(1) Field of the Invention
This invention relates to a method for continuously producing a low molecular weight polymer of conjugated diolefin.  More particularly, the invention relates to a method for continuously producing a liquid low molecular weight polymer of conjugated diolefin by continuously polymerizing conjugated diolefin with maintaining a polymerization system in a substantially oxygen-free state. 
(2) Description of the Prior Art
It is well know in the conventional art that a polymer of a relatively lower polymerization degree is produced by polymerizing diolefin using an organic sodium compound as a polymerization catalyst and an alkylaryl compound as a chain transfer agent. 
For example, U.S. Pat.  No.  3,789,090 discloses that butadiene is polymerized to produce a low molecular weight polymer.  All instances on polymerization as well as those in examples disclosed in this reference are carried out by batch operation. 
Furthermore, in U.S. Pat.  No.  4,313,019, it is disclosed that butadiene is polymerized in the presence of an oxygen-treated catalyst.  In this reference, it is described that polymerization is done continuously.  However, in examples of this reference, what is called continual batch operation, i.e., the repetition of batch operation is done.  That is, the polymerization is nothing but batch-wise operation. 
The inventors of this application tried the methods of the above two references to polymerize butadiene continuously.  As a result, the molecular weight of polymer could be well regulated reproducibly, however, the inside wall surface of reaction vessel was fouled and the lack of cooling of the reaction vessel was caused to occur. 
That is, the polymerization of butadiene was carried out repeatedly according to the method of U.S. Pat.  No.  3,789,090.  As a result, a low molecular weight polymer of well regulated molecular weight, that is, those having sharp molecular weight distribution could be obtained but the inside wall surface of reaction vessel was fouled causing the lack of cooling of the reaction vessel. 
Furthermore, the polymerization of butadiene was carried out according to the method of U.S. Pat.  No.  4 313 019 using a polymerization catalyst which was treated and reacted with oxygen.  As a result, a low molecular weight polymer of well regulated molecular weight could be obtained but the inside wall of reaction vessel was fouled more seriously. 
At present, the mechanism that the inside wall of reaction vessel is fouled with dirt has never been made clear.  Anyway, the substance of the dirt is the polymer of butadiene.  This dirt substance is a polymer which cannot be dissolved by ordinary organic solvents, much less hydrocarbon solvents.  Furthermore, the dirt substance is not a simple gel.  In other words, the dirt substance is not the aimed low molecular weight polymer of butadiene.  This dirt substance grows fast to the inside wall of reaction vessel and outer surface of an inner coil for cooling to inhibit the heat transfer through the wall of reaction vessel and the surface of an inner coil, which causes the lack of cooling.  The lack of cooling in a reaction vessel brings about the rise of reaction temperature and, as a result, it gives serious influences upon the degree of polymerization, the hue of aimed product and so forth. 
In the foregoing prior art reference, the dirt substance of this kind is not referred to, therefore, any countermeasure to the problem on this dirt substance is neither disclosed nor suggested. 
Incidentally, with regard to several kinds of materials used for the reaction, especially, on the solvents which are used in large quantities and a chain transfer agent, any measure to avoid the dissolution of oxygen into these materials during storage and transportation is not generally taken.  Accordingly, when aromatic hydrocarbon solvents such as benzene and toluene are used for polymerization, oxygen on a maximum level (several tens of ppm) is often dissolved in these solvents. 
However, in the methods as disclosed in the foregoing patent publications to produce the low molecular weight polymer of butadiene, no attention was paid to the influences of oxygen in polymerization process except for catalysts.  So that, solvents are generally used without any operation to eliminate oxygen, for example, by positively displacing oxygen with nitrogen.