Short stopping

Evaluate methods for controlling runaway reactions (e.g., short stop, inhibitors)... 

Short-Stop Agent A material added to a reaction mixture to stop or greatly reduce the reaction rate. This is usually done to prevent a runaway reaction. 

Failure to add material, e.g. short-stop or inhibitor, at eorreet stage. 

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

In the manufacture of synthetic rubber, the short stop or stopping agent is the substance added to the reactor to stop the polymerisation reaction. Shrinkage... 

The principle of the short-stop method of determining Y.xr, in cationic polymerisations is that to the polymerising solution a basic reagent Q is added which combines with the growing centre and thus forms an end-group in the dead polymer ... 

The short-stop method of measuring [Pn ] was developed to produce an easily determinable end-group in systems in which the natural end-groups are unsuitable for accurate, quantitative determination. It was first used by Jaacks et al., (1968) to determine the concentration of tertiary oxonium ions during the polymerisation of 1,3-dioxolan, and by Saegusa et al., (1968) for similar studies on tetrahydrofuran. For the polymerisation of alkenes it has only been used on two occasions. 

There are two approaches to the separation of pp into the individual kp and kp values. One approach involves the experimental determination of the individual concentrations of free ions and ion pairs by a combination of conductivity with short-stop experiments or UV-visible spectroscopy. Conductivity directly yields the concentration of free ions that is, only free ions conduct. Short-stop experiments yield the total of the ion-pair and free-ion concentrations. UV-visible spectroscopy for those monomers (mostly aromatic) where it is applicable is also used to obtain the total of the free-ion and ion-pair concentrations. It is usually assumed that ion pairs show the same UV-visible absorption as free ions since the ion pairs in cationic systems are loose ion pairs (due to the large size of the negative counterions see Sec. 5-1). This approach is limited by the assumptions and/or experimental difficulties inherent in the various measurements. Conductivity measurements on systems containing low concentrations of ions are difficult to perform, and impurities can easily lead to erroneous results. The short-stop experiments do not distinguish between ion pairs and free ions, and the assumption of the equivalence of free ions and ion pairs in the spectroscopic method is not firmly established. The second approach involves determination of the... 

Typical emulsion polymerization recipes involve a large variety of ingredients. Therefore, the possibilities of variations are many. Among the variables to be considered are the nature of the monomer or monomers, the nature and concentration of surfactants, the nature of the initiating system, protective colloids and other stabilizing systems, cosolvents, chain-tranfer agents, buffer systems, short stops, and other additives for the modification of latex properties to achieve the desired end properties of the product. 

What provisions are made for handling impending runaways and for short-stopping an existing runaway ... 

Reaction quench or chemical short stop systems to stop or slow the reaction by removing heat sources, cooling the reactants, deactivating any catalyst, or interfering with reaction chemistry ... 

If necessary, during the reaction and short stop at the specific average particle size desired for a given latex. 

The mechanism of initiation is based on an examination of reaction products in an early stage of the polymerization. After short-stopping the polymerization by addition of sodium methoxide/methanol solution, the main product subsequently identified by gas chromatography was C2H5OCH2CH2OCH2OCH3. The rate of polymerization is presumed to follow the standard eqn. (6) for an equilibrium polymerization without termination and with rapid initiation. It is acknowledged that there is an induction period (presumably due to reaction of catalyst with adventitious water, since rigorous drying reduced the induction time to only... 

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