Exploring Self-Condensing Vinyl Polymerization of Inimers in Microemulsion To Regulate the Structures of Hyperbranched Polymers Robert W. Graff, Xiaofeng Wang, and Haifeng Gao* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States * S Supporting Information ABSTRACT: A synthetic method was successfully developed to produce structurally defined hyperbranched polymers using confined micellar space in microemulsion to regulate atom transfer radical polymerization (ATRP) of inimers. Systematic exploration of experimental variables, including five methacrylate-based inimer species, two ATRP ligands, and varied amounts of inimers and catalysts, produced a series of hyperbranched polymers that encompassed a broad range of molecular weights (M n = 194-1301 kg/mol), high degrees of branching (DB = 0.26-0.41), and narrow molecular weight distribution (M w /M n = 1.1-1.7). The ATRP of inimers in the microemulsion media showed a fast polymerization rate with quantitative conversion of methacrylate groups within 0.5 h. At high conversion, there was essentially one hyperbranched polymer per discrete latex particle, whose dimension (hydrodynamic diameter D h = 10.95-20.13 nm in water) and uniformity directly determined the molecular weight and polydispersity of the hyperbranched polymer. The DB of hyperbranched polymers was quantitatively determined using inverse gated 13 C NMR spectroscopy, and its value was affected by several parameters, all related to the effective amount of copper catalysts in the polymerization loci for dynamic ATRP exchange reactions. The use of inimers and ligands that showed high copper complex solubility and a high feed ratio of copper to inimer could increase the concentration of copper catalyst in the discrete particles and consequently the DB value. Within the investigation, the polymerization of inimer 3 using 4,4′-dinonyl-2,2′-dipyridyl (dNbpy) as ligand produced hyperbranched polymers with the highest DB = 0.41 due to the high solubility of Cu(II)/(dNbpy) 2 in inimer 3. When acetal group as a linker was incorporated into the inimer, the produced hyperbranched polymers exhibited complete degradation in acidic environment, indicating potential utility in biomedical applications. ■ INTRODUCTION Both dendrimers and hyperbranched polymers are considered as important types of highly branched polymers that show compact structures, large interior volumes, and multiple chain-end groups. 1-5 However, these two types of polymers differ on many aspects. Dendrimers feature a perfectly branched and monodisperse structure by means of multistep iterative syn- thesis. 2,6,7 In contrast, hyperbranched polymers have a straightforward one-pot synthesis but limited control over their molecular weight, molecular weight distribution, and branching density. 3,4,8 It has been a challenging problem for years to develop robust synthetic methods that can produce hyper- branched polymers in one-pot with better controlled structures. 9 Hyperbranched polymers are traditionally synthesized in solution using the step-growth polymerization of AB f monomers (containing one A group and f (≥2) B groups) 3,4,10,11 and/or self-condensing vinyl polymerization (SCVP) of AB* inimers (containing initiator fragment B* and monomer vinyl group A in one molecule). 12-14 In both cases, the polymers present undesirable structures with broad molecular weight distribu- tions, 15 mainly due to the random polymer-polymer coupling reactions occurring in the continuous reaction media throughout the polymerization. 12 In the past two decades, considerable efforts have been reported to decrease the polydispersity 16,17 and increase the degree of branching (DB) of the hyperbranched polymers using multifunctional cores, 18,19 tuning the monomer addition speed, 18,20 and varying the monomers’ reactivity. 21,22 However, all these methods require either delicate monomer addition or sophisticated monomer synthesis, 9 which limit the synthetic ease and the production of hyperbranched polymers with high molecular weights. Recently, our group developed a new approach to synthesize hyperbranched polymers with both high molecular weight and narrow molecular weight distribution using confined nanospace, e.g., micelles, to regulate the polymer-polymer coupling reactions. 23 The key concept in this new method is to segregate the polymerization of reactive monomers into discrete micelles, so that the monomers inside each micelle or polymerizing locus can react/polymerize with each other completely, while there is no intermicelle reaction. In this present report, we systematically Received: February 9, 2015 Revised: March 11, 2015 Published: March 24, 2015 Article pubs.acs.org/Macromolecules © 2015 American Chemical Society 2118 DOI: 10.1021/acs.macromol.5b00278 Macromolecules 2015, 48, 2118-2126