Macromolecular Architectures Designed by Living Radical Polymerization with Organic Catalysts

Well-defined diblock and triblock copolymers, star polymers, and concentrated polymer brushes on solid surfaces were prepared using living radical polymerization with organic catalysts. Polymerizations of methyl methacrylate, butyl acrylate, and selected functional methacrylates were performed with a monofunctional initiator, a difunctional initiator, a trifunctional initiator, and a surface-immobilized initiator.


Introduction
Living radical polymerization (LRP) has attracted increased attention as it allows for the rational design of polymer architectures with predictable molecular weights and narrow molecular weight distributions [1][2][3].LRP can offer, not only well-defined linear homopolymers, but also diblock copolymers, triblock copolymers, star polymers, and surface-grated brush polymers with sophisticated structures, which have many useful applications.
RTCP uses a reversible chain transfer of Polymer-I with a catalyst radical to generate Polymer• and a catalyst (deactivator) (Scheme 1b).RTCP consists of a dormant species, a catalyst (deactivator), and a radical source that supplies Polymer•.The catalysts include germanium, phosphorus, nitrogen, oxygen, and carbon-centered iodides including N-iodosuccinimide (NIS) (Figure 1) [17] used in the present work.
RCMP utilizes a reversible coordination of Polymer-I with a catalyst (activator) to generate Polymer• and a catalyst-iodine complex.RCMP consists of a dormant species and a catalyst (activator).The catalysts include amines and organic salts such as tetrabutylammonium iodide (BNI) (Figure 1) [27] and methyltributylphosphonium iodide (BMPI) (Figure 1) [27] used in the present work.
The attractive features of RTCP and RCMP include no use of special capping agents or metals.The catalysts are inexpensive, relatively non-toxic, easy to handle, and amenable to a wide range of monomers including styrenes, methacrylates, acrylates, acrylonitrile, and those with various functional groups.RTCP and RCMP can be facile and pervasive methodologies for various applications.We previously reported the use of RTCP and RCMP in preparing well-defined linear polymers including homopolymers, random copolymers, and diblock copolymers [16][17][18][19][20][21][22][23][24][25][26][27].In this paper, we report new examples of diblock copolymers and summarize the diblock copolymers prepared in previous and current works.We also report the syntheses of triblock copolymers, 3-arm star polymers, and surface-grafted brush polymers.Macromolecular designs of diblock, triblock, star, and brush architectures are important to widen the range of RTCP and RCMP applications.The structures and abbreviations of the studied monomers, catalysts, and initiating alkyl iodides (dormant species) are provided in Figure 1.

GPC Measurements
Gel permeation chromatography (GPC) analysis was performed on a Shodex GPC-101 liquid chromatograph (Tokyo, Japan) equipped with two Shodex KF-804L mixed gel columns (300 × 8.0 mm; bead size = 7 μm; pore size = 20-200 Å).The eluent was tetrahydrofuran (THF) or dimethyl formamide (DMF) with a flow rate of 0.8 mL/min (40 °C).Sample detection and quantification were conducted using a Shodex differential refractometer RI-101 calibrated with known polymer concentrations in solvent.The monomer conversion was determined from the GPC peak area.The column system was calibrated using standard poly(methyl methacrylate)s (PMMAs).For the homopolymerizations of BA and LMA and the homopolymerization of MMA from a trifunctional initiator, the samples were also detected using a Wyatt Technology DAWN EOS multiangle laser light-scattering (MALLS) detector (Santa Barbara, CA, USA) equipped with a Ga-As laser (λ = 690 nm).The refractive index increment dn/dc was determined with a Wyatt Technology OPTILAB DSP differential refractometer (λ = 690 nm).

Polymerization
In a typical run, a Schlenk flask containing a mixture of MMA (3 mL), CP-I, and a catalyst was heated at 60 °C under an argon atmosphere with magnetic stirring.For block copolymerization, the second monomer was subsequently added, and the solution was heated under an argon atmosphere with magnetic stirring.After the polymerization, the solution was quenched to room temperature, diluted with THF to a known concentration, and analyzed by GPC.

