Polymer Chemistry PAPER Cite this: Polym. Chem., 2016, 7, 5512 Received 21st July 2016, Accepted 12th August 2016 DOI: 10.1039/c6py01265e www.rsc.org/polymers Synthesis of acid-degradable hyperbranched polymers by chain-growth CuAAC polymerization of an AB 3 monomer† Lei Zou, Yi Shi, Xiaosong Cao, Weiping Gan, Xiaofeng Wang, Robert W. Graff, Daqiao Hu and Haifeng Gao* A tetrafunctional AB 3 monomer that was composed of one alkynyl group, three azido groups and one acetal linker was used in the one-pot copper-catalyzed azide–alkyne cycloaddition (CuAAC) poly- merization for producing acid-degradable hyperbranched polymers (HBPs). In various feed ratios of the AB 3 monomer to a B 3 core, the polymerizations demonstrated a chain-growth mechanism with a linear increase of molecular weight versus conversion, low polydispersity and a high degree of branching (DB). The large amount of terminal azido groups on the HBPs periphery were further modified via reaction with an alkynyl-terminated poly(ethylene glycol) (PEG) to produce water-soluble PEGylated HBPs. Under acidic conditions, both the HBPs and the PEGylated HBPs exhibited clean and fast degradation into low-molecular weight compounds, confirming the labile acetal linkers in the backbone of HBPs. Introduction Hyperbranched polymers (HBPs) have received considerable interest in the past few decades mainly because of their analogous structures to dendrimers and effortless one-pot synthesis under robust conditions. 1–6 Unlike dendrimers, HBPs can be easily synthesized using different monomers in one pot, including the polymerization of AB m (m ≥ 2) monomers, 7–26 the copolymerization of A n and B m monomers (e.g.,A 2 +B 3 ), 6,27–32 the copolymerization of divinyl cross- linkers with monovinyl monomers, 33–40 and the self- condensing vinyl polymerization (SCVP) of polymerizable initiators (e.g., inimer). 41–52 However, the current challenge on these convenient syntheses is the lack of structural control of the HBPs products, 53,54 which significantly undermines the physical properties and the potential applications of HBPs. Very recently, our group developed a one-pot synthesis of polytriazole-based HBPs with both low polydispersity and a high degree of branching (DB) using copper-catalyzed azide– alkyne cycloaddition (CuAAC) polymerization of AB 2 -type monomers. 22,23,54 An intriguing feature in this CuAAC polymerization was that the complexation of Cu(I) to the in situ formed triazole groups confined all added Cu in the polymer at a low monomer conversion. This confinement of the Cu catalyst directly resulted in subsequent selective polymer– monomer CuAAC reactions to demonstrate the chain-growth polymerization features, i.e., a linear increase of polymer mole- cular weights versus monomer conversions and clean chain extension via sequential monomer addition in one pot. 55 As compared to the vast reports in the literature on the use of AB 2 -type monomers to produce various HBPs, the polymer- ization of AB 3 -type monomers has been much less studied. 25,56 It is generally expected that the use of AB 3 monomers, com- pared to the AB 2 monomers, should produce HBPs with higher DB and more peripheral functional groups. Herein, we designed an AB 3 -type monomer by incorporating one alkynyl group, three azido groups and one acid-degradable acetal linker into one monomer structure (Scheme 1). The one-pot CuAAC polymerization of this AB 3 monomer demonstrated a chain-growth feature and produced HBPs with low polydisper- sity and very high DB (close to unity). The acid-triggering hydrolysis of the acetal linker in the HBP was further investi- gated, resulting in fast degradation of the HBPs into small molecular fragments. Results and discussion The AB 3 monomer containing an acetal linker was synthesized in three steps with detailed procedures discussed in the ESI (Fig. S1 and S2†). Subsequent CuAAC polymerization of this monomer using a highly reactive triazido core B 3 , 30 under the conditions of [AB 3 ] 0 : [B 3 ] 0 : [CuSO 4 ·5H 2 O] 0 = 1000 : 1 : 10 was † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c6py01265e Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556-5670, USA. E-mail: hgao@nd.edu 5512 | Polym. Chem. , 2016, 7, 5512–5517 This journal is © The Royal Society of Chemistry 2016 Published on 15 August 2016. Downloaded by University of Notre Dame on 22/11/2016 03:53:42. View Article Online View Journal | View Issue