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.     

Atom Transfer Radical Polymerization Using Multidentate Amine Ligands Supported on Soluble Hyperbranched Polyglycidol

Summary: Controlled polymerization of styrene in toluene was achieved by atom transfer radical polymerization (ATRP) using hyperbranched polyglycidol‐supported multidentate amine ligands/Cu^IBr catalyst systems. These catalyst systems with nanoscopic dimensions were more active than the corresponding low‐molecular‐weight ligands. The controlled/living nature of the polymerization is supported by linear first order plots (ln[M]_o/[M] versus time) and a linear increase of versus conversion as well as low polydispersity. Similar controlled polymerization of methyl methacrylate was possible in acetonitrile. Up to 97% of total copper used for polymerization could be removed from the polymer by simple precipitation in methanol and filtration.

Molecular weight characteristics for styrene polymerization using PG‐triamine as a ligand.     

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.

     

Synthesis and Characterization of Styrene/Butyl Acrylate Linear and Star Block Copolymers via Atom Transfer Radical Polymerization

Summary: Well‐defined styrene (S) and butyl acrylate (BA) linear and star‐like block copolymers are synthesized via atom transfer radical polymerization (ATRP) using di‐ and trifunctional alkyl halide initiators employing the Cu/PMDETA (N,N,N′,N″,N″‐pentamethyldiethylenetriamine) catalyst system. Initial addition of Cu^II deactivator and utilization of halogen exchange techniques suppresses the coupling of radicals and improves cross‐propagation to a large extent, which results in better control over the polymerization. Two types of star‐like PBA/PS block copolymers are prepared by using core‐first techniques: a trifunctional PBA or PS macroinitiator extended with the other monomer. Block copolymers with a well‐defined structure and low polydispersity (PDI = ) are obtained in both cases. A trifunctional PBA₃ macroinitiator with = 136 000 g · mol^?1 and PDI = 1.15 is extended to (PBA‐PS)₃ star‐like block copolymer with = 171 100 g · mol^?1 and PDI = 1.15. A trifunctional PS₃ macroinitiator with = 27 000 g · mol^?1 and PDI = 1.16 g · mol^?1 is extended to (PS‐PBA)₃ with = 91 500 g · mol^?1 and PDI = 1.40. The individual star‐like macromolecules as well as their aggregates are visualized by atomic force microscopy (AFM) where the PS and PBA adopt the globular and extended conformation, respectively. For the PBA core star block copolymers, PS segments tend to aggregate either intramolecularly or intermolecularly. PS core star block copolymers form aggregates with a PS core and emanating PBA chains. Most aggregates have ‘n × 3’ arms but minor amounts of ‘defective’ stars with 4, 5, 8, or 11 arms are also observed. The AFM analysis shows that PS core star block copolymers contain about 92% three‐arm block copolymers, and the efficiency of cross‐propagation is 97.3%.

Schematic representation of the synthesis of BA/S star‐like block copolymers by ATRP, and their resultant AFM images.     

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.     

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.     

Modeling of Atom Transfer Radical Polymerization with Bifunctional Initiators: Diffusion Effects and Case Studies

Summary: A mathematical model for atom transfer radical polymerization (ATRP) with bifunctional initiators was developed. The model was validated with three case studies in bulk and solution polymerization. We used only polymer yield data to estimate some of the model parameters, while others were obtained from the literature. The model fits the polymer yield data and also predicts weight‐average molecular weights and polydispersities very well. The free volume theory was also incorporated to the model to study the effect of diffusion‐controlled reactions. The adjustable parameters in the free volume theory for the termination, propagation, activation, and deactivation reactions were varied to show the effect on monomer conversion, polymer chain length, and polydispersity. The model shows that diffusion‐limited termination reactions produce polymer with smaller polydispersities, while diffusion‐limited propagation reactions have the opposite effect. Both models, considering and neglecting diffusion effects on the kinetic rate constants, were compared with experimental data. Even though the model predictions for monomer conversion, number‐average molecular weight, and polydispersity are good in both cases, the simulations indicate that diffusion‐controlled reactions can be ignored for the cases studied in the three case studies described in this paper.

Comparison between model predictions and experimental data for BA polymerization of number‐average molecular weight in bulk at 90 °C.     

Activation–Deactivation Equilibrium of Atom Transfer Radical Polymerization of Styrene up to High Pressure

Atom transfer radical polymerization (ATRP) equilibrium constants, K_ATRP, are measured for copper‐mediated styrene polymerizations in acetonitrile solution using several different ligand and initiator systems. Application of high pressure increases the rate of an ATRP by enhancing both K_ATRP and the propagation rate coefficient. This favorable effect is not counterbalanced by an increase in dispersity. A comparison of the values of K_ATRP measured on monomer‐free model systems shows close agreement with values measured during styrene polymerization.     

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.     

Parallel Synthesis of Polymer Libraries Using Atom Transfer Radical Polymerization (ATRP)

Parallel atom transfer radical polymerization (ATRP) of styrene and t‐butyl acrylate (t‐BA) was investigated. A series of ATRP polymerizations were carried out in parallel with varying targeted molecular weight ( ) and showed excellent reproducibility of , polydispersity, and conversion. In addition, polymerizations were done to determine the effect of inhibitor on styrene polymerization, yielding similar results. In synthesizing a library of polymers having varying using ATRP, polymers having lower reach their target values earlier and the others continue to react under heating, resulting in peak broadening for the low polymers. Reinitiation experiments indicated that termination reactions were taking place resulting in “dead” polymer chains which were unable to reinitiate polymerization.