Preparation of Poly(methyl methacrylate) Iodide (PMMA-I)
A 100-mL round-bottom flask containing a mixture of MMA [(20 mL (8 M)], CP-I (80 mM), and BMPI (40 mM) was heated at 60 °C for 2.75 h under an argon atmosphere with magnetic stirring.After purification by reprecipitation from cold hexane twice, PMMA-I with M n = 5100 and PDI = 1.15 was isolated.The polymer was then used as a macroinitiator for block copolymerizations (entries 1, 4, 7, and 8, in Table 1).

Preparation of Poly(butyl acrylate) Iodide (PBA-I)
A 100-mL round-bottom flask containing a mixture of BA [20 mL (8 M)], CP-I (80 mM), and BNI (320 mM) was heated at 110 °C for 16 h under an argon atmosphere with magnetic stirring.After purification by reprecipitation from water/methanol (9:1) twice, PBA-I with M n = 10,000 and PDI = 1.33 was isolated.The polymer was used as a macroinitiator for block copolymerization (entry 10 in Table 1).

Surface-Initiated Polymerization
Silicon wafers (Ferrotec Corp., Tokyo, Japan, chemically/mechanically polished on one side, thickness 525 μm) were cleaned by successive sonication in acetone, water, and 1,2-dichloroethane for 8 min each and dried in a stream of nitrogen gas followed by evaporation under reduced pressure prior to use.The silicon wafer was immersed in an ethanol solution containing IHE (1 wt%) and 28% aqueous NH 3 (5 wt%) for 12 h at room temperature under darkness to immobilize the initiating group and then washed with ethanol.For graft polymerization of MMA, the IHE-immobilized wafer was immersed in a solution containing MMA [3 mL (8 M)], CP-I (20 mM), NIS (5 mM), and AIBN (20 mM) in a Schlenk flask and subsequently heated at 70 °C for 4 h under an argon atmosphere.After polymerization, the solution was diluted with THF to a known concentration and analyzed using GPC.The substrate was copiously rinsed with methanol to remove physisorbed free polymers and impurities.The thickness of the brush layer in the dry state was determined using a rotating compensator spectroscopic ellipsometer (M-2000UTM, J.A. Woolam, Lincoln, NE, USA) equipped with D 2 and QTH lamps with a polarizer angle of 45 degrees and an incident angle of 70 degrees.

