Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
Polymers are molecules consisting of many repeating monomer units that are chemically bonded into long chains. They exhibit a broad range of physical properties, which are affected by their chemical structure, molecular weight and degree of polymerization. The polymer industry manufactures thousands of raw materials used in a broad variety of commercial products.1,2
The goal of this video is to perform an addition polymerization reaction and then evaluate the resulting product to understand how viscosity can be used to determine polymer molecular weight. Additionally, this experiment will investigate how molecular weight can be related to monomer conversion.
Many polymers are produced in stirred tank reactors, either batch or continuous. As an example, the polymerization of poly(dimethylsiloxane) (PDMS) is shown in Figure 1. In this reaction, "Me" represents methyl groups and potassium hydroxide is the catalyst. The [Me2SiO]5 is a 5-membered ring which is opened to form the basic building block (the "link") of the polymer. The second product represents finished polymer (it reacts with something called an "endblocker" to stop growth), the first one is a still-growing ("living") polymer. All growth takes place while the chain is attached to the catalyst.
Figure 1: Ring opening polymerization of PDMS.
This is a type of addition polymerization, which is discussed in many kinetics3 and all basic polymer-science textbooks.4 The reaction is mostly thermoneutral and is usually run between 110 - 140 °C and atmospheric pressure. A small amount of molecular weight modifier ("endblocker") is used to stop chain growth, but the catalyst then starts a new chain. Common endblockers are dimethylsiloxanes with trimethylsiloxy end-groups. A "living" chain reacts with the endblocker, forming an end-capped "dead" polysiloxane product with a trimethyl end group.
The Me3SiOK reacts with another polysiloxane to create another trimethylsiloxy end-group. The overall effect is not only the endcapping of the polymer, but also control of the chain length. Average chain lengths (m+n) between 43 - 205 are typical for industrial PDMS in which several different grades of product are synthesized. Because the monomer addition rate >> reaction rate with endblocker (otherwise you would never get to a high molecular weight), the endblocker doesn't influence the reaction kinetics, only the molecular weight distribution.
In analyzing polymerization kinetics, the most difficult step is determining the molecular weight from a physical property, such as kinematic viscosity, and calculating the fraction conversion. The viscosity-average molecular weight, which is measured in this demonstration, is an intermediate measurement with a value in between the number-average and weight-average molecular weight of the polymer. The number average molecular weight is the statistical average molecular weight and indicates that 50 % of the polymer chains are below the number average molecular weight, and 50 % are above. The weight average molecular weight is calculated from the weight fractions in which 50 % of the sample weight consists of chains of lower molecular weight and 50 % consists of chains of higher molecular weight.
Dividing the number average MW by the monomer weight gives the number average degree of polymerization, which is related to fraction conversion. The fraction conversions vs. time are used to determine the order of the reaction as learned in Physical Chemistry and Reactor Design classes.
The system is controlled by running control sequences PS1-PS5 on a standard industrial distributed control system that is operated from a PC. The sequences open/close/adjust valves in the proper sequence and inform when and how to add components to the reactor.
1. Reactor Set-up
- Open the N2 cylinder connected to the reaction vessel.
- Run the control sequence PS1 to test the equipment.
- Then, close the manual valve to the vacuum pump to check that the system is leak-free.
- Wait 5 min and verify that the pressure does not exceed 600 mm Hg.
- Fill the system with N2. Nitrogen is needed for both safety and kinetics reasons. O2 inhibits many polymerizations, and can lead to explosions.
- Run the control sequence PS3 to add monomer to the reactor.
- When prompted by the sequence, add the catalyst solution and endblocker. through a small funnel called the "adder tank".
2. Polymer Fabrication
- Start the PS4 sequence and monitor the reaction temperature.
- Once the temperature reaches >105 °C, frequently collect liquid samples (at least every 8 min) from the sample draw point (caution: HOT, wear thermal gloves).
- Allow the polymerization reaction to run until near equilibrium. Monitor the power usage of the agitator to confirm the reaction has reached equilibrium. Once power has stopped increasing, then the reaction is close to equilibrium.
- Make sure to collect at least 7 samples.
- When done, open the CO2 tank valve and press "RXN COMPLETE" to neutralize the catalyst with CO2. This will occur as part of the PS4 sequence.
- Begin the PS5 stripping sequence.
- Open the manual valve to the vacuum pump and allow stripping to run for 15 min.
- Select "STRIPPING COMPLETE".
- Collect the low boilers from the reaction into a flask.
- Cool the reactor using the automatic cooldown process. Pumpout will be performed much later.
