Recently in my organic chemistry lab we created Poly(methyl methacrylate) (plexi

Question

Recently in my organic chemistry lab we created Poly(methyl methacrylate) (plexiglass) and polystyrene. The plexiglass was created using benzoyl peroxide as its initiator and resulted in a clear, very hard solid as was expected. The polystyrene was created using Azobisisobutyronitrile as the initiator however this reaction was carried out in a toluene solution. The resulting polystyrene at the end of the experiment was a small white ball that was soft and very malleable.

To the best of my knowledge polystyrene should not be malleable and shapable like it was. My question is why, when both reactions are carried out using similar free radical mechanims, were the physical charachteristics of the resulting polymers so different. What causes the plexiglass to be so hard? Why was my polystyrene able to be pulled and stretched? Is it as simple as saying that since it was carried out in solution it was like that and if it were allowed to dry properely it might more colsely resemble styrofoam?

Also my plexiglass has bubbles in it. Is it safe to assume that those are carbon dioxide bubbles since I used benzoyl peroxide?

Explanation / Answer

(a) To study the bulk polymerization of undiluted MMA to PMMA initiated by a single radical initiator at elevated temperature and to determine volumetric shrinkage after conversion is complete. (b) To polymerize MMA in the presence of PMMA at room temperature by addition of a redox pair (double radical initiator) [benzoyl peroxide (BP) -dimethyl paratohidine (DMPT)] and to follow percent conversion by the temperature-time course. (c) To study polymerization of MMA to PMMA as it occurs in a commercial kit used by orthopedic surgeons to make bone cement and to follow the conversion by the temperature-time course. Materials and Apparatus: Prepared master solutions of 100 g PMMA in 500 ml MMA (per vial) MMA monomer DMPT (N,N Dimethyl para toluidine) Benzoil peroxide (BP) Bone cement packets made by Howmedica: Surgical Simplex P vials/testtubes (>10ml) (note: polystyrene is not appropriate for this experiment. Why?) Graduated conical polypropylene centrifuge tubes, 15 ml capacity Vials Al foil Heaters Thermometers Water bath Plastic floats to insulate water bath Brief Background: We have already studied several aspects of free radical polymerization. In this experiment, we shall study the free radical polymerization of MMA to PMMA. This polymerization also allows us to study the Trommsdorf effect because we will do the polymerization in the bulk rather than solution. We shall also see a practical application of this polymerization viz., its use in joint replacement as bone cement. Free radical polymerization: (Read expt. 1, 4 lab instructions "Brief Background" section and re-read Rempp & Merrill, Ch 3) Free radical polymerizations are usually performed using one of four different methods: bulk polymerization, solution polymerization, suspension polymerization and emulsion polymerization. This experiment illustrates certain aspects of bulk free radical polymerization. We shall study suspension polymerization in experiment 13. For a brief introduction to different kinds of free radical polymerization, read the excerpt from Young and Lovell, Introduction to Polymers, Chapman & Hall, 1991, pg. 63-68. Bulk free radical polymerization of MMA to PMMA: In this experiment, we are polymerizing MMA to PMMA: (1) CH3 CH3 H2CC CC O H2 O H3CO H3CO n MMA: MW=100 g/mol PMMA: Tg~100 C Tm = -48 C no Tm Tb = 100 C no Tb, decomposes density = 0.94 g/cm3 The redox system of initiators that we shall use are: (2) OO O O H3C N + DMPT Benzoyl Peroxide OO O O + H3C N + As in experiment 1, the equations describing the process are: conversion X = 1

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