119Sn-NMR evidence for 2,1-initiation in the anionic polymerization of butadiene with trialkyltin lithium

Low molecular weight polybutadienes and styrene butadiene copolymers were anionically prepared with trialkyltin lithium initiator and end-capped with either hydrogen or a trialkyltin group. These polymers were prepared with a variety of microstructures. Analysis by ¹¹⁹Sn-NMR and comparison to model compounds showed no cis-1,4-initiation of the butadiene. The initiation sites found were trans-1,4- and both 2,1- and 1,2-additions of the tin-lithium bound to a 1,3-butadiene. At low levels of added polar modifier, the 2,1-addition predominated. The ¹¹⁹Sn-NMR spectra allowed the assignment of the sequence distribution associated with the nearest eight main chain carbon atoms (2-4 monomer units) adjacent to the tin end groups. No initiation could be detected involving the styrene comonomer, but incorporation of styrene was detected as the first or second unit after initiation. The reaction of the allyl-tin end groups of these polymers with 1,2-napthoquinone was followed by NMR and was used to assign the peaks associated with 1,2-addition of the trialkyltin lithium to 1,3-butadiene. © 1995 John Wiley & Sons, Inc.