Radical Polymerization Of Acrylic Monomers Research Articles

Structure-properties relationships of defined CNF single-networks crosslinked by telechelic PEGs

The high structural anisotropy and colloidal stability of cellulose nanofibrils' enable the creation of self-standing fibrillar hydrogel networks at very low solid contents. Adding methacrylate moieties on the surface of TEMPO oxidized CNFs allows the formation of more robust covalently crosslinked networks by free radical polymerization of acrylic monomers, exploiting the mechanical properties of these networks more efficiently. This technique yields strong and elastic networks but with an undefined network structure. In this work, we use acrylate-capped telechelic polymers derived from the step-growth polymerization of PEG diacrylate and dithiothreitol to crosslink methacrylated TEMPO-oxidized cellulose nanofibrils (MATO CNF). This combination resulted in flexible and strong hydrogels, as observed through rheological studies, compression and tensile loading. The structure and mechanical properties of these hydrogel networks were found to depend on the dimensions of the CNFs and polymer crosslinkers. The structure of the networks and the role of individual components were evaluated with SAXS (Small-Angle X-ray Scattering) and photo-rheology. A thorough understanding of hybrid CNF/polymer networks and how to best exploit the capacity of these networks enable further advancement of cellulose-based materials for applications in packaging, soft robotics, and biomedical engineering.

Systematic studies on polyacrylates containing side‐chain hydrogen bonding units: Significant effects of the pendant functions on thermal behaviors and solubility of polymers

AbstractThe synthesis of polyacrylates containing various pendant hydrogen bonding or non‐hydrogen bonding functions and the effects of side‐chain functions on their physical properties are described. The polyacrylates possessing urea, urethane, N‐substituted urea, or carbonate groups are prepared by the free radical polymerization of acrylate monomers possessing the corresponding functional groups. The presence of stronger hydrogen bonding interactions both in bulk and solutions of a highly polar solvent such as dimethyl sulfoxide (DMSO) is supported by the infrared (IR) spectra, whereas the urethane‐based polyacrylate does not exhibit the interaction in the solution. The presence of the stronger hydrogen bonding interactions of the urea unit is discussed by the IR and nuclear magnetic resonance spectra and the density functional theory calculations of the corresponding model compounds. The solubility in organic solvents and the glass transition temperature (Tg) of polymers are also dependent on the structure of the pendant groups. Polar aprotic solvents such as DMSO can only dissolve the urea‐based polyacrylate, while the other polymers are soluble in a much wider range of solvents such as tetrahydrofuran. The urea‐based polyacrylate shows the highest Tg among the polymers studied owing to the presence of plural hydrogen bonding sites in the urea groups.

Triphenylamine-substituted BODIPY dye based photoinitiators for free radical and cationic photopolymerization under visible LED irradiation and application in 3D printing

Until now the photopolymerization process is largely dependent on the hazardous UV or near UV light irradiation. Herein this work, a series of boron dipyrromethene (BODIPY) derivatives substituted by electron-donating triphenylamine moiety on different positions were introduced and investigated thoroughly. Markedly, their photoinitiation abilities under mild conditions, e.g. visible LEDs irradiation and room temperature, were examined for the first time. In combination with an iodonium salt and an amine as the co-initiators, these BODIPY dyes could form three-component photoinitiating systems (PISs) for the purpose to initiate both the free radical polymerization of acrylate monomers and cationic polymerization of epoxy monomers. Interestingly, under the irradiation of LED@405 nm, LED@530 nm or LED@660 nm, some of these BODIPYs-based PISs could conduct high monomeric conversion and rapid photopolymerization rates, as reflected by the relevant photopolymerization kinetics. Then the notable photochemical reactivity of these BODIPY dyes was examined by both steady state photolysis and fluorescence quenching experiments, and theoretical calculation was also performed on them in details. Finally, photocuring 3D printing technique was applied to the acrylate/epoxy mixture resin, as the result the BODIPYs-based PISs demonstrated great potential to fabricate truly three-dimensional objects in macroscopic scale with excellent spatial resolution.