     

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.     

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.     

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 Telechelic Oligomers via Atom Transfer Radical Polymerization, 3

Summary: ATRP of butyl α‐fluoroacrylate (FABu) was carried out at 90 °C using methyl 2‐bromoisobutyrate (2‐MBiB) as initiator and the homogeneous catalyst CuBr/1,1,4,7,10,10‐hexamethyltriethylenetetramine (HMTETA). Telechelic oligomers were obtained by coupling the bromo terminated polymers in the presence of Cu⁽⁰⁾ and 2,2′‐bipyridyl ligand. The ¹H and ¹⁹F NMR, SEC and MALDI TOF analyses show that the poly(FABu) chains recombination was the main reaction (80% yield). About 20% of disproportionation reaction also occurs in the process, whereas transfer reactions can be neglected.

Compounds produced by recombination, disproportionation and transfer during the coupling reaction of poly(FABu) oligomers.     

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 Copolymerization of α‐Olefins with Methyl Acrylate: Determination of Activation Rate Parameters

Summary: The activation rate parameters of three model compounds for an acrylate/α‐olefin atom transfer radical copolymerization were investigated. It appeared that the activation of the model compound for the dormant acrylate chain (methyl‐2‐bromopropionate) was quite fast (k_act = 0.018 (±0.001) L · mol^?1 · s^?1 at 0 °C). The activation of the model compound for the dormant α‐olefin chain end (2‐bromobutane) did not take place at any appreciable rate (0 and 35 °C). The introduction of a penultimate acrylate unit in the model compound for the dormant α‐olefin chain end (H‐MA‐hexene‐Br) did not result in any activation at 0 and 35 °C. Only at 70 °C some activation was visible after 18 h reaction time. By inference, these results confirm our earlier conclusion that the ATR copolymerization of acrylates and α‐olefins is possible due to the fast crosspropagation of an α‐olefin chain end radical with an acrylate.

¹H NMR spectra for the MBrP/Cu(I)Br/PMDETA/hydroxy TEMPO (1/10/10/10) mixture.     

Photochemically Mediated Atom Transfer Radical Polymerization Using Polymeric Semiconductor Mesoporous Graphitic Carbon Nitride

Photochemically mediated atom transfer radical polymerization of vinyl monomers is successfully activated by ecofriendly heterogeneous mesoporous graphitic carbon nitride (mpg‐C₃N₄). This method pertains to the use of mpg‐C₃N₄ as photoactivator for reduction of initially loaded copper(II) species, thus promoting the in situ formation of the copper(I) species. The controlled nature of the polymerizations in both natural sunlight and UV‐light irradiation at ambient temperature is confirmed by the good agreement of the kinetics of the ­polymerization with theoretical values. The light on–off experiments ­demonstrate that polymerizations are clearly initiated and moderated by either UV light or sunlight.

     

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.     

Kinetic Analysis of the Atom Transfer Radical Polymerization of Butyl Acrylate Mediated by Cu(I)Br/N,N,N ′,N ′,N ″‐Pentamethyldiethylenetriamine

The kinetics atom transfer radical polymerization (ATRP) of n‐butyl acrylate, using the Cu(I)Br/N,N,N ′,N ′,N″‐pentamethyldiethylenetriamine catalytic system and methyl‐2‐bromopropionate as initiator, is studied. The orders of the initiator, activator and deactivator are estimated for two systems – one close to bulk conditions (5 vol.‐% toluene added), and another with 10 vol.‐% N,N‐dimethylformamide added. The former system gave non‐integer orders for initiator and activator, while the latter gave orders of unity for both of these reactants. The second system gave a non‐linear response for the deactivator.     

Effect of [Pyridylmethanimine]/[CuI] Ratio,Ligand, Solvent and Temperature on the Activation Rate Constants in Atom Transfer Radical Polymerization

Summary: The effect of [L]/[Cu^I] ratio, ligand, the choice of solvent and temperature on the activation rate constants in ATRP with catalyst complexes formed with two pyridylmethanimine ligands [PMI, i.e., N‐propyl‐pyridylmethanimine (NPPMI) and N‐octyl‐pyridylmethanimine (NOPMI)] were studied. Maximal values for the apparent rate constants were observed at an [L]/[Cu^I] ratio of ∼2/1 and 1/1 in polar and nonpolar solvents, respectively. This is similar to the results obtained with the Cu^I/bpy system and was attributed to the formation of [Cu^I/L₂]⁺Br⁻ and [Cu^I/L₂]⁺Cu^IBr species. The Cu^I/NPPMI system was only soluble in polar reaction media; however, the Cu^I/NOPMI complex was soluble in both polar and less polar reaction media. The activation rate constants increased with increasing temperature and the value of activation energy (E_a) for Cu^I/NPPMI in the activation process was 58.1 kJ · M ⁻¹ · K⁻¹.

Arrhenius plot of ln k_a versus 1/T for the activation process for NPPMI and bpy in acetonitrile; [Cu^IBr/NPPMI₂]₀ ([Cu^IBr/bpy₂]₀)/[EtBriB]₀/[TEMPO]₀/[TCB]₀ = 20/1/10/2 × 10⁻³ M .