- Follwong the manufacturer's instructions, measure the collected samples with a rotational (cup and bob) viscometer.
- If the rotational speed is set too high, no viscosity reading will be obtained, and a lower speed is chosen. These viscosity values will be used to determine the molecular-weight distribution of the polymer.
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.
The molecular weight can be determined by empirical relationships, such as Barry's relationship for polydimethylsiloxanes with molecular weights above ~2,500.5
This gives the viscosity-average molecular weight. For molecular weight predictions < 2,500, interpolate the experimental data found in Kuo,6 using the kinematic viscosity of the DC-245 monomer for chain length 1. Divide the viscosity (cP) by the polymer density (g/cm3) to obtain the kinematic viscosity in cSt. Divide the viscosity average MWs by 1.6 (empirical factor for PDMS) to get the number average molecular weight, and divide this value by the monomer molecular weight to get the average chain length, (PN)avg, which includes the unreacted monomer.
To get the fraction conversion (fm), start with the mass balance for the average of PN (polymer only):
The left-hand side is the average of PN (polymer only) up to time t, where f = fm. But the average PN that you measure includes the monomer. To account for monomer in (PN)avg, recall that by definition:3-4
The average polymer and (PN)avg for the entire batch are almost equal at the last batch, where fm approaches 1. Compute fm for the last point using a mass balance and the amount of low boilers that was collected. Solve for . For many addition polymerizations, is constant for the entire batch, which allows fm to be computed at all other times from Equation 2. Also, compute the equilibrium constant K (first-order reversible kinetics model) for the reaction by mass balance.
Once fm has been determined as a function of time, assume irreversible kinetics and determine the reaction order with respect to monomer. Use statistical analysis to determine the quality of the fits and the confidence limit on the rate constant kp. Determine the fit for first-order kinetics (expected from theory),3-4 and test if the two fits actually differ.
At similar conditions, others have reported a first-order rate constant of 10-3 s-1 for the DC-245 monomer, and a K > 60.
Figure 2. Typical polymerization results."DOP" = degree of polymerization. The MW's were computed from available data (see ref. 6) or Barry's equation (>2500).5
The workup of representative raw data is shown in Figure 2. These data are for the polymerization of Dow Corning DC-245 monomer. The reaction conditions were: 0.04 wt% catalyst solution, 12 wt% endblocker (modifier), 130 °C and 1 atm pressure. With a relatively large amount of endblocker used, the final degree of polymerization (DOP) was quite low.
In this experiment, 11.36 L of monomer were reacted, and only 15 mL low boilers were recovered, indicating that the data should follow irreversible kinetics. The fit to first-order (in monomer) kinetics is shown in Figure 3 below. The fraction conversions (f) were determined using Equations 1 and 2 with the assumption that the polymer produced is at constant chain length (PN). The resulting fit is reasonable, but not perfect. Slight deviations from the theoretically expected first-order kinetics can arise for several reasons such as diffusional effects, which is when the viscosity increases and the diffusivities decrease significantly. Two other reasons for deviations are suggested by the raw reaction temperature data (temperature oscillations affect the rate constant) and by small leaks that may be present in the pumps, reactor, and heat exchangers. If there are leaks, some O2 could get into the system and gradually inhibit the reaction.
Figure 3. Kinetics analysis. "F" is the 1st order function, the solution of the batch reactor mass balance for a 1st order irreversible reaction.
Applications and Summary
Polymer science provides many examples of the basic principles of chemical kinetics and reactor design. Simple rate expressions can describe fairly complex chemical processes, as in this experiment. Reactor system design must find the optimal reactor type (batch, stirred tank, plug flow, or hybrid) considering kinetics, capital costs, and molecular weight distribution. In particular, the last factor is usually the most important, because it largely defines the product. Depending on this factor alone the product can often range from a hard brittle solid to a rubber to a liquid. A bulk (no solvent) polymerization, such as the one performed in this experiment, has the advantage that subsequent processing to obtain a pure polymer is simple - just strip out the low boilers and filter out the neutralized catalyst. However, the disadvantage of bulk polymerization is that if one loses control of temperature (too high), even in a thermoneutral polymerization, other reactions will dominate and lead to "runaway", which is an uncontrolled exothermic reaction that may result in explosion. Polymerizations with higher heats of reaction are reacted either in solution, suspension (a continuous water phase is present, and the monomer is in droplet form), or in the gas phase.
The major takeaways from the experiment are how one can process raw data of an easily measurable physical property (viscosity) to ultimately determine the monomer fraction conversions and the kinetics of the reaction. Many other physical properties, e.g., density and particle light scattering, are used for this purpose in other polymerizations.