Unimolecular Benzodioxole-based Photoinitiators for Free Radical and Cationic Photopolymerization Under LED Light Irradiation

Compared to amines as coinitiators applied in photopolymerization, benzodioxole derivatives exhibit similar reactivity but less yellowing and lower toxicity. In this paper, a series of unimolecular type II photoinitiators, containing coumarins as chromophores and benzodioxoles as coinitiators, were synthesized. The substituents with different electron donating/withdrawing abilities were designed for systematical investigation of structure-property relationship. Under LED irradiation, the novel photoinitiators can induce not only free radical polymerization of acrylate monomers, but also cationic polymerization of epoxy monomers via radical promoted cationic polymerization mechanism.

N-naphthalimide ester derivatives as Type Ⅰ photoinitiators for LED photopolymerization

In this study, a series of N-naphthalimide ester derivatives (noted NPIE1-NPIE9) with different substituents were designed as visible light photoinitiators (PIs). These structures exhibited good light absorption properties in the visible range. Photoinitiation abilities of NPIEs were evaluated upon exposure to a light-emitting diode at 405 nm. All the structures demonstrated good performances in free radical polymerization of acrylate monomers. Markedly, the function conversion for acrylate monomers in the presence of NPIE1 was higher than that of the benchmark PI diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO). Upon exposure to a light-emitting diode at 455 nm, NPIE1 could also initiate the polymerization of acrylate monomers effectively, i.e., the benchmark photoinitiator (TPO) was not efficient for these long irradiation wavelengths. Through the calculations of the N–O bond dissociation energy and the excited state energy, the cleavage of the N–O bond from the singlet state proved to be favorable. Interestingly, CO2 was detected during the polymerization experiments, which indicated that there was a decarboxylation reaction during irradiation. Based on the obtained results, a mechanism of Type Ⅰ PI behavior can be proposed. Due to the good performance, NPIE1 was successfully used in 3D printing experiments. In addition, high thermal stability of NPIEs in monomers was found. This can be a great improvement for elaborating stable formulations.

New BODIPY Dyes Based on Benzoxazole as Photosensitizers in Radical Polymerization of Acrylate Monomers.

A series of 2-phenacylbenzoxazole difluoroboranes named BODIPY dyes (1–8) was designed and applied as photosensitizers (PS) for radical photopolymerization of acrylate monomer. The light absorption within the ultraviolet-visible (UV–Vis) range (λmax = 350–410 nm; εmax = 23,000–42,500 M−1cm−1), that is strongly influenced by the substituents on the C3 and C4 atoms of phenyl ring, matched the emission of the Omnicure S2000 light within 320–500 nm. The photosensitizer possess fluorescence quantum yield from about 0.005 to 0.99. The 2-phenacylbenzoxazole difluoroboranes, together with borate salt (Bor), iodonium salt (Iod) or pyridinium salt (Pyr) acting as co-initiators, can generate active radicals upon the irradiation with a High Pressure Mercury Lamp which initiates a high-performance UV–Vis light-induced radical polymerization at 320–500 nm. The polymers obtained are characterized by strong photoluminescence. It was found that the type of radical generator (co-initiator) has a significant effect on the kinetic of radical polymerization of acrylate monomer. Moreover, the chemical structure of the BODIPY dyes does not influence the photoinitiating ability of the photoinitiator. The concentration of the photoinitiating system affects the photoinitiating performance. These 2-phenacylbenzoxazole difluoroborane-based photoinitiating systems have promising applications in UV–Vis-light induced polymerization.

Novel photoinitiators based on difluoroborate complexes of squaraine dyes for radical polymerization of acrylates upon visible light

The present article describes the efficiency of novel two-component photoinitiators based on a typical squaraine dye and its difluoroborate analogues for the radical polymerization of acrylate monomers.

Block copolymer synthesis using free-radical polymerization and thiol-maleimide 'click' conjugation.

A method of making block copolymers utilizing free-radical polymerization and subsequent polymer conjugation is described. A disulphide functional radical initiator was used to polymerize methacrylic acid and 3-acrylamidophenylboronic acid. After purification, the disulphide bond of the end group was cleaved, revealing a thiol group which was used for subsequent conjugation to a polylactide containing the complementary maleimide functional group. The method is versatile and can be applied to the synthesis of various block copolymers without requiring the use of controlled/living radical polymerization methods.

