Experimental Characterization of Structural Features during Radical Chain Homopolymerization of Multifunctional Monomers Prior to Macroscopic Gelation J. Brian Hutchison, † Alex S. Lindquist, † and Kristi S. Anseth* ,†,‡ Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0424, and Howard Hughes Medical Institute, University of Colorado, Boulder, Colorado 80309-0424 Received September 18, 2003; Revised Manuscript Received March 22, 2004 ABSTRACT: An experimental methodology is presented to characterize the evolution of structural heterogeneities in cross-linked polymer networks formed by homopolymerizations of multifunctional monomers. In particular, intramolecularly cross-linked macromolecules (ICMs) were synthesized by individual initiation events and characterized. A set of complementary experimental tools, including atomic force microscopy (AFM), size exclusion chromatography, light scattering, and NMR, is used to provide information about size, intramolecular cross-link density, and kinetic chain length for poly(methacrylic anhydride) (PMA) ICMs. Images of PMA ICMs, which contain ca. 1000-2000 monomers, on average, are captured with AFM. Varying polymerization temperature from -13 to 70 °C led to an increase in the apparent size of the ICMs and an increase in the fraction of doubly reacted monomers. Furthermore, ICMs synthesized in the presence of tetraethylthiuram disulfide (TED), a photoiniferter precursor, were characterized. Iniferter-mediated polymerization of methacrylic anhydride created smaller ICMs with higher intramolecular cross-link density. Introduction Investigations of Structural Evolution. The de- velopment of heterogeneity during the evolution of highly cross-linked networks from homopolymerizations of multifunctional monomers impacts many applications and is an important area of research. 1-12 Current commercial applications of multifunctional monomers that would benefit from a better understanding of their cross-linked network structural evolution include abra- sion-resistant coatings, dental materials, and flexo- graphic printing plates, among others. A more complete understanding of nanoscale evolution of polymeric materials is critically important for their implementa- tion in microelectronics and microdevice technologies as well as higher resolution applications such as molecular templating, 13,14 data storage, and quantum computing that necessitate nanoscale control and functionality. The size and structure of the heterogeneous regions, along with relationships to the monomer structure and initiation conditions, are not well understood or char- acterized. With a better understanding of reaction and reactant parameters that influence structural evolu- tion during multifunctional monomer polymerizations, opportunities exist to control and enhance the struc- ture and properties of the resulting macroscopic net- works. While researchers are making significant progress in this area, most results provide indirect measures of heterogeneous structure evolution. For example, regions of more highly cross-linked polymer within an evolving network have been attributed to enhanced reactivity of functional groups near sites of active radicals. 1,2 The same phenomenon can lead to trapping of propagating radicals within highly cross-linked regionssmicrogels. 3,15 Although the small size of the heterogeneous regions precludes the use of spectroscopic techniques or visible light microscopies, information about the development of heterogeneity in multifunctional monomer polymer- izations has been obtained via electron microscopy, 6 film surface ablation, 16 mechanical property measure- ments, 10,17 and fluorescence and NMR spectros- copies. 12,18-20 Finally, a body of information, gathered from simulations of cross-linked network evolution, has provided substantial insight into structural evolu- tion. 2,3,5,6,12,21,22 However, the rich information available from a simulation requires validation with analogous experimental data and observations. Microgel regions and other structural characteristics within infinite networks at moderate to high conversion have been observed and characterized experimentally. Although formation of microgels with characteristic length scales of 50-100 nm must be preceded by single initiation events, very few experimental analyses of structural evolution at the level of individual initiation events have been demonstrated previously. The aim of this contribution is to develop and present an experimental methodology to characterize macro- molecules formed from single initiation events during homopolymerization of multifunctional monomers. The specific results relate to one monomer system with variation of two reaction conditions. In fact, the specific results are only demonstrating the true significance and innovation of this work. For the first time, a diverse set of complementary experimental tools is used together to provide information about heterogeneous features in cross-linked networks by examining size, intramolecular cross-link density, and kinetic chain length for macro- molecules formed by individual initiation events. Intramolecularly cross-linked macromolecules (ICMs) formed by very dilute initiation of pure methacrylic anhydride at various temperatures are characterized, and several questions are explored: What is the char- † Department of Chemical and Biological Engineering. ‡ Howard Hughes Medical Institute. 3823 Macromolecules 2004, 37, 3823-3831 10.1021/ma035400w CCC: $27.50 © 2004 American Chemical Society Published on Web 04/23/2004