High Molecular Weight Block Copolymers by Sequential Monomer Addition via Cu(0)-Mediated Living Radical Polymerization (SET-LRP): An Optimized Approach

Abstract

The synthesis of well-defined high molecular weight block copolymers by sequential in situ chain extensions via Cu(0)-mediated living radical polymerization is reported. Optimal conditions for iterative high molecular weight block formation were determined using model homopolymer quasiblock systems, including methyl acrylate (MA), ethyl acrylate (EA), and n-butyl acrylate (nBA; each block DP n ≈ 100). The PDI after each chain extension was below 1.2, with good agreement between theoretical and experimental molecular weights, while the conversion of monomer incorporation into each distinct block was 95−100% (up to 6 blocks). To demonstrate this approach for true block copolymer materials, well-defined block polymers containing MA, ethylene glycol methyl ether acrylate (EGMEA), and tert-butyl acrylate (tBA) were prepared in high purity: diblock P(MA-b-EGMEA) and triblock P(MA-b-tBA-b-MA). These were prepared in high yields, on multigram scales, and with purification only required at the final step. To the best of our knowledge, this is the first time that high molecular weight block copolymers have been reported using this novel technique. B lock copolymers display a wide range of interesting and useful properties due to the fact that the combination of monomers with different physicochemical properties, confined in block sequences, allows these systems to undergo selfassembly and phase separation into higher ordered structures. 1−8 The synthesis of AB or ABA amphiphilic block copolymers of high molecular weight is of particular interest for the formation of micelles, vesicles, and so on, in solution, and various morphologies in the solid state. 1−8 The morphology of these self-assembled constructs depends upon a well-controlled synthetic protocol allowing preordained molecular weights and volume fractions (ϕA/ϕB) to be obtained. 9−12 However, while there are many polymerization techniques that have been used to produce block copolymers, a number of drawbacks exist. For example, living anionic polymerization 13 is extremely labor intensive, and the number of functional monomers that can be polymerized using this technique is limited. The development of controlled radical polymerization (CRP) techniques, such as ATRP, 14,15 NMP, 16 and RAFT, 17 has expanded this monomer library but experimental and synthetic limitations remain. The most significant limitation is the loss of "livingness", or end group fidelity, as the polymerization proceeds due to unwanted side reactions. 18,19 This loss of "livingness" of the chain end, leads to a drift in PDI, which can be reflected in the structural polydispersity of resulting higher order polymers.Cu(0)-mediated living radical polymerization 20,21 has recently been demonstrated to yield polymers with extremely high livingness at quantitative conversions and extending into post-polymerization conditions. 22 The versatility of the approach has been demonstrated in a diverse range of polar solvents such as DMSO, 23 DMF, 24 ionic liquids, 25 water, 26,27 alcohols, 28 and even in bio...

Block copolymer synthesis by controlled/living radical polymerisation in heterogeneous systems

“…These approaches are based on maintaining a very high degree of livingness (i.e. high blocking efficiency) throughout the polymerisation either by using Cu(0)-mediated radical polymerisation [26][27][28] It is now possible to conduct CLRP in a very wide range of dispersed systems, including ab initio (and seeded) emulsion, miniemulsion, microemulsion, dispersion, precipitation and suspension polymerisation. 17 The continuous phase for heterogeneous CLRP is typically water, but there exists a significant body of work using organic solvents and supercritical CO2 14,15 , respectively, as well as various inverse systems where the continuous phase is a non-polar organic solvent and the dispersed phase is aqueous.…”

Synthesis of amphiphilic diblock copolymers of isotactic polystyrene-block-isotactic poly(p-hydroxystyrene) using a titanium complex with an [OSSO]-type bis(phenolate) ligand and sequential monomer addition

“…However, this method requires that the same catalytic or initiating species can initiate the living polymerization of both monomers, and that the conversion of the rst monomer must be quantitative. [13][14][15][16][17][18][19][20][21] Macroinitiators were prepared from end-functionalized polymers available, which initiate or catalyze living polymerizations of desired monomers to obtain welldened diblock copolymer. [22][23][24][25][26] Moreover, click coupling reaction of two end-functionalized macromolecular chains has been employed to synthesize diblock copolymers in recent years.…”

Kinetic Studies of Elementary Reactions in SET-LRP / SARA ATRP

“…An interesting side effect of the strong chain length dependence of the termination rate constant is that polymerizations carried out in the presence of additional Cu II show an increased fraction of dead chains. Copper(II) salts are frequently added to SET-LRP polymerizations in order to reduce the dispersity of the final polymer (38)(39)(40). The increased copper(II) concentration leads to faster deactivation and the production of lower molecular weight polymer at the beginning of the reaction.…”