Polymerization Rate Modulated by Tetraarylborate Anion Structure

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Polymerization Rate Modulated by Tetraarylborate Anion Structure: Direct Correlation of Hammett Substituent Constant with Polymerization Kinetics of 2‑Hydroxyethyl Methacrylate Willy G. Santos,* Fernando Mattiucci, and Sidney J. L. Ribeiro* Institute of Chemistry, São Paulo State University - UNESP, CP 355, Araraquara, SP 14801-970, Brazil

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S Supporting Information *

ABSTRACT: A series of para-substituted tetraarylborates were investigated as aryl radical generator for photopolymerization reactions, using 2-hydroxyethyl methacrylate as monomer. The first steps of the photopolymerization process involve the use of tetraarylborate as monocomponent photoinitiator or combination of two-component systems (riboflavin and tetraarylborate anion) to generate the aryl radical species, which is used to initiate the radicalar polymerization of vinyl monomers. Both photoinitiator systems herein investigated are very attractive for vinyl polymerization upon light irradiation. The polymerization and photochemical results indicate that the changes on the para-substituted ligand may be used to improve the formation of aryl radical species and the rate constant of electron transfer into two-component systems. In consequence of this reaction, the kinetics of radical polymerization is also modulated by electron donor/acceptor features of para-substituent ligands.

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form (Rib•−), which goes to nonreactive end-products passing by the photobleaching process.27,28,30 Electron transfer reactions from the photoinduced excited state of riboflavin represent an important route of free radical formation that in the presence of an electron donor or coinitiator (i.e., amine) may initiate vinyl monomer polymerization.28−31,33 Alkylboron derivatives have been used as coinitiators of photopolymerization reactions34−39 as well. The irradiation of the two-component system made by a dye and an alkylboron compound release a free and reactive alkyl radical with subsequent reaction with monomer units, initiating the polymerization process.35,38,40−45 Here we report the use of four different p-phenyl-substituted tetraarylborate anions (Scheme 1) as monocomponent or as co-initiator of methacrylate polymerization, using ultraviolet or visible light irradiation to generate free aryl radicals, respectively. The lifetime of transient species and kinetics of polymerization are also investigated to understand the effect of p-substituent ligands in the photoinduced radical formation and electron transfer process between tetraarylborate (donor) and 3Rib* (acceptor).

INTRODUCTION Radicals photogenerated by light exposure are certainly one of the most interesting and promising ways for controlled radical polymerization.1−8 Temporal control in a photopolymerization reaction relies on a dynamic regulation of the polymerization activator or initiation of reactive species in the presence of light exposition to start a photoreaction. Other controlled/living radical polymerization (CLRP) techniques such as atom transfer radical polymerization (ATRP),8−17 reversible addition−fragmentation chain transfer polymerization (RAFT),8,18−23 and nitroxide-mediated polymerization (NMP)24−26 have been used in visible-lightmediated polymerization for the synthesis of polymers and materials with spatial and temporal control. However, an efficient and controlled photoreaction is not so trivial, and the absence of information about photochemical mechanisms, rate constant of electron transfer, and characterization of transient species is detrimental. Riboflavin or as well-known B2 vitamin is a naturally occurring pigment with absorption bands in the visible region.27−32 The photoreduction mechanism of riboflavin by electron donor compounds involves an initial one-electron reduction of the triplet state (3Rib) giving the semireduced © XXXX American Chemical Society

A

DOI: 10.1021/acs.macromol.8b01361 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Chemical Structures (L = Ligand)

Figure 1. Transient absorption (a) spectra and (b) decay traces for the Rib/borate-1 system in ethanol at 303 K, λlaser = 480 nm (5 mJ cm−2). (c) Stern−Volmer plots generated from the triplet excited-state lifetime (τ) quenching curves of riboflavin measured at various concentrations of tetraarylborate anion. Panel c inset: Hammett plot between kq and σp. (d) UV−vis absorption spectra of riboflavin (3 × 10−5 mol L−1) in the absence and (panel d inset) presence of various concentration of borate-1, using a led as continuous irradiation source at 410 ± 5 nm.

