PTHF is usually prepared industrially by polymerization of tetrahydrofuran, hereinafter referred to as THF for short, over suitable catalysts. The addition of suitable reagents enables the length of the polymer chains to be controlled, and the mean molecular weight can thus be set to the desired value. Control is achieved by choice of type and amount of the telogen.
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Such reagents are referred to as chain termination reagents or “telogens”. Selection of appropriate telogens also enables functional groups to be introduced at one or both ends of the polymer chain. Other telogens act not only as chain termination reagents but are also incorporated into the growing polymer chain of the PTHF. They have not only the function of a telogen but simultaneously act as a comonomer and can therefore be referred to as either telogens or comonomers with equal justification.
Examples of such comonomers are telogens having two hydroxy groups, e.g. Diols (dialcohols). These can be, for example, ethylene glycol, propylene glycol, butylene glycol, 1,3-propanediol, 1,4-butanediol, 2-butyne-1,4-diol, 1,6-hexanediol or low molecular weight PTHF. Further suitable comonomers are cyclic ethers such as 1,2-alkylene oxides, for example, ethylene oxide or propylene oxide, 2-methyltetrahydrofuran or 3-methyltetrahydrofuran. The use of such comonomers leads, with the exception of water, 1,4-butanediol and low molecular weight PTHF, to formation of tetrahydrofuran copolymers, hereinafter referred to as THF copolymers, and in this way makes it possible to achieve chemical modification of PTHF.
In industry, use is predominantly made of two-stage processes in which tetrahydrofuran is, for example, polymerized in the presence of fluorosulfonic acid to form polytetrahydrofuran esters and these are subsequently hydrolyzed to polytetrahydrofuran. As an alternative, tetrahydrofuran is, for example, polymerized with acetic anhydride in the presence of acid catalysts to form polytetrahydrofuran diacetate which is subsequently transesterified, e.g. By means of methanol, to give polytetrahydrofuran. Disadvantages of such processes are that they have to be carried out in two stages and that by-products such as hydrofluoric acid and methyl acetate are formed. It has surprisingly been found that the cascaded addition of the telogen and/or comonomer at at least two different addition points in different segments of the polymerization apparatus enables the space-time yield and conversion to be improved significantly.
The number of addition points can be two, three, four, five or more and depends on the polymerization apparatus used, in particular its type and capacity, and also on process engineering and economic boundary conditions. However, preference is generally given to using from 2 to 5 addition points. Examples of suitable polymerization apparatuses are cascades of at least two tank or tube reactors, for example cascades of stirred tanks, cascades of at least two fixed-bed reactors, which may optionally be operated with circulation, and cascades of loop reactors. In these polymerization apparatuses, one segment in which an addition point for the telogen and/or comonomer is located corresponds to a tank or a tube. However, it is not necessary for each segment of the polymerization apparatus in which an addition point for the telogen and/or comonomer is located to be a single unit such as a stirred tank.
Willy Chirino Son Del Alma Rar File on this page. Depapepe Passion Of Gradation Rarlab. Rather, a reactor can be configured so that it fulfills the function of a plurality of reactor elements connected in series. It is therefore also possible to use a single reactor, in particular a fixed-bed reactor, which is divided into at least two, preferably from 2 to 5, segments by means of suitable internals, for example orifice plates or sieve trays.