Improving the Stability in Aqueous Media of Polymer Brushes Grafted from Silicon Oxide Substrates by Surface‐Initiated Atom Transfer Radical Polymerization

Hydrophilic polymer brushes grown via surface‐initiated ATRP from silicon oxide surfaces are susceptible to detachment via hydrolytic cleavage of the anchoring siloxane bond. This paper investigates the influence of the structure of the ATRP initiator on the stability of these brushes and seeks for strategies to further enhance their stability. It is found that increasing the hydrophobicity of the organosilane modified ATRP initiator reduces the susceptibility of the brushes toward cleavage. Robust, hydrophilic polymer brushes are prepared, which are obtained by introducing a short, hydrophobic PMMA or PEHMA block between the silicon oxide substrate and the hydrophilic polymer brush.

     

Surface Functionalization of Electrospun Poly(butylene terephthalate) Fibers by Surface‐Initiated Radical Polymerization

A facile surface modification procedure for electrospun poly(butylene terephthalate) (PBT) fibers by surface‐initiated atom transfer radical polymerization (SI‐ATRP) is reported. Initiators are introduced through aminolysis and chemical vapor adsorption. SI‐ATRP is subsequently carried out to prepare a polymer‐grafted layer at the PBT fiber surface without altering the fiber geometry. After modification with a zwitterionic poly(sulfobetaine), poly(3‐(N‐2‐methacryloyloxyethyl‐N,N‐dimethyl) ammonatopropanesulfonate), the surface is superhydrophilic. The surface properties are thermally stable due to the high melting temperature of the PBT crystallites and are maintained for a prolonged period.

     

Polystyrene End‐Labeled with 2,7‐Dibromofluorene Synthesized Using an Adaptation of Reverse Atom Transfer Radical Polymerization

Summary: Polystyrene with high amounts of end‐labeling was synthesized using initiating systems comprised of conventional radical initiators and 2,7‐dibromofluorene or other fluorene derivatives in an adaptation of reverse atom transfer radical polymerization (RATRP). Benzoyl peroxide (BPO) or 2,2′‐azoisobutyronitrile (AIBN) were decomposed and allowed to react with 2,7‐dibromofluorene, 2‐bromofluorene, or fluorene in the presence of ligand‐bound CuX₂ allowing for abstraction of the 9‐H from the fluorenyl species and the establishment of an equilibrium between the subsequent active radical and the dormant alkyl halide. Gel permeation chromatography (GPC) traces indicated CuCl₂‐catalyzed reactions produced polymers possessing narrow polydispersity index (PDI) values <1.3 with AIBN and 2,7‐dibromofluorene systems, while analogous reactions catalyzed using CuBr₂ were less controlled (PDI > 1.7). Analysis of the polymers using UV‐vis spectroscopy and UV‐GPC demonstrated competition between initiation from both the conventional radical initiator and fluorenyl species generating polymers end‐labeled with both the 2,7‐dibromofluorene and isobutyronitrile groups. Fluorene or 2‐bromofluorene as co‐initiators led to lowered amounts of end‐labeling, but the polymers generally possessed lower PDI values compared to 2,7‐dibromofluorene systems.

     

Gel Formation and Molecular Characteristics of Poly(N‐isopropylacrylamide) Prepared by Free‐Radical Redox Polymerization in Aqueous Solution

The molecular characteristics of poly(N‐isopropylacrylamide) (PNIPA), prepared by free‐radical polymerization using an aqueous redox initiator and reaction conditions comparable to those used in the synthesis of nanocomposite gels, were investigated by altering the monomer concentration ([NIPA]) and the polymerization temperature (T_p) across the transition temperature (LCST). When T_p<LCST, there is a critical [NIPA] (=n*) above which PNIPA partially forms gels in the absence of a chemical crosslinker, and the gel fraction increases with increasing [NIPA] and decreasing T_p. In the range of n<n*, the molecular weight of soluble PNIPA correlated well with [NIPA]. When T_p>LCST, gels were not formed regardless of [NIPA]. The structure and mechanism of formation of self‐crosslinked PNIPA gels are discussed.

