Synthesis Of Molecular Brushes Research Articles

Atom transfer radical polymerization as a powerful tool in the synthesis of molecular brushes

Macromolecules with complex architectures such as molecular brushes (MBs) have received much attention over the last two decades. This interest is largely due to their potential applications in many fields, such as drug delivery and electronics, among others. The synthesis of such complex macromolecules can only be achieved via the use of controlled polymerization techniques such as atom transfer radical polymerization (ATRP). This review highlights the use of ATRP in the synthesis of MBs. © 2014 Society of Chemical Industry

Molecular brushes with extreme grafted side chain densities

A series of densely grafted poly(n-butyl acrylate) (PBA) molecular brushes with four different grafting densities were synthesized by the “grafting-from” approach using atom transfer radical polymerization (ATRP). A novel monomer, isopropylidene-2,2-Bis(methoxy)propionic hydroxyethylmethacrylate (IMPHMA), was synthesized and copolymerized with methyl methacrylate (MMA) under different monomer feed ratios to yield a series of linear poly(methyl methacrylate-stat-IMAPA), [PMMA-s-(PIMPHMA)]. The resulting copolymers were deprotected and transformed to macroinitiators, [PMMA-s-(PHEMA-IMPHMA-Br)]. n-Butyl acrylate (BA) was grafted from these macroinitiators to yield a series of molecular brushes, [PMMA-s-{(PIMPHMA)-g-PBA}], with various side chain lengths. Molecular brushes were characterized by gel permeation chromatography (GPC) and 1H NMR. PBA side chains were cleaved by acid hydrolysis, and the resulting linear PBA polymers were characterized by GPC to study initiation efficiency during the synthesis of molecular brushes. The initiation efficiency increased with polymerization time and decreased with macroinitiators that had more initiation sites. Atomic force microscopy (AFM) measurements demonstrated the characteristic molecular structure by resolving individual brush molecules.

Synthesis of basic molecular brushes: ATRP of 4-vinylpyridine in organic media

A poly(2-(2-bromopropionyloxy)ethyl methacrylate) (PBPEM) was used as macroinitiator in the synthesis of molecular brushes with poly(4-vinylpyridine) side chains, (P(BPEM- g-4VP). Atom transfer radical polymerization (ATRP) was employed as the polymerization technique. The polymerizations were carried out in DMF at 30 °C using a copper–chloride-based ATRP catalyst, which converted all the dormant polymer chain ends to alkyl chloride groups, thus minimizing branching and crosslinking, which occurred when a copper bromide-based catalyst was employed. Tris(2-pyridylmethyl)amine was selected as the ligand due to the high activity of its Cu I complex in ATRP as well as its strong binding to both Cu I and Cu II, which prevented competitive complexation of the monomer or polymer to the metal center. In order to prevent crosslinking via radical coupling, the monomer conversion was kept low (under 3%) and the alkyl chloride end groups of P4VP side chains were converted to alkoxyamines upon activation followed by a reaction with TEMPO radical. Dynamic light scattering measurements showed the hydrodynamic diameter ( D H) of the brushes was pH-dependent. Aggregation of single P(BPEM-g-4VP) brushes in water was very pronounced at high pH values but was observed even when the amount of added HCl was enough to completely protonate the pyridine units ( D H = 278 nm).

Synthesis of Molecular Brushes by “Grafting onto” Method: Combination of ATRP and Click Reactions

