Chapter 9
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 12, 2016 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch009
Synthesis and Photochemistry of Monodisperse Oligomeric-Polymeric Photoinitiators 1
1
Zhiqiang Liu , Matthias Weber , Nicholas and Ben O'Shaughnessy 1
J.
1,2,*
Turro ,
2
2
Departments of Chemistry and Chemical Engineering, Columbia University, 3000 Broadway, M C 3119, New York, NY 10027
Monodisperse polystyrene with a photoinitiator end group was prepared using living free radical polymerization. Nanosecond pulsed laser irradiation was used to generate the oligomeric/polymeric radicals, and the relaxation time constants of the electron spin polarized signal were measured using Time-Resolved Fourier-Transform EPR (TR FT EPR). The preliminary results demonstrate the applicability of this method in the direct experimental study of the properties of the oligomeric/polymeric radicals. This method is expected to allow the study of the chain length dependence of free radical polymerization kinetics with improved accuracy and a larger range of concentration and chain length.
Introduction Free radical polymerization (FRP) is an important method for the industrial preparation of polymers. Typically, the thermal or photochemical decomposition of initiators produces radicals, which can attack monomers and start the growth of polymer chains. In addition to the propagation process in which the chain extends by one monomer unit, the active chains can lose activity and become
© 2003 American Chemical Society
Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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106 dead chains through recombination, disproportionate and chain transfer processes. It is of significant academic and industrial importance to study the details of all of these competing reactions in a systematically varied environment in order to achieve a thorough understanding of the FRP process. The kinetic details depend on many factors including the polymer chain length and concentration. A number of research groups have reported studies of the initiation steps of FRP and their dependence on a series of factors, e.g. solvent viscosity using typical spectroscopic methods. Recent progress in the determination of propagation rate coefficients was made primarily by Pulsed-Laser Polymerization-Size Exclusion Chromatography (PLP-SEC). Measurements of termination rate coefficients have been focused mainly on the Single-Pulse Pulsed-Laser Polymerization (SP-PLP) method. These methods have provided invaluable insights into the kinetics of FRP, but they are typically based on the time evolution of monomer conversion or product molecular weight distribution analysis and, therefore, do not offer direct information on the identity and molecular weight distributions of oligomeric/polymeric radicals. The oligomeric/polymeric systems analyzed were primarily of low conversion. Additionally, in the PLP experiments, time and molecular weight averaged values were obtained during the measurements, and the lack of control on monodispersity of the macro radicals was problematic especially at high conversion. Our interests focus on the direct spectroscopic measurements of the properties of macro radicals with controlled monodispersity, especially the dependence on chain length and environment in the entangled region characterized by a high concentration of long chains. The dynamic properties of polymeric systems in the entangled region display a dramatically different relationship to the chain length compared to the systems below the entanglement threshold. Although the entanglement effects on the properties of macro radicals have received considerable theoretical treatment, experimental results have been largely unavailable. The difficulty in the preparation of large quantities (ca. at millimole scale) of monodisperse polymer samples with suitable photoinitiator groups as end labels using conventional ionic or free radical mechanisms has been one of the limiting factors. These samples can be prepared using the living free radical polymerization (LFRP) methods developed in the past decade. LFRP combines the advantage of living anionic polymerization in the excellent control of monodispersity of the polymer samples and the advantage of conventional free radical polymerization in the ease of operation and tolerance toward a large variety of functionalized end groups. It has been shown that polystyrenes and poly(alkyl methacrylates) with a large variety of end labels and a low polydispersity index (PDI
