Kinetics of Addition Polymerization to Polydimethylsiloxane

Overview

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.

Procedure

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 sy

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Results

The molecular weight can be determined by empirical relationships, such as Barry's relationship for polydimethylsiloxanes with molecular weights above ~2,500.⁵

Equation 2

This gives the viscosity-average molecular weight. For molecular weight predictions < 2,500, i

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Application 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

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References

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, 3^rd Ed., Prentice-Hall, 2001, pp. 354-382 (sections 7.1.2-7.1.5).
Odian, G., Principles of Polymerization, 4^th 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).

Tags

Kinetics Addition Polymerization Polydimethylsiloxane Molecular Weight Degree Of Polymerization Bulk Properties Viscosity Density Light Scattering Batch Polymerization PDMS Initiator Monomer Polymer Chain Addition Polymerization Mechanism Reactor Types

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.
1. Open the manual valve to the vacuum pump and allow stripping to run for 15 min.
2. 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.
1. 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.

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0:07

Overview

1:18

Principles of Polymerization

3:48

Polymer Synthesis

6:06

Results

8:08

Applications

9:40

Summary

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