Thermochemistry of Acrylamide Polymerization: An Illustration of Auto-acceleration and Gel Effect

Nedal Y Abu-Thabit

doi:10.12691/wjce-5-3-3

Abstract

Addition polymerization of alkenes is an exothermic process due to the relief of the bond strain from C-C double bonds (π bonds) in the monomer molecules. Control of exothermic reactions is crucial for preventing runaway reactions/explosions in the industrial setting. This experiment was designed for undergraduate students to demonstrate the exothermic nature of addition (chain-growth) polymerization and to estimate the enthalpy of polymerization using adiabatic calorimetry technique. Acrylamide (AAM) was polymerized using ammonium persulfate (APS) and Tetramethylethylenediamine (TMEDA) as redox initiators at room temperature. The concentration of AAM monomer was varied in the range of 5-25 wt. %, and the heat of polymerization was calculated from the recorded temperature rise (ΔT). The enthalpy of AAM polymerization (∆Hp) was estimated to be ≈ 16.5–19 kcal/ mole, depending on the monomer concentration. Enthalpy of polymerization was increased with increasing the monomer concentration due the auto-acceleration effect, which was clearly observed for solutions with AAM concentrations of ≥ 20 wt. %. The auto-acceleration phenomenon was identified from the fast and non-linear increase in the rate of temperature rise which reached a maximum of (0.5°C/ s) for solutions with AAM of 25 wt. %. For AAM concentrations of ≥ 20 wt. %, the auto-acceleration was accompanied by the polymer gelation (gel effect) due to the formation of the high molecular weight polyacrylamides.

Highlights

Addition polymerization of polyolefins represents one of the major industrial processes with annual polymer production of almost 180 million ton and around $200 billion profits value [1]
Students were distributed into four groups to carry out the polymerization reactions with different starting concentration of acrylamide monomer (5, 10, 20 and 25 percent by weight)
The polymerization was carried out in transparent polystyrene double cup calorimeter to enable the direct observation of viscosity changes, water evaporation and gelation

Summary

Introduction

Addition polymerization of polyolefins represents one of the major industrial processes with annual polymer production of almost 180 million ton and around $200 billion profits value [1]. Addition (chain-growth) polymerization is characterized by its exothermic nature with enthalpy of polymerization in the range of 15-20 kcal/mole depending on the monomer structure and the employed polymerization conditions [2,3]. Addition polymerization is driven by enthalpy change due to the relief of bond strain from the strained C-C double bonds (π bonds), which results in releasing high amounts of heat from the exothermic polymerization. One of the most important aspects of olefin polymerizations is the control of reactor temperature to avoid heat transfer problems resulted from the highly exothermic alkene polymerization. Estimation of the polymerization enthalpy is crucial and very useful for optimization of heat transfer as well as scale-up of polymerization reactions.

Methods

Results

Conclusion