     

Temporally Controlled Ultrasonication‐Mediated Atom Transfer Radical Polymerization in Miniemulsion

Due to the increasing requirement for more environmentally and industrially relevant approaches in macromolecules synthesis, ultrasonication‐mediated atom transfer radical polymerization (sono‐ATRP) in miniemulsion media is applied for the first time to obtain precisely defined poly(n‐butyl acrylate) (PBA) and poly(methyl methacrylate) (PMMA) homopolymers, and poly(n‐butyl acrylate)‐block‐poly(tert‐butyl acrylate) (PBA‐b‐PtBA) and poly(n‐butyl acrylate)‐block‐poly(butyl acrylate) (PBA‐b‐PBA) copolymers. It is demonstrated in the reaction setup with strongly hydrophilic catalyst copper(II) bromide/tris(2‐pyridylmethyl)amine (Cu^IIBr₂/TPMA) responsible for two principal mechanisms – interfacial and ion‐pair catalysis reflecting single‐catalyst approach. This solution turns out to be an excellent tool in controlled preparation of well‐defined polymers with narrow molecular weight distribution (up to Ð = 1.28) and preserves chain‐end functionality (DCF = 0.02% to 0.32%). Temporal control over the polymer chain growth is successfully conducted by turning the ultrasonication on/off. Taking into consideration long OFF stage (92.5 h) during ultrasonication‐induced polymerization in miniemulsion, synthesis is efficiently reinitiated without any influence on controlled characteristics maintaining the precise structure of received PBA homopolymers, confirmed by narrow molecular weight distribution (Ð = 1.26) and high retention of chain‐end functionality (DCF = 0.01%). This procedure constitutes an excellent simple and eco‐friendly approach in preparation of functional polymeric materials.     

Synthesis of 6‐Armed Amphiphilic Block Copolymers with Styrene and 2,3‐Dihydroxypropyl Acrylate by Atom Transfer Radical Polymerization

The controlled free radical polymerization of (2,2‐dimethyl‐1,3‐dioxolan‐4‐yl)methyl acrylate (DMDMA) was achieved by atom transfer radical polymerization (ATRP) in tetrahydrofuran (THF, 50%, v/v) solution at 90°C with the discotic six‐functional initiator, 2,3,6,7,10,11‐hexakis(2‐bromobutyryloxy) triphenylene (HBTP). The 6‐armed polyDMDMA with low polydispersity index (M̄_w/M̄_n = 1.52–1.32) was obtained. The copolymerization of DMDMA with styrene (St) using 6‐armed polySt‐Br as macroinitiator was carried out, and the GPC traces of the copolymers obtained were unimodal and symmetrical, indicating complete conversion of the macroinitiator into block copolymer. The star‐shaped block copolymers with different segment compositions and narrower polydispersity (1.21–1.24) were synthesized, and subsequent hydrolysis of the acetal‐protecting group in 1 N HCl THF solution produced poly[St‐b‐(2,3‐dihydroxypropyl)acrylate] [poly(St‐b‐DHPA)], which was verified by IR and NMR spectroscopy.     

Novel Atom Transfer Radical Polymerization Method to Yield Copper‐Free Block Copolymeric Biomaterials

A new catalyst is reported for atom transfer radical polymerization (ATRP) for the synthesis of copper‐free poly(2‐diethylaminoethyl methacrylate (PDEAEM)‐based pentablock copolymeric biomaterials that have been shown to be effective gene delivery vectors. Biocompatibility is an increasing concern with growing applications for functional polymers with potential applications in drug and gene delivery, because of the residual soluble copper salts used as catalysts. The reported ATRP synthesis method utilizes novel copper(I) oxide nanoparticles as catalysts that can be easily removed after polymerization, with X‐ray spectroscopy (XPS) showing no residual copper in the final product.

     

Atom Transfer Radical Polymerization of Glycidyl Methacrylate: A Functional Monomer

Summary: A detailed investigation of the polymerization of glycidyl methacrylate (GMA), an epoxy‐functional monomer, by atom transfer radical polymerization (ATRP) was performed. Homopolymers were prepared at relatively low temperatures using ethyl 2‐bromoisobutyrate (EBrIB) as the initiator and copper halide (CuX) with N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) as the catalyst system. The high polymerization rate in the bulk did not permit polymerization control. However, homopolymerization in solution enabled us to explore the effects of different experimental parameters, such as temperature, solvent (toluene vs. diphenyl ether) and initiator concentration, on the controllability of the ATRP process. SEC analysis of the homopolymers synthesized confirmed the importance of solvent character on molecular weight control, the lowest polydispersity indices ( ) and the highest efficiencies being found when the polymerizations were performed in diphenyl ether in combination with a mixed halide technique. A novel poly(glycidyl methacrylate)‐block‐poly(butyl acrylate) (PGMA‐b‐PBA) diblock copolymer was prepared through ATRP using PGMA‐Cl as a macro‐initiator. This chain growth experiment demonstrated a good living character under the conditions employed, while simultaneously indicating a facile synthetic route for this type of functional block copolymer. In addition, the isotacticity parameter for the PGMAs obtained was estimated using ¹H NMR analysis which gave a value of σ_GMA = 0.26 in agreement with that estimated in conventional radical polymerization.

