1-Propanol, shortstop

The apparent partial reversion of the polymerizing mixture after torque peaking, its dependence on shortstop concentration, and its ultimate decline with polymerization time are interesting phenomena. The desired shortstopping chemistry is pictured in Equation 3. It involves the reaction of the hydroxyl group of the added 1-propanol shortstop molecules with the unreacted terminal isocyanate groups on the poly(ester-urethane) chains. This permanently stops chain growth as desired. 

Data Showing 1-Propanol Shortstop Effect Polymers 5, 14 and 15... 

In Polymers 14 and 15 the shortstop, 1-propanol, was added to the polymerizing reaction mixture in the Brabender Plasticorder reactor 7 minutes after the polymerization had been started (see asterisk in Figure 9). 

Since 1-propanol is monofunctional and reacts with "matched dissociation isocyanate" (at internal urethane chain positions) as well as "unmatched isocyanate" (at polymer chain terminal positions) its effects include cleavage of some polyurethane chains in the process of generating more urethane groups. As a result, polymer DP, melt viscosity, and torque drop until the shortstop is consumed, or escapes the mixture by volatilizaton. Figure 9 shovjs that the more 1-propanol used to shortstop the polymerizations, the more pronounced the polymer reversion was. 

The data of Table X indicate the pronounced effectiveness of 1-propanol as a urethane polymerization shortstop. It does its job in about 1 to 2 minutes under the conditions investigated and DSV and T values decrease with increasing shortstop concentration. 

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