Termination in Photopolymerization of Methacrylic Monomers Using FTIR and EPR Spectroscopy

There are three primary reactions that define a polymerization: initiation, propagation, and termination. Of these three, the termination reaction is the aspect of crosslinking polymerizations that is least understood and of utmost importance to the properties of the final polymer. Despite the numerous studies of termination 1-5], it has not been thoroughly explained, nor has its behavior been accurately predicted. It is the rapid changes in the polymerization environment leading to the formation of a highly viscous crosslinked network very early in these polymerizations that make the comprehension of the termination behavior such a daunting task. Specifically, at very low functional group conversions in crosslinking polymerizations the termination process becomes reaction diffusion controlled. The mobility of the system becomes limited to the extent that radicals can no longer move together via center-of-mass diffusion. The dominant means of termination then becomes movement of radicals through the network by propagation through unreacted double bonds until they encounter one another and terminate. This mode of termination is called reaction diffusion. When reaction diffusion is the dominant termination mechanism, the kinetic constant for termination is proportional to the product of the propagation kinetic constant and the double bond concentration in the 1, 6-8]. The constant of proportionality is called R, the reaction diffusion coefficient. It is desirable to obtain a more complete understanding of the factors that affect the termination kinetics and the reaction diffusion process. With a more complete understanding of how these termination behaviors correlate to polymerization conditions and starting materials, one can envision the ability to
better control such polymerizations. Ideally, one would like to be able to predict the mechanical
properties of a material from the conditions used during the polymerization, or alternatively to create materials with specific properties purely through an understanding of the kinetics.
Advances in the analytical techniques used to obtain such kinetic information are clearly the mainstays of this understanding process. Over the years, many techniques have been used to probe the kinetics of crosslinkirig photopolymerizations. Tools such as differential scanning calorimetry (DSC) and real-time infrared spectroscopy (RTIR) are commonly used to examine the kinetics of such systems 8-14]. These methods enable monitoring of the decay of reactive functionalities, eg. double bonds, as a function of polymerization time. In this contribution, the use of real-time Fourier transform infrared (FTIR) spectroscopy using a horizontal transmission accessory (FTIR-HT) to obtain information analogous to that which can be obtained with the DSC and RTIR techniques has been examined. This technique combines the fast temporal resolution of the RTIR technique with the ability to obtain entire vibrational spectra of the monomer to polymer transition. The horizontal transmission accessory that has been designed allows for irradiation of the monomer sample in the horizontal position in the sample chamber. This accessory not only facilitates measurements of less viscous monomer samples, but also makes kinetic measurements
of thicker samples possible in conjunction with near-IR data acquisition. This has many advantages in that physical and mechanical property studies can now be performed on the exact samples for which kinetic data has been obtained. The kinetics are also strongly dependent on the radical concentration, which is not directly measured by any of the aforementioned techniques. Correspondingly, the environment and the structure of the radical species are also not available for analysis with these methods. Electron paramagnetic resonance (EPR) spectroscopy provides a means for direct observation of the radical populations formed during such. The ability to monitor the radical species directly provides insight into the kinetics of these reactions as well as into the long-term mechanical properties of the resulting networks. The termination process of the radical species can be observed and the extent of radical trapping in the network can be quantified. This information, in conjunction with the conversion and rate information obtainable from other techniques, provides a more complete picture of the polymerization. In this contribution, a rubbery monomer system has been examined using EPR. Termination kinetics of the stable radical concentrations in this system have been measured and compared to those obtained with the FTIR-HT technique. Moreover, data acquisition at a static magnetic field value has been used to
obtain radical concentration profiles at significantly increased collection speeds over those that have been reported previously, making it possible to calculate the short time termination kinetics in a rubbery system where the initial decay occurs quite rapidly.