SEC chromatograms of PGMA‐Cl macroinitiator and PGMA‐b‐PBA diblock copolymer.     

Block Copolymers from Organomodified Siloxane‐Containing Macroinitiators by Atom Transfer Radical Polymerization

Alternating copolymers of 1,3‐diisopropenylbenzene and 1,1,3,3‐tetramethyldisiloxane were synthesized by hydrosilylation–polyaddition. These linear copolymers were functionalized at both ends with 2‐bromoisobutyryl or benzyl chloride moieties. Subsequently, the obtained organomodified siloxane‐containing macroinitiators were successfully used for the preparation of ABA‐type block copolymers by atom transfer radical polymerization (ATRP) of styrene and tert‐butyl acrylate. The high chain‐end functionality of the macroinitiators was confirmed by ¹H NMR analysis of the macroinitiators and GPC measurements of the obtained ABA‐type block copolymers. The macroinitiator peaks disappeared in GPC traces after ATRP, and the obtained block copolymers showed a significantly narrower molecular‐weight distribution than the macroinitiators.

Synthesis of ABA‐type block copolymers by means of ATRP using organomodified siloxane‐containing, benzyl chloride functionalized macroinitiators.     

Synthesis of Polyacrylonitrile‐block‐Polystyrene Copolymers by Atom Transfer Radical Polymerization

Summary: The synthesis of polyacrylonitrile‐block‐polystyrene (PAN‐b‐PS) copolymers by atom transfer radical polymerization (ATRP) is reported. Chain extension of bromine terminated PAN macroinitiators with styrene was performed using a CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine catalyst system and 2‐cyanopyridine as a solvent. The first‐order kinetic plots of styrene consumption showed a significant curvature, indicating a progressive decrease in the concentration of active species during copolymerization. The loss of the bromide end group was mainly ascribed to the elimination of HBr, as shown by ¹H NMR spectroscopy. By varying the molar ratio of either the catalyst or the monomer to the initiator, a series of PAN‐b‐PS copolymers were prepared, with polydispersities as low as 1.3, and molar compositions ranging from 8.6/91.4 to 35.5/64.5.

¹H NMR spectra of PAN‐b‐PS in DMF‐d₇ at 80 °C.     

Atom Transfer Radical Polymerization of Solketal Acrylate Using Cyclohexanone as the Solvent

Summary: Solketal acrylate (SA) was homopolymerized by atom transfer radical polymerization (ATRP) using CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as the catalyst and cyclohexanone as the solvent with controlled molecular weights and low polydispersities. The prepared bromine‐terminated homopolymers, PSA, were used as macroinitiators to initiate polymerization of tert‐butyl acrylate (tBA) under similar ATRP conditions to produce diblock copolymers, PSA‐b‐PtBA, with controlled molecular weights and low polydispersities. ATRP of SA using bromine‐terminated PtBA as the macroinitiator was also carried out and diblock copolymers, PtBA‐b‐PSA, were obtained. The PSA block was selectively hydrolyzed by stirring for 3 h in 6 N HCl/THF (1/9, v/v) at room temperature to form a poly(glycerol monoacrylate) block. Both blocks of PSA and PtBA were hydrolyzed by stirring in anhydrous trifluoroacetic acid (TFA)/dichloromethane for 4 h, then adding water to the system and stirring for another 3 h to form corresponding diblock copolymers of glycerol monoacrylate and acrylic acid.

Kinetic plot for the atom transfer radical polymerization of solketal acrylate at 90 °C.     

Visible Light‐Induced Atom Transfer Radical Polymerization

Visible light‐induced reverse and simultaneous reverse and normal initiation (SR&NI) atom transfer radical polymerizations of vinyl monomers are examined using various dyes and type I photoinitiators. The effect of photoinitiator types on the control of molecular weight and distribution is described. In both dye and type I photoinitiator sensitized SR&NI ATRP systems, the molecular weights increase linearly with conversion. However, the experimental molecular weights are considerably higher than the theoretical values and the polymers show broad‐molecular‐weight distributions ranging from 1.28 to 1.60 in the dye‐sensitized SR&NI ATRP. However, the polymers obtained by SR&NI ATRP using type I photoinitiator system had molecular weight values close to the theoretical ones and very narrow‐molecular‐weight distributions ranging from 1.11–1.18.     