Molecular brushes (densely grafted polymers or bottle-brush macromolecules) were synthesized by the "grafting onto" method via combination of atom transfer radical polymerization (ATRP) and "click" reactions. Linear poly(2-hydroxyethyl methacrylate) (PHEMA) polymers were synthesized first by ATRP. After esterification reactions between pentynoic acid and the hydroxyl side groups, polymeric backbones with alkynyl side groups on essentially every monomer unit (PHEMA-alkyne) were obtained. Five kinds of azido-terminated polymeric side chains (SCs) with different chemical compositions and molecular weights were used, including poly(ethylene glycol)-N3 (PEO-N3), polystyrene-N3, poly(n-butyl acrylate)-N3, and poly(n-butyl acrylate)-b-polystyrene-N3. All click coupling reactions between alkyne-containing polymeric backbones (PHEMA-alkyne) and azido-terminated polymeric SCs were completed within 3 h. The grafting density of the obtained molecular brushes was affected by several factors, including the molecular weights and the chemical structures of the linear SCs, as well as the initial molar ratio of linear chains to alkynyl groups. When linear polymers with "thinner" structure and lower molecular weight, e.g., PEO-N3 with Mn = 775 g/mol, were reacted with PHEMA-alkyne (degree of polymerization = 210) at a high molar ratio of linear chains to alkynyl groups in the backbone, the brush copolymers with the highest grafting density were obtained (Y(grafting) = 88%). This result indicates that the average number of SCs was ca. 186 per brush molecule and the average molecular weight of the brush molecules was ca. 190 kg/mol.

Initiation Efficiency in the Synthesis of Molecular Brushes by Grafting from via Atom Transfer Radical Polymerization

Grafting from via atom transfer radical polymerization (ATRP) was employed for the synthesis of densely grafted molecular brushes. A poly(2-(2-bromopropionyl)oxyethyl methacrylate) (PBPEM) macroinitiator was used to polymerize n-butyl acrylate (BA) with various degrees of polymerization. The brush syntheses proceeded with first-order kinetic behavior, and the molecular weights determined by gel permeation chromatography coupled with multiangle laser light scattering (GPC-MALLS) increased linearly with conversion. The PBA side chains (SCs) were cleaved by acid solvolysis, and the resulting products were characterized in order to study the initiation process in the synthesis of molecular brushes. By comparing the molecular weight of the cleaved SCs to their theoretical molecular weights (calculated assuming quantitative initiation), the grafting efficiency was determined as a function of monomer conversion. In the early stages of polymerization, the initiation efficiency is low but gradually increases to 87% at 12% monomer conversion. The results were compared to an analogous linear ATRP conducted under identical conditions, except that ethyl 2-bromopropionate was employed as the initiator. The initiation efficiency for the linear polymerization was consistently higher than that observed for the brushes. The difference was attributed to the congested environment (high local concentration of initiation sites) encountered in the case of grafting from a macroinitiator backbone, which led to slower deactivation of growing radicals at low conversion. The initiation efficiency was enhanced by increasing the rate of deactivation of the growing species or decreasing the rate of propagation.

Synthesis of Molecular Brushes with Gradient in Grafting Density by Atom Transfer Polymerization

Macromolecular brushes with a gradient of side-chain spacing along the backbone have been synthesized by the “grafting from” approach using atom transfer radical polymerization. A macroinitiator was prepared in two steps, first by conducting a gradient copolymerization of MMA and HEMA-TMS followed by transformation of the resulting poly(MMA-grad-HEMA-TMS) to poly(MMA-grad-BPEM). The gradient composition of the macroinitiator was a forced gradient formed by continuous feeding of HEMA-TMS during MMA polymerization. The gradient structure was characterized by monitoring monomer conversion (GC) and molecular weight evolution (GPC) during copolymerization. AFM measurements demonstrated the characteristic anisotropy of the molecular structure by resolving individual brush molecules with a bulky head and a thin tail.

Synthesis of Molecular Brushes with Block Copolymer Side Chains Using Atom Transfer Radical Polymerization

Brush macromolecules having poly(n-butyl acrylate-block-styrene) and poly(styrene-block-n-butyl acrylate) side chains have been synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). The molecular weights of the resulting polymers were characterized by gel permeation chromatography (GPC) using refractive index and multiangle light scattering detection. The block copolymer side chains were cleaved from the backbone and analyzed by GPC, confirming the synthesis of well-defined copolymer brushes. Visualization of individual molecules by atomic force microscopy (AFM) enabled analysis of the conformation and microstructure of the brush macromolecules on mica surface. The brushes with the pnBuA core were almost fully stretched, while the inverted structure with the pS core exhibited longitudinal contraction compared to the contour length of the main chain. In addition, the poly(n-butyl acrylate-block-styrene) brushes demonstrated a characteristic necklace morphology which...