Polymers made by ring-opening polymerizations include Nylon-6 from caprolactam, acetal copolymers with ethylene oxide and dioxolane, which are used in everything from fuel tanks to sprinklers, poly(ethyleneimines), which are used in detergents and cosmetics, and many other silicon-backbone polymers.With the exception of Nylon-6, most of these polymers are commercially produced by anionic or cationic polymerizations. Other polymers that are made similarly include copolymers of styrene (especially with isoprene), isobutene-isoprene (butyl) rubber and its halogenated variants, and poly(alkyl vinyl ethers), which are typically used in paints and adhesives. For some such polymerizations, the chain terminations are so controlled that an almost homogeneous molecular-weight distribution is possible. Except for certain specialty grades, it has been found that such narrow distributions present other problems, such as extrusion difficulties.
Many polymers are vacuum-stripped as the first part of their purification to a commercial product. Among these are the poly(vinylidene chloride) copolymers, poly(chloroprene), and many grades of poly(styrene) and its copolymers such as SAN (styrene-acrylonitrile).
Silicone polymers are used in many products, including lubricants, personal care products, medical devices, antifoams, sealants, waterproof coatings, and as components of detergents, electrical insulation, and paints.8 Medical devices composed of very high molecular weight crosslinked silicone may be approved by the FDA for implantation. More common medical uses are consumables such as catheters, tubing, gastric bags, and surgical incision drains. Commercial PDMS is non-hazardous with a flash point higher than 300 °C, minimal toxicological effects, and good resistance to moderately concentrated aqueous alkali and acids.8,9 It does not corrode most common materials. But like many polymers it can oxidatively decompose, in this case above ~150 °C.
|Rotational (cup and bob) viscometer||Brookfield||Use to measure the viscosity of polymer samples|
|Stirred tank reactor||custom||20 L|
|Reactor Agitator||McMaster-Carr||46-460 RPM; 6-blade, flat turbine (Rushton) type, ~4” diameter.|
|Dimethlysiloxane monomer||Dow Corning||DC-245||specific gravity = 0.956 at 25 °C; viscosity = 4.2 cSt; m = average number of dimethylsiloxanes = 5|
|Endblock A||Dow Corning||10082-147||specific gravity = 0.88 at 25°C; m = 4.5 (not counting the two end groups)|
|KOH catalyst||VWR||470302-140||45 wt% solution in water|
|Nitrogen||Airgas||UHP grade||Used to blanket the system|
|Carbon dioxide||Airgas||Tech. grade||Used to neutralize the catalyst|
Viscosity and Density Data at Low Molecular Weight
Data originally from: Dow Corning.10
|Viscosity, cs, 25 °C||0.65||2.0||10||1000|
|Specific gravity, 25 °C||0.760||0.872||0.935||0.970|
- http://www.essentialchemicalindustry.org/polymers/polymers-an-overview.html and http://www.pslc.ws/mactest/maindir.htm (both accessed 8/22/16).
- MatWeb, Material Property Data, http://www.matweb.com/ and Plastics General Polymers Brand Name Listing, http://www.plasticsgeneral.com/BRAND-NAMES-LIST1.htm (both accessed 8/25/16).
- Fogler, F.S., Elements of Chemical Reaction Engineering, 3rd Ed., Prentice-Hall, 2001, pp. 354-382 (sections 7.1.2-7.1.5).
- Odian, G., Principles of Polymerization, 4th Ed., Wiley-VCH, 2004 (ch. 3), or Rodriguez, F., Principles of Polymers Systems, 2nd Ed., McGraw-Hill, 1982 (ch. 4); Fried, J.R., Polymer Science and Technology, Prentice-Hall, 1995 (ch. 2).
- Barry, A.J., Viscometric Investigation of Dimethylsiloxane Polymers, J. Appl. Phys., 1946, 17, 1020-1024.
- Kuo, A.C.M. Poly(dimethylsiloxane), in Polymer Data Handbook, Oxford University Press, 1999, 411.
- Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2012 or the Encyclopedia of Polymer Science and Technology, 3rd Ed., Wiley-Interscience, Hoboken, 2003-04.
- http://www.dowcorning.com/content/discover/discoverchem/properties.aspx (accessed 8/25/16)
- Shin-Etsu Silicone Fluid Technical Data, Shin-Etsu Silicones of America, Akron, 2005.
- Dow Corning, Product Information, Silicon Fluids, http://www.dowcorning.com/applications/Product_Finder/Products.aspx (accessed 9/23/16).