On the Activation Energy of Termination in Radical Polymerization, as Studied at Low Conversion
Year: 2024
Type: article
Abstract: The chain-length-dependent nature of the termination reaction in radical polymerization (RP) renders the overall termination rate coefficient, , a complex parameter in the usual situation where the radical chain-length distribution is non-uniform. This applies also for the activation energy of termination, Ea( ), which we subject to detailed mechanistic investigation for the first time. The experimental side of this work measures Ea( ) for the dilute-solution, low-conversion, chemically initiated homopolymerization of styrene (ST), methyl methacrylate (MMA), butyl methacrylate, and dodecyl methacrylate. Values of 25–39 kJ mol−1 are obtained, consistent with strong chain-length-dependent termination (CLDT) for short chains. On other hand, the reanalysis of analogous bulk polymerization data for ST and MMA finds Ea( ) values of 18–24 kJ mol−1, consistent with weak CLDT for long chains. Both these results are as expected from the so-called composite model for CLDT. A simple analytic framework for understanding and predicting Ea( ) values is presented for the standard RP situation of continuous initiation. All the results of this work can be rationalized via this framework, which clearly establishes that Ea( ) is determined by far more than just the Ea of radical diffusion. This framework is extended to activation energy for the number-average degree of polymerization, Ea(DPn), which we measure and successfully scrutinize via our CLDT model. In the final section of this work, we make interesting, testable predictions about Ea( ) and/or Ea(DPn) in various RP systems of different natures to those studied here, most notably, systems involving acrylates, continuous photoinitiation, or dominant chain transfer.
Source: Polymers
Authors: Majed M. Alghamdi, Gregory T. Russell
Institutions: University of Canterbury, King Khalid University
Language: English
Subfield: Organic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,069
Funder: King Khalid University
OpenAlex ID: W4404568624