1. Field of the Invention
The present invention relates to a novel anionic polymerization for polymerizing a monomer in a solvent in the presence of a polymerization initiator. More particularly, it relates to an anionic polymerization for producing a polymer having an average molecular weight being substantially the same with the predetermined one, in high reproducibility by substantially removing a polymerization inhibitor in the polymerization system by a special manner before charging the monomer and the polymerization initiator.
2. Description of the Prior Arts
A living anionic polymerization is a significantly important polymerization process for producing a block or graft copolymer for a thermoplastic elastomer; an oligomer having both terminal functional groups being useful as a liquid elastomer or a polymer having uniform molecular weights useful as a macromolecular standard for determining molecular characteristics.
In an ideal living anionic polymerization in the absence of any side reaction beside the initiation and the propagation reaction, a molecular weight M of a polymer corresponds to a ratio of an amount of a charge monomer m(g) to an amount of a polymerization initiator I(mole). That is, the molecular weight of the polymer M is given by the equation (1): EQU M=m/I (1)
However, in general, it has been difficult to remove completely a polymerization inhibitor which deactivates a polymerization initiator and an active living end from a solvent for polymerization, a monomer for polymerization and a polymerization atmosphere.
In such case, a molecular weight of the polymer M is given by the equation (2): EQU M=m/(1-.alpha.) (2)
wherein .alpha. (mole) represents an amount of the polymerization inhibitor; which is a sum of .alpha.sa of an amount of the polymerization inhibitor remained in the solvent and the atmosphere and .alpha.m of an amount of the polymerization inhibitor remained in the monomer.
When an anionic polymerization is carried out in a small scale in a laboratory, the solvent and the monomer used for the polymerization are small enough to attain a complete purification of the solvent and the monomer. A purification process is carried out in high vacuum so as to remove substantially the polymerization inhibitor remained in the solvent and the monomer by repeatedly contacting with a metal hydride hydrate, an alkali metal and an organometallic compound whereby the value .alpha. can be remarkably reduced. Therefore, in such case, it is possible to obtain a polymer having a molecular weight being substantially the same with the molecular weight given by the equation (1) so far as the molecular weight is not too high.
On the other hand, in a case of a bench scale or a factory scale of the polymerization, an amount of the solvent is too much to carry out in high vacuum and accordingly, the polymerization is carried out in an inert gas and it is impossible to purify completely the solvent as in high vacuum. The value .alpha. can not be negligible in comparison with an amount of the polymerization initiator I. Therefore, a molecular weight of the resulting polymer is higher than a predetermined molecular weight given by the equation (1).
When a predetermined molecular weight of the object polymer is high, such as higher than 500,000, the molecular weight of the resulting polymer is remarkably higher than the predetermined molecular weight given by the equation (1). Sometimes, a polymerization does not proceed.
In a large scale process, it is difficult to maintain constant of the amount of the polymerization inhibitor .alpha.sa remained in the solvent and the atmosphere for polymerization in polymerization batches whereby it is difficult to expect a reproducibility on the relation of the predetermined molecular weight and the molecular weight of the resulting polymer.