In this reaction the free radical attacks the monomer and adds to it. The double bond is broke open and free radical reappears at the far end.
2- Propagation: After initiation reactions, many monomer molecules are added rapidly, perhaps in a fraction of a second. On the addition of each monomer, the free radical moves to the end of the chain.
3- Termination: In the termination reaction, two free radicals react with each other. Termination is either by combination where R represents a long chain of portions or by disproportionation where hydrogen is transferred from one chain to the other. This latter result produces in a two final chains. While the normal mode of addition is a head-to-tail reaction, this termination step is normally head-to-head.
Methodology
Materials used:
The following materials were used in order to conduct the experiment:
- Styrene monomer
- Benzoyl Peroxide (initiator)
- Benzoquinone (inhibitor)
- Nitrobenzene (retarder)
Equipments used:
The following equipments were used in order to conduct the experiment:
- Abbe refractometer
- Sodium light
- Separating funnel
- Vacuum oven
Procedure to be followed:
Note: Before starting the experiment, following measurements were taken to ensure safety:
- Safety glasses were worn at all times while conducting the experiment
- When handling the hot screw-cap jars, gloves were always worn
Stage 1: Firstly, a 110 cm3 of the styrene solution is shaken with an equal amount of dilute sodium hydroxide solution in a separating funnel. The mixture is then shaken twice with an equal volume of deionised water. Once this procedure has been carried out, the aqueous solution is disposed off. By carrying out this procedure, the inhibitor is removed from the styrene.
The monomer left in the funnel, is then dried by shaking it with anhydrous sodium sulphate. This is then allowed to settle in a stoppered conical flask. This leaves a 100ml of Styrene monomer.
Stage 2: Secondly, 1g of Benzoyl Peroxide (initiator) is added to the flask with the 100ml of styrene left from Stage one of this experiment. The mixture is then split into three equal 30ml aliquots.
Sample A: One mixture is left as it is without adding any inhibitor or a retarder.
Sample B: 0.1 wt % of Benzoquinone (inhibitor) is added in to the second mixture of Styrene and Benzoyl Peroxide.
Sample C: 0.5 wt % of Nitrobenzene (retarder) is added in to the third mixture of the Styrene and Benzoyl Peroxide solution.
The three solutions are then placed separately in a small screw-cap jar and placed in an oven heated at 85° C. All three solutions were observed at ten-minute intervals: a few drops from each solution were withdrawn and its refractive index was measured with an Abbe-refractometer. Between every measurement, the prisms of the refractometer were cleaned with a tissue soaked in acetone. This cleared the surface of the refractometer so that the experiment remains fair.
Result and Discussion
Temperature = 85°C
Refractive index of styrene = 1.5468
Refractive index of polystyrene = 1.59 – 1.60
Discussion
As shown in the graphs above, it was found that the induction period of polymerisation was almost steady for all samples within the first 20 minutes. From then the rate of polymerisation changed in each sample.
In sample A where only initiator was used, the rate of polymerisation was spectacularly increased. This means the Styrene was polymerised at a particular rate. Whereas in sample B where an inhibitor called Benzoquinone was added to the solution, the rate of polymerisation is decreased after induction period and the polymerization inhibited for the time. However in Sample C where a retarder called Nitrobenzene was used, demonstrated that the polymerisation takes place but the rate of polymerization is different from Sample A and Sample B.
The results found in this experiment bring the following questions to one’s mind: the roles played by Benzoyl Peroxide, Benzoquinone and Nitrobenzene in this experiment? Why did Benzoquinone and Nitrobenzene slow down the rate of polymerisation? And how does one work out the rate constant for decomposition of Benzoyl Peroxide in polymerisation? These questions are discussed in detail below:
Role of Initiator (Benzoyl Peroxide):
The first step in this experiment was the initiation of Styrene monomer. For this we used Benzoyl Peroxide – this was used to start a chain reaction. In free radical polymerisation the first step is the generation of primary free radicals (free radicals are added electron species and are formed by splitting of a bond symmetrically, so that one of the electrons goes to the other part). The primary radicals are produced by thermal decomposition of Benzoyl Peroxide. When the free radicals are formed it reacts with the monomer. However the dissociation of the initiator is a slow reaction requiring high activation energy. The concentration of the initiator should be low, on the order of 0.1–0.5 percent by weight on monomer content and moderately elevated temperature are employed at 50-85° C.
Role of Retarder/Inhibitor –Nitrobenzene and Benzoquinone:
A retarder is defined as a substance that can react with a radical to form products incapable of adding monomer. The retarder is very effective, no polymer may be formed. This condition is called inhibition. The action of a retarder is twofold: it both reduces the concentration of radicals and shortens their average lifetime. The mechanism of retardation is simply the combination or disproportionation of radicals.
During this experiment, it was noticed that the rate of polymerisation of the Styrene monomer was reduced by adding Benzoquinone (Sample B) and Nitrobenzene (Sample C). The effect is expressed graphically in the graphs on the page above (please refer to page 5) where it was noticed that polymerisation was inhibited later than the induction period. Past experiments conducted have found that Benzoquinone strongly inhibits polymerisation with free radical initiators. The exact mechanism is in doubt, however it can be said that the mechanism involves a transfer reaction between the initiating radicals and Benzoquinone.
The new radical species is inactive so far as the initiation of a polymerisation is concerned because of the delocalisation of the odd electrons. So inhabitation by Benzoquinone is regarded as a test for polymerisation initiated by radicals.
Furthermore, Nitrobenzene in Sample C acted as a retarder without giving rise to induction period. Here again, the transfer is supposed to occur with the production of radical species of lower activity then the original initiator.
Decomposition of Benzoyl Peroxide:
Decomposition of an initiator can usually be followed by analytic methods. The most direct method of finding the initiator efficiency depends upon finding the polymer for initiator fragments. As known previously, the refractive index of polystyrene is 1.59 to 1.60 (please refer to page 4). Through our observations, it has been found that the refractive index in Sample A increased progressively. This shows the decomposition rate of Benzoyl Peroxide takes place in polymerisation.
Conclusion
By performing this experiment it can be summarised that the amount of inhibitor and retarder added to the Styrene solution can affect the rate of polymerisation greatly. Therefore, the greater the content of the polymer, the higher the viscosity of the reaction system and the more rapid the rise in the rate of polymerisation system. The experimental evidence of this study suggests it is possible to control the synthesis of Polystyrene from monomer to polymer.
In addition, it can be concluded that an important application of inhibitors is in the stabilising of monomer. It is dangerous to store polymerised monomers in its pure state on a large scale, since if oxidation producing peroxides or irradiation by ultra-violet light occurred, free radicals could be formed which would then result in polymerisation. This may accelerate to an explosive degree. This is prevented by the addition of a few parts of inhibitors such as the ones used in this experiment (Benzoquinone). Such inhibitors may be removed before the use of the monomer, by distillation, washing with alkali or passing down an ion exchange column.
References
Billmeyer, FW., Text Book of Polymer Science, 3rd ed., John Wiley and Sons, New York, 1984
Cowie, Blakie, J.M.G., Polymers: Chemistry & Physics of Modern Material, 1991
Parker, D.B.V., Polymer Chemistry, 3rd ed., Applied Science Publishers, London, 1974
Williams,D.J., Polymer Science and Engineering, 1971
www.bradely.edu/las/chm/course/351/polymerisation.htm, 14 October 2007, 21:15
Billmeyer, FW., Text Book of Polymer Science, 3rd ed., John Wiley and Sons, New York, 1984, pp. 51 – 55
