Structural Control in ATRP Synthesis of Star Polymers Using the Arm-First Method and their Applications to the Medical Field
Polymers are large molecules composed of small repeating units called monomers. There is a unique type of polymer called star polymers because of their shape.1 They consist of a central core with attaching monomer arms that can form branches.1 Interest in the synthesis of star-shaped polymers began in the 1950’s because of living anionic polymerization.1 By living I mean that the polymers ability to terminate has been removed. Recently, star-shaped polymers have attracted even more interest because of the development of Controlled Radical Polymerization ...view middle of the document...
2 Star polymers are usually characterized using size exclusion chromatography (SEC), gel permeation chromatography (GPC), vapour phase osmometry (VPO), and nuclear magnetic resonance (NMR).2
Star polymers can be synthesized using a variety of techniques. The “core-first” method uses a multifunctional initiator which is a compound that can act in multiple ways to initiate polymerization.1 In this method the number of arms per star polymer is determined by the number of initiating functionalities on each initiator.1 In comparison to this method, the “arm-first” method involves the synthesis of straight preformed polymer arms.1 This method can be subdivided into two more groups: the first is linear chain coupling with a multifunctional terminator, and the second is a chain extension of the linear arm precursor with a multivinyl cross-linking agent. Employing the “core-first” method, the average number of arms per star is determined by the function of the terminator, and the resulting star polymer contains no more initiating sites.1 This reaction method is particularly slow and results in a low coupling efficiency.1 With this method there are usually multiple stars with varying numbers of attached arms.1 On the other hand, the “arm-first” method of star polymer synthesis contains a core that is cross-linked with a microgel and each star polymer has approximately the same number of arms.1 Star polymers from this method can also initiate polymerization of other monomers due to the initiating sites of their cross-linked core are still “living.”1 The star polymers synthesized from this method are called miktoarm star copolymers.1 The “arm-first” method is much more effective than the “core-first” method because the multivinyl cross-linker is simpler for the synthesis of star polymers.1 Star polymers for the “arm-first” method contain arms that vary in length and initiating sites that can be further modified.1 Another strategy for synthesizing star polymers that just recently starting to become more common is the “Coupling onto” process.2 In this method a linear telefunctional molecule is mixed with a premade core molecule having complementary functionality.2
The atom transfer radical polymerization (ATRP) is a very common way to synthesize star polymers using the “arm-first” method.1 By using ATRP, star polymers can protect their terminal halogen atoms within their core.1 These halogen atoms can be effectively used as multifunctional macroinitiators (MI) to extend the chains from the core by forming a second set of arms.1
During the first step of the experiment, ATRP of tert-butyl acrylate (tBA) was done using ethyl 2-bromopropionate (EBrP) as the initiator and CuBr/N,N,N’,N’’,N’’-pentametyldiethylenetriamine (PMDETA) as a catalyst.1 At specific conversions of the tBA monomer, nitrogen-purged divinylbenzene (DVB) was injected into the system as a cross-linker.1 This cross-linker copolymerized the remaining tBA monomer and produced the intended star...