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Kinetics of Addition Polymerization to Polydimethylsiloxane
 

Kinetics of Addition Polymerization to Polydimethylsiloxane

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Polymers are a ubiquitous class of compounds found in all facets of industry and manufacturing. Two of their most important characteristics, molecular weight and degree of polymerization, must be derived from other bulk properties. Unlike other substances, whose physical characteristics are defined solely by their chemical structures, polymers are also affected by their degree of polymerization and molecular weight. Chemically identical polymers can vary from liquids to rubbers to hard, brittle solids, all based on these physical properties. Since microscopic attributes, such as molecular weight, are difficult to measure directly, bulk properties, such as viscosity, density, and light scattering, can be used to infer these important characteristics. This video will illustrate a batch polymerization of polydimethylsiloxane, or PDMS, and determine its molecular weight and degree of polymerization from its viscosity.

To begin, let's focus on the bulk fabrication of polydimethylsiloxane, or PDMS. Polymerization reactions are classified by their mechanisms, reactor types, product characteristics, and more. In the case of PDMS, an initiator reacts with the monomer to produce the polymer chain, which in turn can be extended through further reactions with the monomer. This reaction mechanism is known as addition polymerization, and is characterized by the absence of by-products. The choice of reactor depends on reactant properties and affects the characteristics of the product. Batch reactors, which typically consist of a tank, agitator, and heating or cooling system, operate as closed systems in which the reactants are added in a discreet step and then allowed to react over time. Batch reactors are preferred for small-scale reactions when low quantities of reactants are used or a new process is being developed, or to synthesize several grades of product. They are frequently used for polymerizations. PDMS is synthesized from a monomer, an initiator, and an end-blocker without any solvent, a condition known as bulk polymerization. The absence of solvents simplifies polymer processing, since the by-products and catalyst are easily separated from the polymer. However, the temperature must be carefully controlled, as with a water cooling jacket, to prevent exothermic runaway that may result in an explosion. Regardless of the reaction conditions, the measured physical properties of the product, such as the viscosity, are used to estimate the number-average molecular weight and weight-average molecular weight. Dividing the number average molecular weight by the molecular mass of the monomer yields the average chain length or degree of polymerization, which is related to conversion and reaction order. Now that you know the basics of polymerization, let's see how to operate a small-scale batch reaction of PDMS and determine the reaction kinetics.

To begin the procedure, open the nitrogen cylinder connected to the reaction vessel. Run the first sequence, which verifies that the equipment is operational and in good working order. Next, test the system for leaks by closing the manual valve to the vacuum pump. Wait five minutes and verify that the pressure rise does not exceed 600 millimeters of mercury. Reopen the valve to remove any remaining atmosphere. Finally, close the manual valve and fill the system with nitrogen. The third module of the program adds the cyclic monomer to the reactor. The lower quantity ingredients, the catalyst and end blocker, are added through a small funnel called the adder tank. The reactor is now full and ready for polymerization. Start the fourth process and monitor the temperature. Once it rises above 105 degrees, begin collecting liquid samples from the sample draw point. Collect aliquots at intervals of at least every eight minutes. To know when the polymerization reaches equilibrium, monitor the power usage of the agitator. Once power has stopped increasing, the reaction is complete. At this point, open the carbon dioxide tank and valve and press the reaction complete push button to neutralize the catalyst. To begin the stripping sequence, open the manual valve to the vacuum pump and allow it to run for 15 minutes at a higher temperature. At this point, select stripping complete and collect the low boilers from the reaction into a flask. Allow the automated cool down process to run. Using the manufacturer's instructions, measure the collected samples with a rotational viscometer. If the speed is set too high, no reading will be obtained and a lower speed will be chosen. These values will be used to determine the molecular weight distribution of the polymer.

A lot of information can be obtained from the relatively simple viscosity measurement. Dividing the viscosity of the PDMS sample by its density yields its kinematic viscosity. Empirical equations, such as Barrie's relationship, relate kinematic viscosity to the viscosity-average molecular weight. Dividing the viscosity-average molecular weight by 1.6, another empirical factor for PDMS, yields the number-average molecular weight, the average weight per polymer chain. Dividing this by the weight of the monomer yields the average chain length or degree of polymerization, the number of monomer units in the polymer. However, since the calculated chain length includes the un-reacted monomer, it will be artificially low. A correction that accounts for the fractional conversion must be applied. Here are typical results for the viscosity-average molecular weight and degree of PDMS polymerization with reaction time. In this reaction, a large amount of end blocker, which stops chain growth and forms a trimethyl end group, was used, resulting in a low final degree of polymerization. The fractional conversion can also be determined as a function of time. By assuming irreversible kinetics and that the polymer was produced at a constant chain length, the reaction order with respect to monomer was determined to be first order, as confirmed by the reasonable fit. A rate constant of 0.054 inverse minutes was calculated, which agrees with other studies that report a first order rate constant of 0.06 inverse minutes for this monomer under similar conditions.

Synthetic polymers are found in a wide range of products, both on the industrial and commercial scale. Let's look at a few common examples. Siloxane polymers, such as PDMS, can be industrially formed via several techniques, such as injection molding. They are suitable for diverse applications, including lubricants, sealants, detergents, electrical insulation, paints, and medical devices. Medical implants and probes, such as this prototype, are of particular note, as PDMS is non-hazardous, has minimal toxicological effects, and resists moderately concentrated acids and bases. For these reasons, the FDA has approved the use of PDMS in the medical field. PDMS synthesis is an example of ring-opening polymerization, a common form of chain-growth polymerization. In ring-opening polymerizations, the chain iteratively opens cyclic monomers to form successive reactive centers on the polymer. Depending on the system, the reactive center can be radical, anionic, or cationic. This process allows strict control of molecular weight distribution, though this can, in turn, cause issues with extrusion. It has been shown that having some higher molecular weight polymer in the mixture provides a more uniform extrudate.

You've just watched Jove's introduction to addition polymerization. You should now understand the concepts of both polymerization and how viscosity can determine monomer conversion and kinetics. Thanks for watching.

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