Article pubs.acs.org/Macromolecules
Intramolecular Cyclization Dominating Homopolymerization of Multivinyl Monomers toward Single-Chain Cyclized/Knotted Polymeric Nanoparticles Yongsheng Gao,† Dezhong Zhou,† Tianyu Zhao,† Xuan Wei,† Sean McMahon,† Jonathan O’Keeffe Ahern,† Wei Wang,†,¶ Udo Greiser,† Brian J. Rodriguez,‡,§ and Wenxin Wang*,†,¶ †
Charles Institute of Dermatology, School of Medicine and Medical Science, ‡School of Physics, and §Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland ¶ School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China S Supporting Information *
ABSTRACT: Polymerization of multifunctional monomers could produce polymers with different functionalities and novel macromolecular architectures. However, the ability to control the homopolymerization of multivinyl monomers (MVMs) has always been a challenge. Here we demonstrate that the homopolymerization of acrylate based MVMs can be kinetically controlled via Cu0-mediated controlled/living radical polymerization in the presence of additional CuII, which enables the efficient promotion of intramolecular cyclization and suppression of intermolecular cross-linking. The gelation is effectively delayed over ca. 40% monomer conversion in the concentrated polymerization system ([M] = 40.9 wt %), which is far higher than the Flory−Stockmayer theory predicts. Moreover, closer inspection of the synthesized polymers reveals that single-chain cyclized/knotted polymeric nanoparticles (SCKNPs) are formed due to the nature of one-pot in situ intramolecular reaction and self-cyclization of the propagating polymer chains. This facile method opens a new avenue to the design and synthesis of a broad range of novel single-chain cyclized/knotted polymeric materials.
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INTRODUCTION Polymerizations of multifunctional monomers are of great importance to polymer chemists, given that the resulting products should, in principle, contain multiple functional groups and form novel macromolecular architectures by either the involvement of multiple reactive groups during the polymerization process or the postmodifications.1,2 Despite the increased sophistication in the design of the controlled polymerization techniques, the main challenge associated with this process is the use of widely available multifunctional monomers in a convenient, efficient, and environmentally friendly manner to yield the desired topological structure. For instance, the polymerization of multivinyl monomers (MVMs)one kind of the most heavily used monomers would inevitably lead to gelation even at low monomer conversion owning to the presence of significant intermolecular cross-linking reactions according to the classical Flory− Stockmayer theory (F−S theory).3−5 This prevents the formation of controlled macromolecular structures and high monomer conversion rate, which have been verified experimentally numerous times.6,7 There have been continued efforts to control the polymerization process and delay the gelling point for the polymerization of MVMs. Sherrington and co-workers8−10 introduced a large amount of chain transfer agents into the conventional radical copolymerization of monovinyl monomers and MVMs © XXXX American Chemical Society
to inhibit cross-linking and got high monomer conversion. Recently, the evolution of controlled/living radical polymerization (CRP) has also contributed to this field. The CRP of monovinyl monomers and small amounts of MVMs via nitroxide-controlled free radical polymerization (NMP),11,12 atom transfer radical polymerization (ATRP),13−15 and reversible addition−fragmentation chain transfer polymerization (RAFT)16−18 have been utilized to delay the gelling point and attempt to synthesize a soluble polymers with high monomer conversion. Nevertheless, most results indicated that the intermolecular cross-linking degree at the gelling point was still in close alignment with the F−S theory,11−14,18 while others had a slight discrepancy from F−S theory were attributed to the existence of partial intramolecular cyclization.15−17 Compared to the copolymerization of monovinyl monomers and MVMs, the controlled homopolymerization of MVMs appears significantly more challenging, given that each MVM is a potential cross-linker, and hence there are much more “cross-linkers” in the homopolymerization reaction compared to the copolymerization system. Unsurprisingly, most published results indicate that gelation occurred at extremely low monomer conversions (