Dual Polymerization Pathway for Polyolefin-Polar Block Copolymer Synthesis via MILRad: Mechanism and Scope.

This work explores the mechanism whereby a cationic diimine Pd(II) complex combines coordination insertion and radical polymerization to form polyolefin-polar block copolymers. The initial requirement involves the insertion of a single acrylate monomer into the Pd(II)-polyolefin intermediates, which generate a stable polymeric chelate through a chain-walking mechanism. This thermodynamically stable chelate was also found to be photochemically inactive, and a unique mechanism was discovered which allows for radical polymerization. Rate-determining opening of the chelate by an ancillary ligand followed by additional chain walking allows the metal to migrate to the α-carbon of the acrylate moiety. Ultimately, the molecular parameters necessary for blue-light-triggered Pd-C bond homolysis from this α-carbon to form a carbon-centered macroradical species were established. This intermediate is understood to initiate free radical polymerization of acrylic monomers, thereby facilitating block copolymer synthesis from a single Pd(II) complex. Key intermediates were isolated and comprehensively characterized through exhaustive analytical methods which detail the mechanism while confirming the structural integrity of the polyolefin-polar blocks. Chain walking combined with blue-light irradiation functions as the mechanistic switch from coordination insertion to radical polymerization. On the basis of these discoveries, robust di- and triblock copolymer syntheses have been demonstrated with olefins (ethylene and 1-hexene) which produce amorphous or crystalline blocks and acrylics (methyl acrylate, ethyl acrylate, n-butyl acrylate, and methyl methacrylate) in broad molecular weight ranges and compositions, yielding AB diblocks and BAB triblocks.

Cover Picture: Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers (Angew. Chem. Int. Ed. 8/2020)

Cover Picture: Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers (Angew. Chem. Int. Ed. 8/2020)

Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers

AbstractDevelopment of photocatalysts (PCs) with diverse properties has been essential in the advancement of organocatalyzed atom transfer radical polymerization (O‐ATRP). Dimethyl dihydroacridines are presented here as a new family of organic PCs, for the first time enabling controlled polymerization of challenging acrylate monomers by O‐ATRP. Structure–property relationships for seven PCs are established, demonstrating tunable photochemical and electrochemical properties, and accessing a strongly oxidizing 2PC.+ intermediate for efficient deactivation. In O‐ATRP, the combination of PC, implementation of continuous‐flow reactors, and promotion of deactivation through addition of LiBr are critical to producing well‐defined acrylate polymers with dispersities as low as 1.12. The utility of this approach is established through demonstration of the oxygen‐tolerance of the system and application to diverse acrylate monomers, including the synthesis of well‐defined di‐ and triblock copolymers.

Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers.

Development of photocatalysts (PCs) with diverse properties has been essential in the advancement of organocatalyzed atom transfer radical polymerization (O-ATRP). Dimethyl dihydroacridines are presented here as a new family of organic PCs, for the first time enabling controlled polymerization of challenging acrylate monomers by O-ATRP. Structure-property relationships for seven PCs are established, demonstrating tunable photochemical and electrochemical properties, and accessing a strongly oxidizing 2 PC.+ intermediate for efficient deactivation. In O-ATRP, the combination of PC, implementation of continuous-flow reactors, and promotion of deactivation through addition of LiBr are critical to producing well-defined acrylate polymers with dispersities as low as 1.12. The utility of this approach is established through demonstration of the oxygen-tolerance of the system and application to diverse acrylate monomers, including the synthesis of well-defined di- and triblock copolymers.

Structural effect of oxazolone derivatives on the initiating abilities of dye-borate photoredox systems in radical polymerization under visible light.

Three photoinitiating systems based on new oxazolone derivatives have been developed and their performance in initiation of radical polymerization of acrylate monomers has been tested by differential scanning calorimetry. The absorption characteristics of the oxazol-5(4H)-ones is compatible with the emission characteristics of different light sources like diode pulse solid state lasers. Thus, the dyes were used as sensitizers which are photoreduced during a photochemical reaction in the presence of phenyltriethylborate salt. Results showed that the increase in the dimensionality of the molecule extends the range of light absorption and increases the efficiency of the photoinitiation process. The photoreduction of the oxazolone–borate complex was studied using steady-state and nanosecond laser flash photolysis. The dye singlet and triplet were found to be quenched by the electron donor via an electron transfer process. Rate constants for the quenching of the excited states were high and were found to depend on the dye structure.

