Method for continuously producing low equivalent weight polyols using double metal cyanide catalysts
Polyether polyols having equivalent weights of up to 500 are continuously prepared in the presence of a double metal cyanide catalyst. A first step of the reaction is performed at a temperature of at least 150° C., while controlling the hydroxyl content and unreacted alkylene oxide content of the reaction mixture to within certain ranges. A portion of that reaction mixture is withdrawn and permitted to react non-isothermally to consume the unreacted alkylene oxide. This process is highly efficient, does not result in catalyst deactivation, as is commonly seen in previous processes, and does not produce a significant ultra high molecular weight tail.
1. A process for continuously polymerizing an alkylene oxide in the presence of a double metal cyanide polymerization catalyst to form a polyether polyol having a hydroxyl content of from about 3.4 to 20% by weight, wherein:
in a first step a), an initiator compound that has at least two hydroxyl groups per molecule and a hydroxyl equivalent weight of up to 115 and at least one alkylene oxide are fed to a continuous reactor containing a double metal cyanide catalyst, and a partially polymerized mixture is withdrawn from the continuous reactor, wherein (i) the continuous reactor is maintained at a polymerization temperature in excess of 150° C., (ii) the concentration of unreacted alkylene oxide in the contents of the continuous reactor is maintained at a level of from 1.3 to 5% by weight, and (iii) the hydroxyl content of the reactor contents is maintained at 5 to 10% by weight and
in a subsequent step b), the partially polymerized mixture is withdrawn from the reactor in step a) is permitted to further polymerize exothermically without applied heating until the unreacted alkylene oxide content of the mixture is reduced to 0.5% or less by weight,
wherein the build ratio in step a) is from 2.5 to 15 and the overall build ratio is from 2.8 to 16, and further wherein the reaction mixture in step (b) increases by 10 to 25° C. as a result of the exothermic polymerization reaction.
2. The process of claim 1 wherein the alkylene oxide is propylene oxide or a mixture of propylene oxide and ethylene oxide that contains at least 85% by weight propylene oxide.
3. The process of claim 2 wherein the unreacted alkylene oxide in the contents of the continuous reactor is maintained at a level of from 2 to 5% by weight.
4. The process of claim 3 wherein the initiator is glycerine.
5. The process of claim 1 wherein step a) is performed in a loop reactor and step b) is performed in a pipe reactor.
6. The process of claim 1 wherein step a) is performed in a continuously stirred tank reactor and step b) is performed in a pipe reactor.
7. The process of claim 1 wherein the concentration of the double metal cyanide catalyst in step a) is from 10 to 100 ppm, based on the weight of the product.
8. The process of claim 1 wherein the temperature in step a) is from 160 to 190° C.
9. The process of claim 1 wherein the double metal cyanide catalyst is a zinc hexacyanocobaltate catalyst complex.
10. The process of claim 2 wherein the product polyether polyol has a hydroxyl content of from 8.5 to 11.3% by weight.
11. The process of claim 1 wherein the initiator is glycerine and the overall build ratio is from 8 to 11.5.
12. The process of claim 1 wherein in step b), the temperature of the partially polymerized reaction mixture increases by from 10 to 25° C.