Diblock Copolymers
Table 1 summarizes examples of diblock copolymerization, including previously reported [18,27] and new results as indicated.
Table 1 (entries 1-3) shows the block copolymerizations of MMA and GMA.The polymerizations were conducted using three methods.The first method involved polymerization from a purified macroinitiator (polymer-iodide) (entry 1).We prepared the PMMA-I macroinitiator in the bulk RCMP of MMA (8 M) using CP-I (80 mM) as an initiating dormant species and BMPI (40 mM) as a catalyst at 60 °C for 2.75 h.After purification from hexane, we obtained a purified PMMA-I macroinitiator with M n = 5500 and PDI = 1.15.The iodine elemental analysis indicated that this macroinitiator included a high fraction (95%) of active polymer possessing iodine at the chain end (with ± 5% experimental error).Such high chain-end fidelity of this macroinitiator can lead to high block efficiency for the subsequent block copolymerizations.Using this macroinitiator, the RCMP of GMA yielded a well-defined diblock copolymers with M n = 17,000 and PDI = 1.23 at 90% monomer conversion for 6 h at 60 °C.
The second method involved successive addition of two monomers starting from a purified low-mass dormant species (entry 2).When CP-I was used as a low-mass dormant species, a low-polydispersity diblock copolymer (PDI = 1.34) was obtained.Toluene was used as a solvent to prevent solidification of the first block solution, which facilitated mixing with the second monomer.To overcome the slow polymerization due to dilution, a small amount of an azo compound (V65) was added in the first block.V65 can supply Polymer• and, thus, increase the polymerization rate R p .Azo compounds have often been used to increase R p in other LRP systems [4][5][6][7].The R p was sufficiently increased (90% monomer conversion over 6 h) without causing significant broadening of the molecular weight distribution.Because the amount of V65 was approximately 0.15 equivalents compared with CP-I, the obtained block copolymer could include approximately 15% of dead first-block homopolymer.
The third method involved successive addition of two monomers starting from molecular iodine (I 2 ) and an azo compound (R-N=N-R) (entry 3).An alkyl iodide (R-I) formed in situ in the polymerization serves as the initiating dormant species.This I 2 /azo method was originally invented by Lacroix-Desmazes et al. for iodide-mediated LRP [29,30].This method was also effective for RTCP [17,18] and RCMP [25,27].A low-polydispersity diblock copolymer (PDI = 1.39) was obtained (entry 3).In the three cases studied (entries 1-3), the total polymerization time for the first and second blocks was below 12 h.Figure 2 provides the full molecular weight distributions (GPC chromatograms).A large fraction of the first block polymers was extended to block copolymers in all three cases, confirming the high block efficiency due to the high chain-end fidelity demonstrated above.
RTCP and RCMP provide a variety of diblock copolymers, including amphiphilic diblock copolymers consisting of water-insoluble and water-soluble segments.Table 1 shows amphiphilic diblock copolymers with MMA and DMAEMA (water-soluble basic) segments (entries 4-6) and MMA and MMA/MAA (water-soluble acidic) segments (entry 7).The second segment in entry 7 is a random copolymer of MMA (60%) and MAA (40%), which is water soluble at this monomer composition.Both the basic and acidic segments are accessible as water-soluble segments.
Table 1 (entries 8-11) shows diblock copolymerizations of MMA and BA.We can start with both MMA and BA (as the first block) to obtain well-defined diblock copolymers.In certain LRP systems, the synthetic order of the two blocks is crucial when two different monomer families are used.Thus, the lack of restriction in the synthetic order of the two blocks in the present system is of interest.Figure 3 shows the GPC chromatograms, confirming high block efficiency.These examples demonstrate the high accessibility of RTCP and RCMP to a variety of block copolymers.

Triblock Copolymers
We attempted to use a difunctional initiator in addition to the above-described monofunctional initiator.We synthesized a difunctional initiator, EMA-II, with two methacrylate-type chain ends (Figure 1). Figure 4 shows the polymerizations of MMA with EMA-II.Low polydispersity polymers of predetermined molecular weight were obtained in both the RTCP with NIS at 40 °C and the RCMP with BMPI at 80 °C. Figure 5 shows the polymerizations of BA.Again, low polydispersity polymers of predetermined molecular weight were obtained in the RCMP with BNI at 110 °C.These results demonstrate the high initiation efficiency of EMA-II for both MMA and BA polymerizations.
The high initiation efficiency of EMA-II and the aforementioned lack of restricted synthetic order for the two blocks encouraged the preparation of two different types of triblock copolymers of MMA and BA, i.e., BA-MMA-BA and MMA-BA-MMA triblock copolymers [Table 2 (entries 1-3)].Starting with MMA (as the first block), we obtained a low-polydispersity BA-MMA-BA triblock copolymer (PDI = 1.31), and starting with BA, we obtained a low-polydispersity MMA-BA-MMA triblock copolymer (PDI = 1.3-1.4).The accessibility of two different types of triblock copolymers is an attractive feature.
The MMA-BA-MMA triblock copolymer is a hard-soft-hard triblock copolymer with a variety of applications including use in elastomers.We also prepared a well-defined MMA-LMA-MMA triblock copolymer [Table 2 (entry 4)] as another hard-soft-hard triblock copolymer.This copolymer is an all-methacrylate copolymer and was easier to prepare with a shorter polymerization time (12 h) than the MMA-BA-MMA copolymer (21-27 h).