Mid-Chain Radical Migration in the Radical Polymerization of n-Butyl Acrylate.

The occurrence of intramolecular transfer to polymer in the radical polymerization of acrylic monomers has been extensively documented in the literature. Whilst it has been largely assumed that intramolecular transfer to polymer leads to short chain branches, there has been some speculation over whether the mid-chain radical can migrate. Herein, by the matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry (MS) of poly(n-butyl acrylate) synthesized by solution polymerization under a range of conditions, it is shown that this mid-chain radical migration does occur in the radical polymerization of acrylates conducted at high temperatures, as is evident from the shape of the molecular weight distribution. Using a mathematical model, an initial approximation of the rate at which migration occurs is made and the distribution of branching lengths formed in this scenario is explored. It is shown that the polymerizations carried out under a low monomer concentration and at high temperatures are particularly prone to radical migration reactions, which may affect the rheological properties of the polymer.

A multifunctionalized macromolecular silicone-naphthalimide visible photoinitiator for free radical polymerization

Abstract Multifunctionalization strategy in the photoinitiator design keeps in step with the requirement of the green chemistry in photopolymerization techniques. Therefore, a multifunctional macromolecular silicone-naphthalimide visible photoinitiator (SND) has been designed and prepared by a convenient thermal polymerization process of 4-(1,3-dioxo-6-(piperidin-1-yl)-1H-benzo[de]isoquinolin-2(3H)-yl)phenyl acrylate (ND-PA) and 3-bis(trimethylsilyoxy)methylsiyl-propylmethacrylate (TSMSPM). Its structure has been confirmed by 1H NMR, 13C NMR, FT-IR and GPC analysis. On one hand, SND has been evidenced as a visible photoinitiator by its successfully initiating the free radical polymerization of acrylate monomers in the presence or absence of a hydrogen donor under the irradiation of different LEDs (405 nm, 455 nm and 470 nm). Furthermore, SND/MDEA (N-methyldiethanolamine) system shows higher photoinitiating activity than that of the commercial CQ (camphorquinone)/MDEA system. On the other hand, this multifunctionalization strategy also brings about some desired modification for cured-materials. After employing only 5 wt% SND as the photoinitiator, for example, a photo-cured film of urethane diacrylate prepolymer (PUA) increases its water contact angles from 70.1° to 101.2°, implying that SND could change the surface property of the PUA cured film from hydrophilic to hydrophobic. Meanwhile, the use of SND as an initiator could significantly improve the resistance against water and ethanol. When the mass fraction of SND increases to 15%, the water and ethanol absorption ratios of the cured PUA films decrease from 4.4% to 1.0% and 8.9% to 2.8%, respectively.

The effect of tunable morphology on the potential application of p(acrylic acid-co-2-ethylhexyl acrylate)/silica nanohybrids

AbstractThe present study deals with the one-pot synthesis of acrylate copolymers/silica latexes through the use of simultaneous radical polymerization of acrylic monomers and a silica sol-gel precursor. In presence of 3-methacryloxypropyltrimethoxysilane (γ-MPS) and 3-aminopropyltriethoxysilane (APTS), compatibility of acrylate chains to the silica species was improved by the chemical bonds as revealed by Fourier transform infrared spectroscopy (FTIR) analysis. Transmission electron microscopy (TEM) demonstrated the successful formation of p(acrylic acid-co-2-ethylhexyl acrylate)/silica nanohybrids with core-shell morphology. The nanohybrids have been used to modify a glassy carbon electrode (GCE). Cyclic voltammetry and electrochemical impedance spectra were utilized to investigate the properties of the modified electrode in a 1.0mKCl solution that contained 1.0 mmK4[Fe(CN)6]/K3[Fe(CN)6], and the interface properties of electrode surfaces. The result showed a dramatic decrease in redox activity as compared to the bare GCE electrode. This revealed a slight increase in electron transfer resistance and the conductivity of the copolymer oligomers and silica species in the hybrid nanostructure. All the electrochemical results illustrated that the p(acrylic acid-co- 2-ethylhexyl acrylate)/silica nanohybrids could immobilize the selective analytes on the electrodes, which had electrochemical catalytic activity. The barrier properties of the hybrid films were also examined via ultraviolet (UV) absorption capacity of the films. It could be concluded that the adsorption capacity was a function of the silica content and uniform dispersion of the nanoparticles in the resultant films.

