Styrene-Diene Random Copolymers, 'Random-Diblocks' and A-(A7B)-B Triblocks

摘要

Anionic copolymerizauons of styrene and diene monomers with alkyl lithium initiators in hydrocarbon media are known to result in block copolymers due to the large differences in copolymerization reactivity of the monomers1. It is often desirable to have diene polymers with high 1,4 content (>85%) since these materials have lower glass transition temperatures. The anionic synthesis of dienes with high 1,4 content requires the use of hydrocarbon solvents since polar additives typically produce increasing levels of 1,2 content even at very low concentrations6"^ Polar compounds, such as ethers, amines, crown ethers, etc., impact the relative 1,2/1,4 contents of diene polymers at concentrations comparable to the concentration of the initiating alkyl lithium species (-0.01-0.0001 M), These polar additives also often result in side reactions with the alkyl lithium propagating species resulting in broadening of the molecul ar weight distribution and loss of the ability to prepare functional polymers and block copolymers. Unfortunately, anionic , polymerization initiators that are soluble in hydrocarbon media are limited to a few alkyl lithium initiators in contrast to a broader variety of initiators (alkyl sodiums, potassiums, magnesiums, difunctional alkyl lithiums, etc.) that are soluble in polar media. The numerous studies of the random and block nature of styrene-butadiene copolymers have been reviewed by Tanaka.10