1.2.1 General considerations about multiblock copolymers by RAFT
polymerization
As mentioned above, RAFT polymerization has living character originating from the ability of the S=C(Z)S- moiety to transfer radicals between dormant and active chains.5 Considering most of the chains generated from RAFT process possess a S=C(Z)S- end group (as minimum amount of initiator was used for the polymerization), polymerization will be able to continue in the presence of additional monomers to afford block copolymers.5 Synthesizing
block copolymers is one of the main advantages of RAFT polymerization over a free radical polymerization. Multiblock copolymers which have a higher level of structural control are polymers in which the sequence of the large number of monomers/functionalities is regulated along the polymer chain. This characteristic of multiblock copolymers enables them to mimic the properties and structural features of biopolymers, such as proteins and nucleic acids.42-44
The highly specific functions of these macromolecules originate from the elegantly controlled primary structure along the polymer backbone.45 By incorporating a wide range of functional groups in specific domains of a polymer chain, highly ordered materials with unique properties will be generated.
A basic requirement for the synthesis of block copolymers with a narrow molecular weight distribution in a RAFT polymerization is that the polymeric thiocarbonylthio compound (Macro-CTA) formed in the former block should have a high chain transfer constant in the following polymerization to form the subsequent block. This will require the propagating radical Pm• (formed from the former block) to have a comparable or higher leaving ability than
that of the new (later formed) propagating radical Pn• under the polymerization conditions
(Scheme 1.2).5
Scheme 1.2 Simplified mechanism and structures of the propagating radicals and intermediate radicals in RAFT polymerization.
However, since free radical intermediates are formed in the process of RAFT polymerization, radical-radical termination is unavoidable.46 Therefore, dead polymer chains derived from initiator radicals will be formed ultimately. In order to synthesize multiblock copolymers, keeping a high fraction of living chains is of major significance. To achieve polymers with narrow polydispersities and a high number of living chains, the priority is to minimize the consumed initiator and, as a result, to minimize the number of initiator-derived chains.6 The polymerization usually needs to be stopped at low to moderate monomer
conversions to preserve a high livingness, which will severely limit the practice of the synthesis of multiblock copolymers.47 There are several different parameters which can be optimized in order to maintain a high level of livingness in a RAFT polymerization process, e.g. monomer concentration,48 solvent,49-52 reaction temperature,51, 53 and pressure.53, 54 For example, increasing the monomer concentration will increase the Arrhenius pre-exponential factor which will accelerate the speed of polymerization.48, 55 Choosing a polar solvent (i.e. water) will help stabilizing the transition state of the propagating radicals, which will lower the termination rate and the activation energy, therefore increase the speed of propagation.49 Furthermore, the propagation rate constant (kp), is considered to be chemically affected and therefore to be
1.2.2 Calculation of the theoretical livingness in multiblock copolymers
The livingness (the number fraction of living chains which have the CTA end group, L) in the synthesis of multiblock copolymers is calculated according to Equation 1.4.𝐿 = [CTA]0
[CTA]0+2𝑓[𝐼]0(1−e−𝑘𝑑t)(1−𝑓𝑐2)
(Equation 1.4)
where L is the number fraction of living chains, [CTA]0 and [I]0 are the initial concentrations
of CTA and initiator, respectively. The term ‘2’ represents that one molecule of azo initiator degrades into two primary radicals with a certain efficiency f (taken as 0.5 in this thesis). kd is
the decomposition rate coefficient of the initiator. The term (1 ─ fc/2) represents the number of
chains produced in a radical–radical termination event with fc the coupling factor (fc = 1 means
100% bimolecular termination by combination, fc = 0 means 100% bimolecular termination by
disproportionation). Using this equation, it is possible to predict the amount of chains possessing the CTA end group and the structures of generated block copolymers.
1.2.3 Advances and highlights of multiblock copolymers by RAFT
polymerization
Multiblock copolymers have received considerable attention in recent years as they aid to understand the relationship between the monomer sequence of a polymer chain and the resulting structure and functionality.57-62 For instance, the self-assembly of block copolymers
in solution or in bulk can lead to the formation of different types of objects with tailored microstructures.63-71 Although having great promises, the synthesis of sequence controlled multiblock copolymers by RAFT polymerization still remains challenging. This is attributed to the inevitably loss of chain ends during polymerization as a result from the termination reactions due to the requirement of an external radical source which will cause the overall livingness to be lower than in other RDRP techniques. Considering that the amount of dead
chains only corresponds to the amount of radicals derived from the decomposed initiator in the RAFT polymerization (depicted in Equation 1.1), it is possible to reach quantitative monomer conversions with a high fraction of living chains by keeping the decomposed initiator at a minimum concentration. However, lowering the initiator concentration will slow down the polymerization rate and will therefore prolong the reaction time.2 This drawback can be overcome by optimizing other reaction parameters.
Recently, our group developed an approach which enables the synthesis of highly complex multiblock copolymers with near full monomer conversion for each block extension and good control of molecular weight distributions. This strategy employed a one-pot approach
via sequential monomers addition to synthesize multiblock copolymers using RAFT polymerization by optimizing the reaction parameters.18 A dodecablock and an amphiphilic hexablock copolymer were synthesized by utilizing carefully selected conditions to polymerize acrylamide (high kp) monomers using AIBN as the initiator and a trithiocarbonate as the CTA
(low retardation) using dioxane as solvent at 65˚C. Near full monomer conversion (> 99%) was obtained within 24 h for each chain extension with a final theoretical livingness of up to 95%. In order to avoid the lengthy reaction time to reach full monomer conversion, an initiator which decomposes much faster than AIBN (2,2’-azobis[2-(2-imidazolin-2- yl)propane]dihydrochloride (VA-044)) was employed. This approach demonstrated the high versatility by synthesizing the first reported icosablock (20 blocks, 2 h per block at 70˚C in H2O) copolymer with ~ 98 – 99% conversion for each chain extension and a remarkable control
of the molecular weight with a final dispersity of 1.4 and a 93.8% theoretical livingness. This study illustrates the great potential of using RAFT polymerization to the synthesis of highly complex functional polymers with precisely distribution of monomers sequence along the polymer chain. These approaches, however, still have limitation, i.e. not suitable for monomers with low kp (styrene and methacrylates).
Very recently, the Haddleton group reported the synthesis of sequence-controlled multiblock copolymers using various methacrylate monomers via a sulfur-free emulsion RAFT polymerization technic.72 Despite the significant challenge of using methacrylates for the fabrication of complex multiblock copolymers, several higher order multiblock copolymers (up to 24 blocks) were successfully obtained with low molecular weight distribution (Ð < 1.5) in a facile, rapid, quantitative, and scalable approach (up to 80 g).
All the above approaches offer new perspective for building sequence controlled synthetic polymers which have great potential in the wide range of applications, including nanostructured materials, microphase separation, and single chain folding.