Intermolecular Transfer to Polymer in the Radical Polymerization of n-Butyl Acrylate

Transfer to polymer in the radical polymerization of acrylic monomers results in chain branching and has significant implications for both reaction kinetics and the microstructure of the resulting polymer. While intramolecular transfer to polymer is more prevalent than the intermolecular pathway, intermolecular transfer to polymer is of particular importance for understanding polymer microstructures. Despite this, the magnitude of the rate coefficient is uncertain, and therefore, predicting the effects of intermolecular transfer remains a challenge. Herein, we seek to provide an estimate of the rate of intermolecular transfer of butyl acrylate by conducting reversible addition–fragmentation chain transfer (RAFT) polymerizations in the presence of a low molecular weight dead polymer. In these experiments, intermolecular transfer to polymer yields a characteristic low molecular shoulder in the polymer detected by UV in the SEC. We obtain a value significantly higher than those previously estimated and discu...

Effect of nanoconfinement on kinetics and microstructure of poly(butyl acrylate) synthesized by microemulsion polymerization

It is now well documented that the radical polymerization of acrylic monomers is heavily influenced by transfer to polymer reactions. In this article we show that transfer to polymer impacts uniquely on the kinetics and microstructure of poly(n-butyl acrylate) synthesized by microemulsion polymerization where the polymer chains are confined to a small volume. Intramolecular transfer to polymer results in a distinctive kinetic profile with a maximum in the rate of conversion at ≈25% monomer conversion in batch microemulsion polymerizations, and this effect is explained by development of a mathematical model. Furthermore, the importance of intermolecular transfer to polymer in seeded microemulsion polymerizations is explored. It is shown that intermolecular transfer to polymer followed by termination reactions lead to the majority of chains in each individual particle being linked together. However, because of the small size of the particles formed in microemulsion polymerization, a limit on the maximum molecular weight of the polymer prevents formation of gel, and allows for control over the polymer microstructure. Finally, we show that this control over microstructure can be used to tune the adhesive properties of films cast from the latex dispersions.

Reevaluation of the Formation and Reactivity of Midchain Radicals in Nitroxide-Mediated Polymerization of Acrylic Monomers

In radical polymerization of acrylic monomers, intramolecular transfer to polymer, and the reactions of the resulting midchain radical, can have a strong impact on both the kinetics and microstructure of the resulting polymer. It has previously been stated that, in nitroxide-mediated polymerization, a substantial proportion of midchain radicals reacts with free nitroxide to form a capped species and that this can lead to a reduction in branch points, in the resulting polymer. In this article, we show that, contrary to previous evidence, the nitroxide capped midchain species cannot be observed in nitroxide-mediated polymerization of butyl acrylate. In addition, we show that, in nitroxide-mediated polymerization, lower than expected concentrations of products arising from transfer to polymer are observed.

Synthesis and studies of novel homodimeric styrylbenzimidazolium dyes applied as photoinitiators for the radical polymerization of acrylic monomers

A series of homodimeric styrylbenzimidazolium dyes were synthesized and applied as visible light initiators for the polymerization of acrylic monomer. In the photoinitiating system, hemicyanine dye cation acts as an electron acceptor whereas borate anion is an electron donor. The photochemistry of the series of bichromophoric hemicyanine borates: 1,3-bis{2-(4-(N-alkylamino)styryl)-1-methylbenzimida- zolium}propane, 1 ,5-bis{2-(4-( N-alkylamino)styryl)-1-methylbenzimidazolium}pentane, 1,10-bis{2-(4-(N-alkylamino)styryl)-1-methylbenzimidazolium}decane, was compared to the photoche- mistry of structurally related monochromophoric hemicyanine borates. The photoinitiating efficiency of the donor-acceptor pairs is discussed based on the free energy change for the photoinduced electron tran- sfer from borate anion to dye cation.