Liquid‐Crystalline Polymethacrylates by Atom‐Transfer Radical Polymerization at Ambient Temperature

Side‐on and side‐end liquid‐crystalline (LC) polymethacrylates were synthesized by atom‐transfer radical polymerization at 20°C using monofunctional and difunctional initiators. The polymers have narrow molecular‐weight distributions (M̄_w /M̄_n = 1.15–1.45). The polymerization kinetics were determined for a side‐on LC methacrylate, and appear to be first‐order, whatever the initiator used. However, the measured molecular weight is much larger than expected, probably because the initiation step is slow. The thermotropic properties of the LC polymers were studied by thermal optical polarizing microscopy, differential scanning calorimetry, and X‐ray diffraction. For a side‐end LC polymethacrylate, two LC phases were detected although only one has been reported in the literature. The phase sequence was shown to be Cr–SmA–N–Iso.     

Gradient Density Grafted Polymers on Ground Tire Rubber Particles by Atom Transfer Radical Polymerization

Summary: The graft polymerization of styrene and (meth)acrylic monomers via ATRP from cross‐linked rubber particles, produced by recycled tires (“ground tire rubber”, GTR), is reported. GTR particles, obtained by cryogenic grinding process, still contain C?C unsaturations on the surface, which were first oxidized to hydroxyl groups and then modified by reaction with 2‐bromoisobutyryl bromide to serve as ATRP macroinitiators. Graft polymerizations were carried out using CuBr/CuBr₂/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as catalytic system in bulk or in anisole at high temperature. The grafted particles were analyzed by ATR‐IR (spectra were recorded on particles surface and cross‐sectional slices), TGA, SEM and X‐ray microanalysis.

Arrangement of surface‐grafted polymer assemblies along the particles section.     

Kinetic Behavior of Atom Transfer Radical Polymerization of Dimethacrylates

Summary: The reaction behavior and kinetics of the atom transfer radical polymerization (ATRP) of poly(ethylene glycol) dimethacrylates (PEGDMA) were studied with respect to polymerization rate, vinyl conversion and the development of a crosslinked network. The polymerization rates were much slower than the corresponding conventional free radical polymerizations with the ATRP systems exhibiting milder autoacceleration. The linear relationship of the semi‐logarithmic kinetic plot of ln([M]₀/[M]) vs. time did not provide good evidence for any living nature of the system because of the combined effects of diffusion controlled radical deactivation and diffusion controlled monomer propagation. The influence of the spacer length (CH₂CH₂O)_x between the vinyl moieties of the dimethacrylates on the polymerization kinetics was examined. The polymerization rate and final vinyl conversion increased as value of x decreased from 14 to 9 to 4. These increases in rate and conversion were caused by a more rigid network structure with shorter spacer lengths, and thus more restricted diffusion of the catalyst/ligand complexes that impeded the radical deactivation. The effect of temperature on the polymerization rate and final vinyl conversion were also investigated.

Apparent rate constants versus vinyl conversions for the ATRP of PEGDMA with the different spacer lengths at 100 °C.     

Synthesis of Block Copolymers Containing Polybutadiene Segments by Combination of Coordinative Chain Transfer Polymerization,Ring‐Opening Polymerization,and Atom Transfer Radical Polymerization

Two triblock copolymers, polybutadiene‐block‐poly(ε‐caprolactone)‐block‐poly(methyl methacrylate) (PBD‐b‐PCL‐b‐PMMA) and polybutadiene‐block‐poly(ε‐caprolactone)‐block‐polystyrene (PBD‐b‐PCL‐b‐PS), are synthesized by combination of coordinative chain transfer polymerization (CCTP), ring‐opening polymerization (ROP), and atom transfer radical polymerization (ATRP). The molecular structures of these polymers are determined by ¹H NMR and gel‐permeation chromatography (GPC) analysis. The resulting triblock copolymers are evaluated as compatibilizing agents for highly immiscible binary PBD/PMMA and PBD/PS blends. The presence of these compatibilizers substantially reduces the average PMMA or PS droplet size. Furthermore, static water contact angle experiments are employed to assess the modification of the surface properties of nonpolar PBD.

