A method was developed to synthesize molecular brushes with polymethacrylate backbone and ultrahigh density of grafted side chains (SCs), i.e., 1.34 SCs per backbone carbon atom, using accelerated copper-catalyzed azide alkyne cycloaddition (CuAAC) grafting-onto strategy. This acceleration effect that benefits from the complexation of triazole with Cu was first confirmed in two model CuAAC reactions of (a) 1:1 molar ratio of a diazide compound and an alkynyl-terminated poly(ethylene oxide) (ay-PEO18 with average degree of polymerization DP = 18) and (b) 1:1 molar ratio of a dialkyne compound and an azido-terminated N-3-PEO18. It was found that both model reactions produced ditriazoles as major products, although the former reaction exhibited a higher yield of PEO-PEO dimers, demonstrating better CuAAC acceleration effect. Following this principle, polymethacrylate backbones with multiazido dangling groups were subsequently used for grafting-onto reaction with ay-SCs to prepare an array of molecular brushes with high grafting densities. Within our investigation, all these CuAAC grafting reactions finished within 10 min and introduced different SCs, including PEO18, PEO113, poly(methyl acrylate) (PMA(31)), and polydimethylacrylamide (PDMA(46)). The grafting density was affected by the composition of SCs and the initial molar ratios of ay-SCs to azido groups. When applying linear SCs with thinner structure, such as ay-PEO113, the highest grafting density was obtained (1.34 SCs per backbone carbon atom) on both longer polymethacrylate backbone (DP = 430) and shorter backbone (DP = 180).