The properties of polymers are strongly dependent on their compositions and topological structures. Over the past few decades, polymers with different topological structures have been synthesized. In these years, synthesis of macromolecular brushes (also called comb polymers or bottle brushes) has attracted much interest due to their specific structures and unique properties.[2, 3] Macromolecular brushes are a type of macromolecules with densely grafted side chains on a linear backbone. Because of the steric repulsion between the crowded side chains on the backbone, the brush polymers adopt worm-like cylindrical brush conformation with length from tens to hundreds of nanometers. Macromolecular brushes can be categorized into one-component brushes with homopolymer side chains and multicomponent brushes.[5-10] In the past decade, many different polymerization strategies have been applied in the synthesis of macromolecular brushes,[11-14] among which atom transfer radical polymerization (ATRP) has been the most frequently used technique in the synthesis of macromolecular brushes.
Recently, atom transfer radical coupling (ATRC) has been demonstrated to be a highly efficient coupling reaction.[15-17] ATRP is based on a rapid dynamic equilibration between a minute amount of growing free radicals and a large majority of the dormant species. In contrast to the low radical concentration in an ATRP system, in an ATRC reaction Cu(0) is introduced into the reaction system, and which lead to the reduction of Cu(II) to Cu(I). The increase of Cu(I) in the system shifts the equilibrium between active and dormant species to the side of the active radical species, and an increase in radical concentration leads to the coupling reaction between radical chain ends.[19, 20] ATRC has provided access to the synthesis of cyclic polymers, telechelic polymers,[17, 22] and H-shaped terpolymers.
In these years, intramolecular crosslinking of linear polymer chains has been an efficient route to the preparation of well-defined nanoparticles. After intramolecular crosslinking reaction, a linear polymer chain present a coil-to-globule transition and single-molecular nanoparticles can be prepared. Herein, we report intramolecular ATRC of macromolecular brushes. After the coupling reaction, a brush macromolecule experiences a conformational transition from worm-like to globule-like structure.
The pendant side chains of the macromolecular brushes prepared by ATRP preserve the halogen end functionality, which can be used in radical coupling reaction. Similar to intramolecular crosslinking of linear polymer chains, the intramolecular ATRC of macromolecular brushes can be conducted in a solution at very low polymer concentration and after the reaction pendant loop structures are formed (Scheme 1). In ATRC coupling reaction, any two terminal radical sites on pendant side chains are possibly coupled and formation of globule-like structure will be observed.
The coil-comb macromolecular brushes used in this study were prepared by grafting from method. Poly(ethylene glycol)-block-poly(2-hydroxyethyl methacrylate) (PEG-b-PHEMA) block copolymer was synthesized by ATRP; PHEMA blocks were esterified by 2-bromo-isobutyryl bromide and PEG-block-poly(2-(2-bromoisobutyryloxy)ethyl methacrylate (PEG-b-PBIEM) macroinitiator was obtained. PS polymer brushes on PBIEM backbone were prepared by ATRP. Intramolecular ATRC of PS brushes was conducted at a very low polymer concentration, and pendant PS loop structures were formed. The whole synthetic process is illustrated in Scheme 1.
In this study, PEG macroinitiator was prepared by a reaction of 2-bromoisobutyryl bromide and CH3O-PEG-OH. 1H NMR result demonstrated that complete reaction was achieved (Fig. S1 in the Supporting Information). The PEG macroinitiator was used to initiate ATRP of HEMA, and PEG-b-PHEMA block copolymer was prepared. 1H NMR spectrum of the block copolymer is shown in Supporting Information Figure S2. Based on 1H NMR result, the average repeat unit number of PHEMA is calculated to be about 350. The PHEMA blocks were reacted with 2-bromoisobutyryl bromide, and PEG-b-PBIEM was prepared. 1H NMR spectrum of PEG-b-PBIEM is shown in Supporting Information Figure S3. After esterification, the signals corresponding to methylene protons of PHEMA moved from 3.5–4.0 ppm to 4.1–4.5 ppm, which indicated the successful esterification.
The pendant PS brushes were synthesized by ATRP. The bromide groups on the PBIEM blocks were used as initiators and CuBr/dNbpy complex was used as catalyst. To avoid the possible crosslinked structures and the possible free polymers produced in ATRP process, the monomer conversion was controlled at low level (around 10%). 1H NMR spectrum of the brush polymer is shown in Supporting Information Figure S4. Based on 1H NMR result, the average repeat unit number of PS side chains is 186, and the polymer is assigned as PEG-b-PBIEM350-g-PS186. Gel permeation chromatography (GPC) traces of macromolecular brush and its precursors are presented in Supporting Information Figure S5. It is found that the GPC trace moves to short retention time region after each step. Absolute molecular weight of the brush polymer was determined by GPC equipped with light scattering detector. The absolute molecular weight is about 6.62 × 106, which agrees well with 1H NMR result.
