Shortstopped polymerizations

Isocyanate Levels During 1-Decanol Shortstopped Polymerization of Polymers 17 18... 

Since the principal hazard of contamination of acrolein is base-catalyzed polymerization, a "buffer" solution to shortstop such a polymerization is often employed for emergency addition to a reacting tank. A typical composition of this solution is 78% acetic acid, 15% water, and 7% hydroquinone. The acetic acid is the primary active ingredient. Water is added to depress the freezing point and to increase the solubiUty of hydroquinone. Hydroquinone (HQ) prevents free-radical polymerization. Such polymerization is not expected to be a safety hazard, but there is no reason to exclude HQ from the formulation. Sodium acetate may be included as well to stop polymerization by very strong acids. There is, however, a temperature rise when it is added to acrolein due to catalysis of the acetic acid-acrolein addition reaction. 

The basic constituents of all commercial emulsion polymerization recipes are monomers, emulsifiers, and polymerization initiators. Other common components are modifiers, inorganic salts and free alkaH, and shortstops. The function of these different components and the mechanism of emulsion polymerization have been described (43,44). 

Neutral or alkaline salts, eg, KCl, K SO, K CO, or Na PO, are often present in synthetic latices in quantities of - <1%, based on the weight of the mbber. During emulsion polymerization the salts help control viscosity of the latex and, in the case of alkaline salts, the pH of the system. Many polymerizations are carried out at high pH, requiring the use of fixed alkaH, eg, KOH or NaOH. Very small amounts of ferrous salts can be employed as a component of the initiator system, in which case a sequesteriag agent, eg, ethyldiaminotetraacetic acid (EDTA) may be iacluded to complex the iron. Water-soluble shortstops, eg, potassium dithiocarbamate, may also be iacluded ia very small amounts (ca 0.1 parts). 

In the polymerization of BD by Ti-, Co- and Ni-based catalyst systems the polymerization has to be shortstopped at a specific monomer conversion in order to avoid the formation of gel. In contrast, polymerization catalysis by Nd catalysts does not need control of monomer conversion. As gel formation is particularly low with Nd catalysts full monomer conversion can be accomplished [427,428]. 

If the polymerization is shortstopped at a specific monomer conversion water, acids and alcohols are used. Water has to be properly mixed with the... 

The first three benefits are a direct consequence from the extremely low tendency of the Nd-catalyst to form branches and gel. Because of this remarkable feature, Nd-catalysts allow monomer conversions up to 100%. Therefore, the polymerization reaction does not have to be shortstopped below a critical monomer conversion in order to avoid gel. In addition, polymerization temperature does not have to be controlled within a well-defined temperature range. As the maximum polymerization temperature (at complete monomer conversion) can be as high as 120 °C the polymerization process can be performed in a fully adiabatic manner. In this case energy costs for cooling and for the removal of low molar mass residuals can be very low. Another benefit of the Nd-catalyst is the low tendency to catalyze the Diels-Alder dimerization of BD to vinyl cyclohexene. 

The ampoules were removed from the bath at the desired time and opened. The polymerizations were shortstopped by quickly pouring the polymer cement into 30% of its volume of a 0.15M solution of ammonia gas in THF. This cement was then evaporated in air and finally dried in a vacuum oven to constant weight. 

Butadiene-Styrene Rubber occurs as a synthetic liquid latex or solid rubber produced by the emulsion polymerization of butadiene and styrene, using fatty acid soaps as emulsifiers, and a suitable catalyst, molecular weight regulator (if required), and shortstop. It also occurs as a solid rubber produced by the solution copolymerization of butadiene and styrene in a hexane solution, using butyl lithium as a catalyst. Solvents and volatiles are removed by processing with hot water or by drum drying. 

Polymerizations were carried out in 8-ounce glass bottles with metal caps containing self-sealing butyl rubber gaskets. The capped bottles with their contents were rotated end-oyer-end at 45 rpm at 50°C. in a thermostatted water-bath. Samples for conyersion and for particle size measurements were withdrawn at regular time interyals using hypodermic needle and syringe. Hydroquinone was used as a shortstop. 

Use Fungicide, corrosive inhibitor, rubber accelerator, intermediate, polymerization shortstop. 

Published information on urethane polymerization detail largely concerns thermoset urethane elastomers systems.4 13 In particular, the work of Macosko et. al. is called to attention. The present paper supplements this literature with information on the full course of linear thermoplastic urethane elastomer formation conducted under random melt polymerization conditions in a slightly modified Brabender PlastiCorder reactor. Viscosity and temperature variations with time were continuously recorded and the effects of several relevant polymerization variables - temperature, composition, catalyst, stabilizer, macroglycol acid number, shortstop - are reported. The paper will also be seen to provide additional insight into the nature and behavior of thermoplastic polyurethane elastomers. 

Catalyst and antioxidant when used were thoroughly mixed through the hot macroglycol-chain extender glycol blend before MDI addition. Shortstop when used was added quickly and cleanly to the polymerizing reaction mixture through the mixing chamber throat. 

High acid number (2.4) polyester was used in these runs. Number 3 was made from reactant lots IV, XVI and X. The polyester used also had high acid number (2.00) and the polymerization temperature was 200°C. None of these polymerizations was deliberately shortstopped. 

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). 

Comparison of Polymers 14 and 15 in Figure 9 shows fairly similar polymerizations up to the point of shortstop addition. The control, Polymer 5, was slower but alive and growing at 18 minutes of polymerization time, reaching a final torque value of 660 meter-grams. Its steady intermediate polymerizarion rate is seen to be characteristic of uncatalyzed thermoplastic polyurethane elastomer polymerizations based on low acid number PTAd while Polymers 14 and 15 formed faster than usual from such a low acid number polyester glycol component. 

In Polymer 14 (1.0% SS) the polymerization showed a torque increase of 30 meter-grams peaking at 580 meter-grams one half minute after shortstop addition (+ AT 30/ 0.5, 580 maximum) with 80 units reversion in the subsequent 12.5 minutes of polymerization (-AT 80/12.5). 

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. 

Shortstop molecules, added in substantial excess of the isocyanate groups in the polymerizing mixture (to shortstop quickly and completely) upset the urethane equilibrium shown in Equation 1, driving the reaction to the right by the Law of Mass Action in order to produce more urethane groups and thus re-establish the equilibrium constant (Equation 2) for that temperature. 

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. 

B. 1-Decanol. Figure 10 shows the polymerization viscosity-time-temperature relations for a set of (2.0) ratio thermoplastic poly(ester-urethane) elastomers, Polymers 16, 17 and 18, where 17 and 18 were shortstopped with a higher molecular weight primary alcohol, 1-decanol (XIV - Table I). The polymers of this set were all made from the same reactant lots (VII, IV, and X - Table I). The polyester glycol had a low acid number, 0.15. 

Comparison of Polymer 16 polymerization (control) with 17 shows essentially identical polymerization rates up to the point of shortstop addition after 8 minutes of polymerization time. In Polymer 16 we see the polymerization begin to slow down after about 8 minutes of reaction at 420 meter-grams but to achieve 490 meter-grams in 10 minutes without die-out. 

The behavior of Polymer 17 polymerization (0.24% SS at 8 minutes) showed less than complete shortstop action (+ AT 30/2) and a final torque of 470 meter-grams. Note the total absence of shortstop-induced polymer reversion even after 16 minutes of reaction time. 

Isocyanate analyses of the shortstopped polymers, performed during the polymerization before and after shortstop addition, afforded the results of Table XII. 

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