Amphiphilic PEG-Functionalized Gradient Copolymers via Tandem Catalysis of Living Radical Polymerization and Transesterification

Abstract

Amphiphilic gradient copolymers with poly-(ethylene glycol) pendants were synthesized via tandem catalysis of ruthenium-catalyzed living radical polymerization (LRP) and titanium alkoxide-mediated transesterification. The gradient sequence can be catalytically controlled by tuning the kinetic balance of the two reactions. The tandem catalysis is one of the most efficient and versatile systems to produce amphiphilic gradient and sequence-controlled copolymers. Typically, methyl methacrylate (MMA) was polymerized as a starting monomer with a ruthenium catalyst and a chloride initiator in the presence of Ti(Oi-Pr) 4 and molecular sieves (MS 4A) in poly(ethylene glycol) methyl ether (PEG-OH) as a solvent at 80 °C. Hydrophobic MMA was concurrently transesterified into hydrophilic PEG methacrylate (PEGMA) during LRP to give MMA/PEGMA gradient copolymers. The gradient sequence is directly determined by the instantaneous monomer composition changing from MMA alone to PEGMA-rich mixture in solution. Synchronized catalysis of LRP and transesterification thus affords gradient copolymers whose composition linearly changes from an initiating terminal to a growing counterpart. Additionally, amphiphilic MMA/PEGMA gradient copolymers showed self-assembly, thermoresponsive, and thermal properties specific to the gradient sequence, distinct from amphiphilic random or block counterparts.

Asymmetric Copolymers: Synthesis, Properties, and Applications of Gradient and Other Partially Segregated Copolymers

“…Synchronization of the rates of RDRP and transesterification allows direct access to linear gradient copolymers ( Figure ) . Other architectures such as random, block, gradient block, and bidirectional gradient copolymers are also accessible via tailoring parameters such as the relative rates of tandem catalysis, the timing of reagent introduction (e.g., sequential vs concurrent addition of catalysts and/or monomer(s)), and the functionality of the initiating species …”

“…[134,135] Other architectures such as random, block, gradient block, and bidirectional gradient copolymers are also accessible via tailoring parameters such as the relative rates of tandem catalysis, the timing of reagent introduction (e.g., sequential vs concurrent addition of catalysts and/or monomer(s)), and the functionality of the initiating species. [135,136] A critical point for the success of gradient copolymer synthesis via the tandem catalysis method is a high level of selectivity in transesterification between the monomeric and polymeric esters; ideally the pendent esters on the polymer should be inert. [134,135,137] This constraint is met for methacrylate (co) polymerization, however the technique is less reliable for the synthesis of acrylate-based gradients, [134,135] due to the decreased steric bulk around the polymeric ester moieties and hence increased rate of transesterification.…”

“…[134,135,137] This constraint is met for methacrylate (co) polymerization, however the technique is less reliable for the synthesis of acrylate-based gradients, [134,135] due to the decreased steric bulk around the polymeric ester moieties and hence increased rate of transesterification. For methacrylates the technique appears general with primary alcohols bearing a range of functionalities, with aliphatic alcohols, [134,135] poly(ethylene glycol), [135,136] fluorinated alcohols, [138,139] and alcohols bearing hydrogen-bond motifs [140] being exploited to date.…”

Sequence-controlled polymers via reversible-deactivation radical polymerization

“…The synchronized rate control is essential, and various functional groups can be incorporated by selecting different alcohols. [27][28][29][30] UNIQUE COPOLYMERS VIA CONCURRENT TRANSFORMATION OF ACTIVE SPECIES Different active species can be generated from identical dormant species according to the stimulus. For example, a carbon-halogen bond that is activated into a carbocationic species via Lewis acid catalysis for living cationic polymerization 31,32 is also available as the dormant species in conjunction with one-electron redox catalysis for metal-catalyzed living radical polymerization 4,5 or ATRP.…”

Solubility behaviour of random and gradient copolymers of di- and oligo(ethylene oxide) methacrylate in water: effect of various additives

“…Studies on the solubility behavior of gradient copolymers consisting of a hydrophilic and a hydrophobic monomer show that the solubility behavior changes drastically, depending more on the interaction of the comonomers with water and their hydrophobicity rather than their sequential order in the copolymer structure. 19,20 On the other hand, the thermal phase transition of other temperature responsive gradient copolymers consisting of monomers with similar chemical structure and therefore, similar hydrophobicity has shown considerable differences from the respective random and block copolymers and dependent on their sequential order in the gradient structure. This characteristic makes such gradient copolymers a great potential in biomimetic applications.…”