Investigations utilizing mono- and difunctional organo alkali metal initiators for anionic polymerization
This work focused on the formation and the subsequent use of a difunctional organolithium as an anionic polymerization initiator for primarily diene monomers. The initiator was formed in nonpolar solvent such as cyclohexane and toluene by the addition of two equivalents of sec-butyllithium to one of the nonconjugated diene 1,3-bis(α-phenylethenyl)benzene.
The disappearance of the starting diene was easily followed by UV/Visible spectroscopy. Rate expressions were evaluated based on the assumptions of a consecutive reaction mechanism and association behavior of the alkyl lithium. Arrhenius behavior was observed and an activation energy of 19 Kcal/mole was determined.
Unlike the spectroscopic techniques, gas chromatography was able to separate the unreacted diene, mono-, and diaddition products but dramatic increases in sensitivity were realized by using a capillary column techniques, thus allowing the direct analysis of initiator solutions that were routinely used for high molecular weight polymerizations. Quantitative determination of all reaction components confirmed the existence of a consecutive reaction mechanism for this addition and the data strongly suggested that the second rate constant was an order of magnitude faster than the first in cyclohexane.
Using the chromatographic technique to monitor initiator formation, the dianionic diaddition product was maximized by the simple addition of more sec-butyllithium. Polydiene homopolymers were then routinely prepared with predictable molecular weights and narrow polydispersities. Successive addition of isoprene and styrene resulted in triblock copolymers that incorporated both monomers over a wide compositional range. Novel triblock systems that contained t-butyl methacrylate end blocks and an isoprene center block were prepared by addition of THF which facilitated the efficient crossover.
A new trialkyl sodium magnesiate initiator was evaluated as an alternative to alkyllithiums for the polymerization of vinyl and cyclic monomers. Homopolymerizations of isoprene and styrene showed slow initiation rates and uncontrollable molecular weights relative to sec butyllithium. This initiator also polymerized methyl methacrylate in high yields without modification of its basicity and steric bulk.