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Electron Paramagnetic Resonance Measurements. X-band (9.5 GHz) EPR measurements were performed at 303 K using a TE102 rectangular cavity with 100 kHz magnetic field modulation and 0.5 G modulation amplitude (EMX plus, Bruker BioSpin, Rheinstetten, Germany). Light irradiation of the initiator system was performed using a LED laser at 410 ± 5 nm (1200 mW cm−2) placed at 0.3 m from the cavity optical window. Measurements were performed in free oxygen solution. Density Function Theory (DFT) Calculations. DFT calculations, performed with the Gaussian 98 program using the threeparameter hybrid functional B3LYP with the 6-31G* basis set, were performed to obtain data on the spin distributions on the investigated aryl radical form. Polymerization Measurements. The polymerization of HEMA photoinitiated by one-component (tetraarylborate) or two-components (Rib + tetraarylborate) was measured by photo-DSC (TA DSC Q100 instrument) and a TA-PCA photo unit equipped with an UV− vis light source (200−520 nm). All photopolymerizations were

EXPERIMENTAL SECTION

Chemicals. Riboflavin, sodium tetra(phenyl)borate, sodium tetra(p-toluyl)borate, sodium tetra(biphenyl)borate, tetra(pchlorophenyl)borate, 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO), and ethanol were used as purchased from Sigma-Aldrich. The monomer 2-hydroxyethyl methacrylate (HEMA) from SigmaAldrich containing 50 ppm of hydroquinone was vacuum distilled before use. UV−Vis Absorption and Emission Measurements. Absorption and fluorescence measurements were recorded on a Shimatzu UV 2550 spectrometer and a Hitachi F-4500 spectrofluorometer at 298 K, respectively. Laser Flash Photolysis. Transient absorption spectra and time decay were measured on an Edinburgh instruments LP-920. A Quantel Brilliant Nd:YAG laser with third harmonic (355 nm) coupled to an Opotek Inc. Rainbow optical parametric oscillator was used to obtain excitation wavelength at 480 nm with 5 ns pulse. B


Article

Macromolecules Table 1. Polymerization Rate and Quenching Rate Constant Values monocomponent system (tetraarylborate) tetraarylborate (ligand) borate-2 borate-3 borate-1 borate-4

(CH3) (phenyl) (H) (chloro)

Rp(1%) × 10−4 (min−1) 10.0 9.2 8.7 6.8

ij R p(borate) yz jj zz k R p(borate‐1) { 1.3 1.1 1.0 0.6

two-component system (tetraarylborate + riboflavin) 2

ij 3kq(borate) yz jj 3 z j kq(borate‐1) zz k { 1.8 1.1 1.0 0.3

3 kq × 109 (L mol−1 s−1)

6.0 3.7 3.2 1.2

RP (1%) × 10−5 (min−1) 4.0 3.3 3.2 2.5

ij R p(borate) yz jj zz k R p(borate‐1) { 1.6 1.1 1.0 0.6

2

Hammett parameter (σp)a −0.17 −0.01 0 0.23

σp values obtained from the literature.49

a

performed under N2 flux (50 mL min−1) in an aluminum pan with 15 mg of monomer solution, and 5 W cm−2 irradiation was performed between 220 and 500 nm spectra range. A cutoff filter at 390 nm was used for measurements with visible irradiation. Gel Permeation Chromatography (GPC) Analysis. Molecular weight (Mn and Mw) and dispersity of the polymer were determined by a GPC instrument (Shimadzu Prominence LC system) via a refractive index detector and DMF as eluent. The flow rate of the system was 1 mL min−1 at 40 °C. The molecular weights were calculated using polystyrene standards.

sion of the 720 nm transient absorption (Figure 1b,c). The quenching rate constant for each borate anions was found around 109 L mol−1 s−1, which is 100× faster than the safranine/2-ethylamino diphenylborinate44 or riboflavin/amine systems.29−31 Encinas and co-workers have reported the exited singletstate deactivation of riboflavin by amine derivatives,30 which may indicate an electron transfer process in the excited singlet state. However, for the riboflavin/tetraarylborate system here investigated, the emissive singlet state is not affected by presence of borate anions ruling out the possibility of electron transfer in the excited singlet state (1Rib*). Figure 1d (graph inset) shows clearly that riboflavin is decomposed in the absence of tetraarylborate anion. In the presence of various concentration of tetraphenylborate, the decomposition is accelerated through a redox process leading to a faster degradation of the riboflavin. Photogeneration of Aryl Radical. By use of a new approach of the Hammett equation for the radical aryl stabilization, the effect of the EDG character of aryl ligands on the rate-determining step involved in the electron transfer was developed as an effective method to predict the generation of aryl radical species.

