Well-defined long sub-chain hyperbranched poly(methyl methacrylate) (lsc-hp PMMA) was obtained under reaction conditions that disfavour self-cyclization of the seesaw macromonomer, and its formation was triggered by the intrinsic hindrance of the 1,1-disubstituted chain ends of the seesaw macromonomer. Firstly, seesaw-type alkynyl-(PMMA-Br)₂ with one alkynyl group at the chain center and two bromine groups at each chain end was synthesized by atom transfer radical polymerization (ATRP). After the azidation of alkynyl-(PMMA-Br)₂, the seesaw macromonomer of alkynyl-(PMMA-N₃)₂ underwent a click reaction in a high concentration in a good solvent to produce lsc-hp PMMA, almost without intra-chain cyclization of the macromonomer. Compared with our previous reports, the steric hindrance of 1,1-disubstituted MMA units caused almost no self-cyclization of the alkynyl-(PMMA-N₃)₂ macromonomer. Therefore, lsc-hp PMMA with an exact sub-chain length was obtained except the polydispersity of overall molecular weight. The chemical structure of lsc-hp PMMA was fully confirmed through the combination of gel permeation chromatography (GPC) with different detectors, proton nuclear magnetic resonance spectroscopy and Fourier transform infrared analyses. Furthermore, the formation kinetics for lsc-hp PMMA was monitored based on GPC with a multi-angle laser light scattering detector and followed the equation: ln[(DP_w + 1)/2] = [A]₀k_AB,0(1 − e^−αt)/α, where DP_w is the absolute weight-average amount of macromonomers in the hyperbranched polymers.