Three-Arm Star Polymer
In addition to linear polymers, we also prepared a 3-arm star polymer from a trifunctional initiator.We synthesized EMA-III as a trifunctional initiator (Figure 1). Figure 6 shows the RCMP of MMA from EMA-III with BNI at 60 °C.The molecular weights were determined by GPC-MALLS.The M n was in good agreement with the theoretical value and PDI was about 1.2 at a later stage of polymerization.A 3-arm star polymer with M n = 34,000 and PDI = 1.22 was obtained as an example.

Surface-Initiated Polymerization-Concentrated Polymer Brush
A surface of material plays a crucial role in many important properties such as mechanical, thermodynamic, and chemical properties; thus, surface modification is an important issue [31][32][33].Surface-initiated graft polymerization is among the most effective surface modification methods.Fukuda and Tsujii et al. [31,34] were the first to use LRP in graft polymerization to obtain a polymer brush with a surface density that was one order of magnitude higher than those of conventional brushes.The surface occupancy reached 40%.This so-called concentrated polymer brush takes a highly extended conformation, with extension up to 80% of all-trans conformation length in good solvents [31].Such an extended conformation affords many new properties not attainable by conventional semi-dilute and dilute brushes, such as high elasticity, ultra-low friction, and excellent repellency of proteins and cells.Therefore, concentrated brushes may have useful applications [31][32][33].
Given these results, surface-initiated RTCP was studied.We synthesized a surface-immobilizing initiator IHE (Figure 1) consisting of an alkyl iodide (initiating) moiety and a triethoxysilyl (TEOS) group.IHE was synthesized from BHE (the previously reported corresponding bromide) [28] via a halogen exchange reaction with NaI.IHE was fixed on a silicon wafer through the TEOS group.The IHE-immobilized silicon wafer was immersed in a mixture of MMA, a non-immobilized free initiator CP-I, a catalyst NIS, and a radical source AIBN, and heated at 70 °C for 4 h for polymerization (Figure 7).The free initiator CP-I was added because its addition can improve control over M n and PDI of the immobilized graft polymer [31][32][33].The M n and PDI of the free polymers generated from free initiators, which are usually in good agreement with those of the immobilized graft polymers [31][32][33], were 15,000 and 1.31, respectively.The thickness of the graft polymer was measured to be 10.5 nm using ellipsometry.Assuming the same M n for the graft and free polymers, the surface density of the graft polymer was calculated to be 0.51 chains/nm 2 .This density is high and located in a concentrated polymer brush region [31], indicating the successful controlled synthesis of a concentrated polymer brush.Another example of the graft polymerization is depicted in Figure 8, in which IHE was fixed on the surface in a patterned manner.For demonstration purposes, we carried out an RTCP of BzMA, using a non-toxic catalyst vitamin E, without a priori purge of inert gas for the short time of 5 min at 85 °C.After the polymerization, the polymer brushes were observed as black square spots (Figure 8), demonstrating patterning and a robust and quick polymer brush synthesis.

Conclusions
Diblock copolymers were synthesized using a purified macroinitiator and the successive addition of two monomers.Well-defined triblock and star polymers and concentrated polymer brushes on solid surfaces were synthesized from difunctional, trifunctional, and surface-immobilized initiators.We prepared two different types of triblock copolymers using MMA and BA: MMA-BA-MMA and BA-MMA-BA copolymers.Access to a variety of macromolecular architectural designs may be beneficial to a variety of applications.

Figure 7 .
Figure 7. Surface-initiated RTCP of MMA.The experimental conditions are indicated in the figure.

Table 2 .
Syntheses of triblock copolymers.Determined by GPC with a multiangle laser light-scattering detector (MALLS) for the first block of entries 2-4 and PMMA-calibration for others.d Diluted in toluene (50% toluene and 50% MMA for the first block of entries 1 and the second block of entries 2 and 3 and 40% N,N-dimethyl 2-methoxyethylamide and 60% LMA for the first block of entry 4).e With the addition of I 2 (4 mM).
b M = monomer.c