic polymerization after full consumption of the RLi molecules, a simple kinetics shown in Scheme 1 seems to have been accepted. In this kinetics, the anionic polymer chains, PLi, are in chemical equilibrium with their star-like f -mer aggregates (PLi) f bound at the Li ends (as detected from scattering experiments [10][11][12][13][14][15][16] ), and the propagation occurs only through the dissociated PLi chains because the aggregates are considered to be well stabilized and thus inert for the propagation. The time (t) evolution of the molar concentration of the monomer, [M], is then described by…”