AGGREGATION BEHAVIOR BELOW THE GELATION THRESHOLD FOR METAL-OLEFIN COORDINATION IN DIENE POLYMERS

This study focuses on aggregation below the gelation threshold in tern ary solutions containing diene polymers, atactic 1,2-polybutadiene (PB u) 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, th e aggregation kinetics correlate with PdCl2, concentration because the polymer concentration is 100 times greater than the salt concentratio n. Aggregation is the process that occurs prior to gelation as the tra nsition-metal salt forms a coordination bridge between two different p olymer chains. Differential analysis of the initial aggregation rate o n the basis of light scattering data at 45 degrees relative to the inc ident throughput beam reveals that the 'true' order of the coordinatio n reaction is close to unity. This suggests that coordination crosslin king 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 tetrah ydorfuran (TNF). Scaling analysis of the weight-average molecular weig ht dependence of the root-mean-square size of isolated scattering part icles yields an effective exponent of 0.7 for polymer/metal-salt compl exes at infinite dilution. This experimental scaling law agrees with l iterature values for crosslinked polymer networks. In all cases, the s ize of the aggregates increases at higher PdCl2 concentration. For pol ybutadiene/PdCl2 mixtures in THF or toluene below the gelation thresho ld, the light-scattering-detected average aggregation number (AN = M-w ,M-complex/Mw,M-pure polymer via Zimm-plot intercepts) for low-viscosi ty solutions is 2, while AN approximate to 9 for viscous THF solutions . In contrast, low aggregation numbers (AN approximate to 2) were calc ulated for viscous ternary mixtures of 3,4-polyisoprene and PdCl2 in T HF. (C) 1998 Elsevier Science Ltd. All rights reserved.