     

Effect of Aromatic Co‐Solvents on the Efficiency of Atom Transfer Radical Coupling Reactions of Fluorinated and Non‐Fluorinated Vinyl Aromatic Polymers

Monobrominated versions of poly(pentafluorostyrene) (PPFSBr), polystyrene (PSBr), and poly(methyl acrylate) (PMABr) are prepared by atom transfer radical polymerization (ATRP) and employed in a variety of atom transfer radical coupling (ATRC)‐type reactions to observe the impact of external aromatic faces on the extent of coupling (X_c). In ATRC reactions assisted with the radical trap 2‐methyl‐2‐nitrosopropane (MNP), X_c is nearly unchanged when the electron‐rich benzene co‐solvent (50% v/v with THF) is replaced with the electron‐poor hexafluorobenzene (HFB) for PSBr and PMABr. In the case of PPFSBr, the addition of benzene to the reaction mixture results in far lower extents of coupling (X_c < 0.2). ¹H NMR spectra of the radical trap MNP in HFB show greater aggregation to the inactive form, compared to the spectra obtained in benzene. To remove the effect of the radical trap interacting with the aromatic co‐solvent and altering the rate of coupling, traditional ATRC reactions are performed with the same co‐solvent systems and, in this case, HFB results in higher X_c values across all polymer types. This is consistent with HFB pushing the position of the K_ATRP further toward the active radical, while benzene increases the reactivity of the MNP radical trap.     

Synthesis of Telechelic Oligomers via Atom Transfer Radical Polymerization, 1

Summary: Atom Transfer Radical Polymerization (ATRP) of styrene was carried out at 110 °C using various substituted 2‐bromoisobutyrates as initiators and the homogeneous catalyst CuBr/1,1,4,7,10,10‐hexamethyltriethylenetetramine (HMTETA). Telechelic oligomers were obtained by coupling the bromo terminated polymers using Cu(0)/PMDETA catalyst at 90 °C. The products were characterized by ¹H NMR and MALDI‐TOF and some unsaturated polymer chains were observed. They could originate from either disproportionation reaction or dehydrohalogenation of halogen terminated oligomers during MALDI analysis. Using this method, polymers with hydroxy, acid, or ester end groups were synthesized in a range of molecular weights from 1 000 to 13 000 g · mol^?1. Coupling efficiency was between 79 and 100%, depending on the structure of the initiator.

Coupling reaction of substituted polystyrenes using Cu(0)/PMDETA system at 90 °C.     

Atom‐Transfer Radical Polymerization: A Strategy for the Synthesis of Halogen‐Free Amino‐Functionalized Poly(methyl methacrylate) in a One‐Pot Reaction

Summary: An initiator containing an alkyl bromide unit and a protected amine functional group is used with CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA), in a 1:2 molar ratio with respect to initiator concentration, in order to obtain amino‐group terminated as well as halogen‐free poly(methyl methacrylate) (PMMA) in a one‐pot atom‐transfer radical polymerization (ATRP). The terminal bromines are replaced by hydrogen atoms of the PMDETA ligand, which acts as a transfer agent. However, terminating side reactions like disproportionation or dehydrobromination occur from the beginning of the polymerization. Kinetic studies by in‐line Raman spectroscopy and off‐line ¹H NMR spectroscopy revealed that the controlled character of the ATRP is lost under these conditions. The measured molecular weights were consistently higher than the theoretical ones and the molecular weight distributions are relatively broad. Thermal analysis of the obtained poly(methyl methacrylate) shows two main degradation steps, one starting from unsaturated end groups (depolymerization), and one caused by main‐chain scission, a further proof for the occurrence of terminating side reactions.

Structural analysis of PMMA by matrix‐assisted laser desorption‐ionization mass spectrometry.     

Physical Properties of PBMA‐b‐PBA‐b‐PBMA Triblock Copolymers Synthesized by Atom Transfer Radical Polymerization

The physical properties of well‐defined poly(butyl methacrylate)‐block‐poly(butyl acrylate)‐block‐poly(butyl methacrylate) (PBMA‐b‐PBA‐b‐PBMA) triblock copolymers synthesized by atom transfer radical polymerization (ATRP) are reported. The glass transition and the degradation temperature of copolymers were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC measurements showed phase separation for all of the copolymers with the exception of the one with the shortest length of either inner or outer blocks. TGA demonstrated that the thermal stability of triblock copolymers increased with decreasing BMA content. Dynamic mechanical analysis was used for a preceding evaluation of adhesive properties. In these block copolymers, the deformation process under tension can take place either homogeneously or by a neck formation depending on the molecular weight of the outer BMA blocks and on the length of the inner soft BA segments. Microindentation measurements were also performed for determining the superficial mechanical response and its correlation with the bulk behavior.

Stress‐strain curves for the different PBMA‐b‐PBA‐b‐PBMA specimens at room temperature and at 10 mm/min.