In ATRC of macromolecular brushes, CuBr/PMDETA was used as the catalyst and Cu0 was used as the reducing agent to reduce CuII into CuI. The coupling reaction of the radicals at the side chain ends will cause the size contraction of the macromolecular brushes and result in the formation of the globule-like structures. Comparison of the GPC traces for the nanoparticles versus the starting macromolecular brushes provides an efficient technique for demonstrating the volume change of the brush molecules from worm-like structure to globule-like structure. Figure 1(a) shows GPC curves of PEG-b-PBIEM350-g-PS186 before and after ATRC, where an increase in retention time is observed on increasing time of ATRC reaction. The increase in retention time is attributed to a reduction in hydrodynamic volume of the brush polymer with intromolecular ATRC reaction. Figure 1(b) shows the changes of the apparent molecular weight and the molecular weight distribution of PEG-b-PBIEM350-g-PS186 with reaction time. Because of the size contraction of the brush polymer backbone, the apparent molecular weight of the polymer decreases with reaction. After 5 h reaction, the apparent molecular weight decreases from 8.0 × 105 to 6.4 × 105. However, the polydispersity index of the brush polymer remains unchanged at around 1.30. The intermolecular coupling reaction would result in higher molecular weight and broad molecular weight distribution, so a decrease in the molecular weight and low polydispersity index of the brush polymer after ATRC indicate that no intermolecular ATRC reaction occurs in the synthesis of the single macromolecular brush nanoparticles. The absolute molecular weight of the brush polymer after 5 h reaction was determined by GPC equipped with light scattering detector. The molecular weight is about 6.38 × 106, which is a little bit smaller than the brush polymer before ATRC reaction due to the loss of the bromide groups in the ATRC process. Elemental analysis results indicated that more than 80% of bromide groups were lost after ATRC reaction.
The glass transition temperatures (Tg's) of linear and cyclic polymers are different. Because of the restriction of the mobility of the polymer chains, higher Tg's are observed for the macrocyclic polymers. When the molecular weight is higher than ten thousands, the difference is about a couple of degrees. The loss in chain mobility by cyclization results in higher Tg's of cyclic polymers. Differential scanning calorimetry (DSC) curves of the brush polymer before and after ATRC are given in Figure 2. After ATRC, the glass transition temperature of the macromolecular brush increases from 103.2 °C to 105.5 °C, which demonstrates the formation of the pendant PS loop structures and a restriction of the mobility of the pendant chains.
Dynamic light scattering (DLS) curves of PEG-b-PBIEM350-g-PS186 before and after ATRC coupling reaction are presented in Figure 2(b). In both cases, the brush polymers possess unimodal size distributions, one with a hydrodynamic diameter distribution ranging from 21 to 100 nm, and the other from 18 to 90 nm. After the coupling reaction, the scattering peak shifts from 37 to 32 nm, demonstrating the size contraction of the brush macromolecules.
Figure 3 shows transmission electron microscopy (TEM) images of PEG-b-PBIEM350-g-PS186 before and after ATRC coupling reaction. The TEM specimens were prepared by depositing dilute solutions of brush polymers on Formvar coated grids, and solvents were evaporated in air. To improve the contrast, PS phases were stained in RuO4 atmosphere. The morphology of a macromolecular brush is determined by the length of the backbone and the length of the pendant side chains. For PEG-b-PBIEM350-g-PS186 the main chain is longer than the side chains, so the brush polymer adopts worm-like structures. The width of the structures is in the range between 10 to 20 nm [Fig. 3(a)]. Figure 3(b) shows a TEM image of brush polymer after ATRC. On the TEM image, single macromolecular brush nanoparticles with diameters from 27 to 36 nm are observed, indicating a conformational transition of the polymer from worm-like structure to globule-like structure [Fig. 3(c)]. Part a in Figure S6 (Supporting Information) shows an atomic force microscopy (AFM) taping mode height image of PEG-b-PBIEM350-g-PS186 dispersed on mica surface from a dilute solution. On the AFM image, worm-like structures of the brush polymer are observed. The height of the brush polymer is about 3.7 nm. However, after ATRC, spherical nanoparticles are observed (part b in Supporting Information Fig. S6), and the height of the brush polymer nanoparticle is about 6.6 nm. The height increase is attributed to the intramolecular coupling reaction and the formation of the globular structure.
In a previous research, Sheiko et al. reported a transition of brush polymer from a rodlike to a globular conformation on lateral compression of a monolayer of poly(n-butyl acrylate) adsorbed on a water surface. In this research, the conformational transition was attributed to the coupling reaction of the terminal radicals on the pendant PS chains [Fig. 3(c)]. Previous researches on the fabrication of single molecular nanoparticles are related to the intramolecular crosslinking and collapse of linear polymer chains. In this study, the synthesis of the single brush polymer nanoparticles was based on terminal radicals coupling, and the PS loops in the single molecular nanoparticles can still be solvated in a good solvent.
In summary, we have described an effective and novel method to synthesize single macromolecular brush nanoparticles. Because of the high-molecular weight of the brush polymer, the sizes of the brush polymer nanoparticles are much bigger than the single linear polymeric nanoparticles. Polymers with many different topological structures will be able to be prepared based on single macromolecular brush nanoparticles.