wA novel AB(n)-type dendrimer/linear polymer block copolymer, i.e., poly(trimethyleneimine) dendrimer-block-(polysarcosine)(64) (1), was synthesized by ring-opening polymerization of sarcosine N-carboxyanhydride initiated with the 64-NH2-terminal poly(trimethyleneimine) dendrimer as a macroinitiator. 1 has narrow molecular weight distributions ((M) over bar(w)/(M) over bar(n) = 1.0(1)-1.0(3), by size exclusion chromatography) and controlled polysarcosine chain lengths (by varying the monomer/dendrimer feed molar ratios). Small-angle neutron scattering (SANS) data obtained in D2O solution of 1 (DP's of polysarcosine = 2.0 and 24) fitted well with a Guinier plot of a spherical particle, and gave diameters of 44 and 100 Angstrom, respectively.