Solution polymerisation of styrene

Matthew Ong 112891 Solution Polymerization of Styrene

Introduction The production of polystyrene is of important significance in industry. The applications of polystyrene can be seen in various places including our household. Among the many products that are composed of polystyrene, we have plastic cups and insulation materials. A wide variety of applications ask for careful emphasis in the polymerization of styrene. In this experiment, we will be producing polystyrene through solution polymerization. Solution polymerization involves dissolving the monomer and initiator into a solvent. The solvent helps diffuse the heat that is needed for reaction to happen. The mixture is then heated at a temperature that will allow polymerization and the initiation process will begin and the propagation and termination will follow. There are some advantages and disadvantages in using this technique in the production of polystyrene. Some advantages would be the continues diffusion of heat for polymerization, therefore temperature control is much easier, and it allows for easy stirring to avoid viscosity build up in the mixture. A disadvantage would be that a pure polymer is hard to produce, as the process of removing the solvent completely is difficult. Also, high molecular weight polymers are hard to come by because the solvent molecules may also act to terminate the growing polymer chain, which then brings up the issue of finding a suitable solvent for the polymer you are trying to produce.

Materials The apparatus used was quite simple. A 400 ml beaker was used for the oil bath consisting of silicon oil. It is then placed on top of a hot plate that is placed under a metal stand to hold the round bottom flask that will contain the solvent, monomer and initiator. This round bottom flask was submerged into the oil bath. A paper clip was put inside the oil bath to stir. The round bottom flask also consists of a stirrer to help for even heat distribution. A separatory funnel was used to wash the styrene of its inhibitors. This was also used near the end of the experiment for collection of the polystyrene produced. The reagents used were, of course, styrene as the monomer, benzoyl peroxide as the initiator, and toluene as the solvent. Calcium chloride and NaOH was also used to obtain dry uninhibited styrene. This will be discussed further in

the procedures. The reagents and the corresponding amounts used of each are shown in the next table.

Reagents

Amount

Styrene

7.3 ml

Benzoyl Peroxide

0.3 g

Calcium Chloride

Approx. 0.15 g

10% NaOH solution

7ml

Toulene

21 ml Procedure

The first thing we did was to obtain uninhibited styrene. This was done by washing it with 7ml of 10% NaOH solution with the use of a separatory funnel. We did this three times. After which, we added approximately 0.15g of calcium carbonate to dry the uninhibited styrene. 5ml of uninhibited styrene was put into the round bottom flask. For the solvent, 20ml of toluene was poured inside. 3g of benzoyl peroxide, the initiator was placed inside along with the toluene and uninhibited styrene. A 490ml bath consisting of silicon oil was prepared. The bath was then placed on top of the heat mantle and under the metal stand, which served as the holder of the round bottom flask. A paper clip was placed inside the silicon oil bath to stir the bath. A small stirrer was also placed in the round bottom flask to help in distributing heat evenly over the whole mixture. The round bottom flask was then submerged in the oil bath with the help of the iron stand. A thermometer was submerged in the oil bath to enable the monitoring of temperature. The heating plate had adjustors that could change the temperature and speed of stirring. Also, it is to be noted that before submerging the round bottom flask into the oil bath, we first heated the oil bath to reach 70-80 degrees Celsius. Once the oil bath reached this temperature, we submerged the round bottom flask into the beaker that contained the silicon oil. We were to wait 2 hours before taking out the mixture from the bath, which meant we had to be constantly aware of the bath’s temperature and make sure that it didn’t reach values outside of 70-80 degrees Celsius.

After 2 hours, we took out the round bottom flask and allowed it to cool for a few minutes. We transferred it into a 400ml beaker and added methanol, which allowed precipitate to show. No precipitate showed as there was an issue with the used methanol (will be discussed in discussion). We were advised to leave the mixture under the hood overnight to evaporate the methanol and leave the precipitate products alone in the flask. We then ended up using the separatory funnel to separate the two and added clean methanol and obtained some precipitate. We measured the mass of the produce and recorded the data.

Results and Discussion The inhibited styrene before the wash was a clear liquid. After washing, we see a separation between the inhibitor and styrene. Styrene produced a greenish yellow color while the NaOH and inhibitor was colored pink. The calcium chloride used to dry the styrene eventually formed lumps at the bottom of the beaker. Some observations after heating the mixture: upon introducing methanol into the beaker, the mixture turned murky white, which suggests that a reaction did take place in the process. Minimal precipitate was found at the bottom of the beaker, which tells us that it’s of low molecular mass. Strangely however, no precipitate was collected when we tried to filter the mixture. The precipitate seemed to have disappeared upon introduction of new methanol. It was then announced that some of the methanol used was contaminated and could have caused the zero yield we obtained. The mixture was then placed under the hood overnight in the hopes of separating the contaminated methanol and the precipitate. No evaporation happened, so the mixture was put through the separatory funnel and added clean methanol. We were able to collect 0.48 g of polystyrene. We only obtained low molecular weight, which could have been caused by the inefficiencies in controlling the temperature or human error in preparing the corresponding chemicals for the reaction. Many factors could have caused the low amount of product we obtained. To get the % yield, we have to divide the collected polystyrene by the initial amount of styrene present before the reaction. 5ml of uninhibited polystyrene was initially placed in the round bottom flask. The density of styrene is said to be 909 kilograms per meters cubed. Doing unit analysis we can obtain the mass of 5ml of uninhibited polystyrene. This value comes out to be 4.55 g of initial uninhibited styrene. We can now obtain the % yield by dividing 0.48 g by 4.55 g. A percent yield of 10.54% is obtained. This is considered to be low percent yield because we only obtained polystyrene of low molecular mass. This again can be attributed to various factors such as the inefficiency of controlling the heat in the bath and basic human errors.

The polymerization process may take a lot longer if the inhibitor is not removed. Inhibitors slow down or prevent the reaction from happening. Inhibitors may cause not only a slower reaction process but also lower percent conversion and therefore will not be able to maximize the resources used for the reaction to happen. A faster reaction will happen if there is no inhibition. It will also obtain a higher % yield than that of a reaction with inhibitors. Other initiators can also be used in the reaction. Cumyl peroxide can be used as an initiator, potassium persulfate as well. There are various techniques for polymerization. Among these are bulk polymerization and solution polymerization. Bulk polymerization should expect a higher % yield, as it uses no solvent for the reaction. This means that there will be no contamination that may serve as catalysts for degrading the polymer. It is important to understand the key advantages and disadvantages of each polymerization technique to be able to produce the desired polymer in an efficient way.

References:

1.) http://web.stanford.edu/class/cheme160/lectures/lecture13.pdf 2.) http://www.slideshare.net/Santachem/polymerization-techniques 3.) http://home.sandiego.edu/~khuong/Publications/ja0448667_styrene.pdf 4.) http://www.ch.ic.ac.uk/local/organic/tutorial/steinke/StructurePorpertyRelat ionships2003.pdf 5.) http://www.chemicalbook.com/ProductChemicalPropertiesCB3415111_EN. htm