Anionic synthesis and characterization of block copolymers from isoprene and 2-vinylpyridine

Diblock copolymers of isoprene and 2-vinylpyridine have been prepared by anionic polymerization techniques in benzene using sec-butyllithium as initiator. In the absence of additives at 5{dollar}spcirc{dollar}C the crossover reaction of poly(isoprenyl)lithium with 2-vinylpyridine produced a block copolymer with multimodal molecular weight distribution. The addition of LiCl (LiCl/sec-BuLi {dollar}>{dollar} 4) produced block copolymers free of homopolymer and dimer, with molecular weights close to those expected from the ratio of monomer to initiator in the feed. A variety of diblock copolymers with molecular weights ranging from {dollar}40.5times10sp3{dollar} g/mol to {dollar}224times10sp3{dollar} g/mol were synthesized by this methodology. The presence of LiCl in the system did not affect the microstructure of the polyisoprene segments, as evidenced by {dollar}sp1{dollar}H NMR spectroscopic analysis, showing 92% 1,4 enchainment.; A hydrocarbon soluble dilithium initiator (MDDPE-sec-BuLi) was prepared by reacting 2 moles of sec-butyllithium with 1 mol of 1,3-bis(1-phenylethenyl)benzene (MDDPE). This difunctional initiator was used to synthesize poly(2-vinylpyridine-b-isoprene-b-2-vinylpyridine) triblock copolymers in benzene at 8-25{dollar}spcirc{dollar}C. In the presence of either LiCl (LiCl/(MDDPE-sec-BuLi) = 4) or lithium sec-butoxide (sec-BuOLi/(MDDPE-sec-BuLi) = 1 to 2) monomodal {dollar}alpha,omega{dollar}-dilithium polyisoprenes were synthesized. The crossover reaction of 2-vinylpyridine with {dollar}alpha,omega{dollar}-poly(isoprenyl)dilithium proceeded rapidly, requiring a maximum reaction time of only 12 minutes. Polymerizations were instantaneously deactivated by addition of acetic acid/methanol (1/10, vol/vol). This methodology provided well-defined triblock copolymers with molecular weights in the range of {dollar}101times10sp3{dollar} g/mol to {dollar}187times10sp3{dollar} g/mol and with M{dollar}rmsb{lcub}w{rcub}/Msb{lcub}n{rcub}=1.2.{dollar}; Poly(styrene-b-isoprene-b-styrene) and poly(4-methylstyrene-b-isoprene-b-4-methylstyrene) triblock copolymers were synthesized by sequential monomer addition in benzene using sec-butyllithium as initiator.; The stress-strain behavior of the poly(2-vinylpyridine-b-isoprene-b-2-vinylpyridine) triblock copolymers indicated that these triblock copolymers exhibit the characteristic properties of thermoplastic elastomers. However, their tensile strengths were significantly lower than those of poly(styrene-b-isoprene-b-styrene) triblock copolymers with equivalent molecular weights.; These triblock copolymers were also analyzed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) techniques. Spherical, cylindrical and lamellar structures were detected from solvent-cast films. The domain size and morphology of the samples determined by TEM were found to correlate with the values determined by SAXS.