The mechanisms of the Michael addition polymerizations of different trifunctional amines with an equimolar 1,4-butanediol diacrylate (BDA) were investigated by using H-1 and C-13 NMR to in situ monitor the polymerization processes. The trifunctional amine monomers adopted were 1-(2-aminoethyl)piperazine (AEPZ) and 4-(aminomethyl)piperidine (AMPD) and linear aliphatic amines with different steric hindrance on the 2degrees amines (original), i.e., N-methylethylenediamine (MEDA), N-ethylethylenediamine (EEDA), and N-hexylethylenediamine (HEDA). For AEPZ, AMPD, and MEDA with low steric hindrance on the 2degrees amines (original), the reactivity sequence of the three types of amines was 2degrees amines (original) > 1degrees amines much greater than 2degrees amines (formed) and linear poly(amino ester)s were formed via AB-type intermediates due to the 2degrees amines (formed) being kept out of the reaction. However, AB-type intermediates were only formed from the 2degrees amines (original) for AEPZ and AMPD, but from both the 2degrees amines (original) and 1degrees amines due to their smaller reactivity difference for MEDA. Nevertheless for EEDA and HEDA, the increased steric hindrance on the 2degrees amines (original) changed the reactivity sequence of the three types of amines to 1degrees amines > 2degrees amines (original) > 2degrees amines (formed) and branched polymers with degrees of branching of ca. 33% and 37%, respectively, were obtained due to all the three types of amines participating in the reaction. The molecular weights, glass transition temperatures, and thermal stability of the linear or branched poly(amino ester)s obtained were characterized by GPC, DSC, and TGA, respectively.