Synthesis of core cross-linked star polymers with adjustable coronal properties

A range of functionalized core cross-linked star (CCS) polymers were synthesized via the "arms first" approach and subsequently modified postsynthesis to manipulate the size, density, and chemical composition of the CCS corona. Poly(epsilon-caprolactone)-b-poly(methyl inethacrylate) (PCL-b-PMMA) macroinitiator was used to synthesize a novel CCS polymer with a degradable outer PCL coronal layer. Subsequent hydrolysis of the PCL layer reduced the CCS arm length and effectively reduced the overall hydrodynamic diameter of the polymer. Surface-functionalized CCS polymers capable of initiating either ring-opening polymerization (ROP) or atom transfer radical polymerization (ATRP) front the periphery of the arms were also synthesized through the use of a multifunctional initiator, 2-hydroxyethyl 2'-methyl-2'-bromopropion ate. ROP and ATRP chain extensions from the CCS surface were shown to be possible, resulting in increased arm lengths and hydrodynamic diameters with the initiation efficiency being found to be adversely affected by the sterically hindered structure of the CCS polymer. Core functionalization to generate novel CCS polymer capable of simultaneously initiating ATRP from the core as well as the periphery of the arms was also investigated. ATRP-initiated chain extension of this core/ surface-functionalized CCS polymer was shown to increase the preexisting arm length as well as the number of arms. This resulted in an increased hydrodynamic diameter, with the protected initiating sites within the core being found to undergo a higher degree of polymerization than the functionalized arms.