Polyamide, commercially named Nylon by DuPont, has been
identified as a crucial class of engineering plastics since 1930s [1–
3]. Polyamide 6 (PA6 or Nylon 6), one of the biggest branches of
polyamide, is broadly used in fibers, autos and composite materials
for the outstanding mechanical properties, corrosion resistance,
chemical inertness and abrasion performance [4,5].
In contrast to the traditional commercial linear PA6 product,
star-branched polyamide 6 (SPA6) has drawn the academic and
industrial interest for the increased performance with potential
advantages in engineering applications [6–9]. Various synthetic
routes to SPA6 have been established through different polymeri-
zation mechanisms. Cationic ring-opening polymerization (ROP) of
e-caprolactam (CL) with starburst poly(ethylenimine) as an initiator
was used to prepare six-arm SPA6 [10]. The resultant polymer
showed decreased viscosity in the melt and in solution and retained
mechanical properties. Dentritic polyamide 6 was reported utilizing
a novel third generation polyamidoamine (PAMAM) dendrimer
initiator [11]. Hydrolytic ROP was the main method to produce PA6
in industry. Three-arm SPA6 was generated in the presence of
trimesinicacid through a hydrolytic mechanism [12,13]. However,
the reaction conditions required long reaction time (>2 h), high
temperature (>200 8C) and pressure (>1 MPa) for both cationic and
hydrolytic polymerizations.