Aggregation behaviour below the gelation threshold for metal-olefin coordination in diene polymers

This study focuses on aggregation below the gelation threshold in ternary solutions containing diene polymers, atactic 1,2-polybutadiene (PBu) or 3,4-polyisoprene (PI), and an inorganic salt, bis(acetonitrile) dichloropalladium(II). Upon mixing, effective coordination crosslinks are formed because the acetonitrile ligands of the palladium salt are displaced by olefinic pendant groups of the polymers. In all cases, the aggregation kinetics correlate with PdCl2, concentration because the polymer concentration is 100 times greater than the salt concentration. Aggregation is the process that occurs prior to gelation as the transition-metal salt forms a coordination bridge between two different polymer chains. Differential analysis of the initial aggregation rate on the basis of light scattering data at 45 degrees relative to the incident throughput beam reveals that the 'true' order of the coordination reaction is close to unity. This suggests that coordination crosslinking is diffusion-controlled in the early stages of aggregation. Half-life analysis of viscous solutions yields an overall reaction order of 1.6 for aggregation of polybutadiene and palladium chloride in tetrahydorfuran (TNF). Scaling analysis of the weight-average molecular weight dependence of the root-mean-square size of isolated scattering particles yields an effective exponent of 0.7 for polymer/metal-salt complexes at infinite dilution. This experimental scaling law agrees with literature values for crosslinked polymer networks. In all cases, the size of the aggregates increases at higher PdCl2 concentration. For polybutadiene/PdCl2 mixtures in THF or toluene below the gelation threshold, the light-scattering-detected average aggregation number (AN = M-w,M-complex/Mw,M-pure polymer via Zimm-plot intercepts) for low-viscosity solutions is 2, while AN approximate to 9 for viscous THF solutions. In contrast, low aggregation numbers (AN approximate to 2) were calculated for viscous ternary mixtures of 3,4-polyisoprene and PdCl2 in THF. (C) 1998 Elsevier Science Ltd. All rights reserved.