Introduction The regeneration of monomers from plastic waste followed by their repolymerization is a — Physical Chemistry — Thermodynamics Chemistry Question

Introduction
The regeneration of monomers from plastic waste followed by their repolymerization is an ideal recycling method, especially when the plastic waste is dirty, variously coloured or contains filling materials. Unfortunately, only a few polyolefins depolymerize into their monomers when heated. One example is polymethylmethacrylate (PMMA, Plexiglas), a plastic that starts to depolymerize into its monomers at 150 °C. At temperatures between 300 °C to 350 °C, the reaction is quantitative, the polymer chains are decomposed consecutively and the formation of the fragments is not statistical:
[VISUAL]
In the case of polymethylmethacrylate, the reformation of monomers proceeds in a high yield, as during the pyrolytic degradation tertiary radicals are formed from the quaternary carbon atoms. These are more stable and chemically less reactive than the corresponding secondary and primary radicals. Hence, degradation is the preferential reaction compared to other radical reactions such as recombination. Subsequent polymerization of the purified monomer gives a product that cannot be distinguished from the starting material.

Equipment
- Bunsen burner
- 2 small test tubes (diameter ca. 3 cm)
- rubber stopper that fits to the test tube with a hole bored through a right-angled bent glass tube (inner diameter ca. 0.5 cm) that is passed through the hole of the stopper
- test tube (diameter ca. 2.0 cm)
- rubber stopper that fits to the test tube with a hole bored through
- a straight glass tube (reflux condenser, inner diameter ca. 0.5 cm) that is passed through the hole of the stopper
- ice water bath (for cooling)
- distillation apparatus with thermometer and 50 cm₃ distillation flask
- sand bath on heating plate or heating mantle (50 cm₃)
- stand

Chemicals
- Polymethylmethacrylate (pulvarized), 30 g, or
- Polymethylmethacrylate-waste, 30 g, (for example covers of rear lamps that have been pulvarized)
- dibenzoylperoxide (C₁₄H₁₀O₄), 0.6 g

The experiment should be carried out in a fume hood. Avoid inhaling methyl methacrylate produced in the experiment and do not allow it to come into contact with the skin.

Procedure
Fill a weighed test tube with small pieces of polymethylmethacrylate-waste to about one third and weigh the filled tube. Set up the apparatus shown in figure 1. The apparatus should be clamped to a stand.
Figure 1: Experiment set-up for the pyrolysis of polymethylmethacrylate. [VISUAL]

Heat the test tube containing the plastic waste carefully with a Bunsen burner (move the Bunsen burner continually to ensure uniform heating of the plastic and to prevent the liquid foaming). If bubbles are formed in some parts of the melt, heat more strongly but do not overheat. Overheating causes effervescence of the melt and the resulting vapour can no longer be condensed. In the cooled test tube a fruity smelling liquid is formed which can have a variety of colours, depending on the nature of the dyes carried over with it.
Transfer the liquid to a distillation flask, add boiling chips and support the flask on a sand bath arranged in a way that the level of the sand is about the same height as the condensate. Distill under atmospheric pressure and collect the methyl methacrylate. The product is a colourless liquid. Determine the boiling point of methyl methacrylate.
Place 8 g of the purified methyl methacrylate into a large, carefully dried test tube, add 0.6 g of dibenzoylperoxide and mix the two components using a glass rod. Place a rubber stop containing a straight piece of glass tubing, that will act as a condenser, in the neck of the test tube and clamp it to a stand. Heat the mixture cautiously with a small Bunsen flame till an exothermic reaction takes place. Within minutes, a hard and bubbly plastic is formed.

Disposal:
Test tubes that were used for the depolymerization can be reused in the same experiment, as any plastic residual in them will not interfere with any subsequent reaction.
Sources of error
In some cases, the repolymerization does not readily take place. If there is no observable reaction, the mixture should be heated in a water bath for about 10 minutes.