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RESULTS AND DISCUSSION Steady and Dynamic Processes of Transient Species. As previously investigated by Santos and co-workers,40,46 during irradiation in the UV-absorption band (λ < 290 nm) of the tetra(phenyl)borate, the 3π,π* transition is excited with subsequent B−C(aryl) bond cleavage, generating a reactive phenyl radical and a low reactive boranyl radical species. By use of visible light as irradiation source, only riboflavin is photosensitive, showing absorption bands up to 500 nm, and the triple state of riboflavin is so obtained with oxidation potential around 1.4 eV, which is higher than other potential values for several tetraarylborates (Eox = 0.5−1.2 eV).47,48 These potential values indicate that the riboflavin in the triple state may be used to oxidize tetraarylborate anions, as predicted by the well-known Rehm−Weller equation i D+ y iAy ΔGet = Ejjjj zzzz − Ejjj − zzz − ΔE00 + wp D kA { k {

log k = [log k 0] + ρ × σp

Hammett equation

where k0 and k are the reference reaction rate of the unsubstituted (hydrogen-ligand) and a para-substituent ligand on the tetraarylborate anion structure, respectively. ρ is a constant value for the reaction, which depends on the type of the intermediate species in the photochemical reaction. σp is the Hammett parameter for the para-substituent constant. The corresponding σp values for each ligand on the tetraarylborate structure are listed in Table 1. Scheme 2 shows the overall mechanisms of radical photogeneration and electron donor/ acceptor influence of ligands on the tetraarylborate structure. A plot of the quenching rate constant vs Hammett parameter (σp), as shown in the graphic insert (Figure 1c), is linear with negative slope (ρ = −0.89). This negative ρ-value demonstrates that a positive charge buildup occurring in the aryl group, predicting the oxidation process involved in the photochemical reaction, precisely in an aryl−boron fragment. This hypothesis provides yet another evidence where a redshift of the Raman band around 1600 cm−1, attributed to the aryl breathing (multiovertone modes), was found for different EDG character of para-substituent ligands and also indicates an interesting effect of the ligand into the boron−aryl bond stability. In the monocomponent system, the borate anion only absorbs in the ultraviolet spectra region. After excitation, the triple state of the tetraarylborate populated and quickly deactivated by boron−aryl homolytic bond cleavage yielding

(1)

where E(D+/D) and E(A/A−) correspond to the donor and acceptor ground state potentials, respectively, ΔE00 is the transition energy between the vibrationally relaxed ground and excited states of the fluorophore, and wp is the Coulomb stabilization energy associated with the intermediate radical ion pair. Figure 1 shows the transient absorption spectra of N2saturated solution of riboflavin (5 × 10−5 mol L−1) in the presence of borate-1 (1 × 10−5 mol L−1), using laser excitation at 480 nm (5 mJ, 5 ns pulse) and ethanol as solvent media. In the absence of tetrarylborate anion, the transient absorption band of 3Rib* is easily observed between 500 and 800 nm with lifetime decay of 14 μs. In the presence of tetraarylborate salt, the transient absorbance of 3Rib decays significantly faster with concomitant increase of a new longlived component around 500−700 nm (see Figure 1b and graphic insert) which remains over 100 μs. This long-lived component is attributed to the semirreduced form of riboflavin (Rib•−) as a consequence of an electron transfer process between tetraphenylborate (donor) and 3Rib* (acceptor). The quenching rate constant of the triplet state of riboflavin by different tetraarylborates was determined by the suppresC


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Macromolecules

was attributed to the aryl radical adduct (TMPO-Aryl•). The coupling constants (aN = 14.50 G and aH = 22.6 G) are in agreement with other reference values for the adduct radical (Ph−DMPO radical) using DMPO (5,5-dimethylpyrrolidone N-oxide) as spin trap molecule.50 In the absence of tetraarylborate salt, no radical adducts are observed by the EPR technique at room temperature. In comparing the red simulation with the experimental spectrum (black line), a second and independent radical adduct was also confirmed with 13 broad bands. This second adduct radical was assigned to the triarylborane−TMPO adduct such as proved by satisfactory simulated spectra (blue line), using aN = 11.40 G, aB11 = 2.90, and aH = 4.00 G as coupling constants. The influence of the boron-10 isotope into the simulated EPR spectra was not significant. From DFT calculations results (see the Supporting Information), it can be noted that the para substituent does not exert a significant effect upon the geometry of the aryl radical. However, the spin density is mainly localized on the ring, whereas the most significant fraction of the spin is on the carbon atom. See Figure 3a for the orbital contour of the borate-3 fragment.

Scheme 2. Photochemical Scheme of Aryl Radical Formation Using Ultraviolet and Visible Light

triphenylborane radical and phenyl radical. The mechanism of radical formation by direct excitation of tetraarylborate anion is shown in Scheme 2. In a two-component system, a photoinduced intermolecular electron transfer from the tetraarylborate to the riboflavin may occur, producing the oxidized species of tetraarylborate, which is quickly fragmented to the stable triarylborane molecule and the nonstable aryl radical species. Comparing the quenching rate constant (kq) values for different riboflavin/tetraarylborate systems, an electron donating group (EDG) such as a methyl ligand (e.g., borate-2) tends to accelerate the rate of the reaction by low electronic stabilization of radical species. On the other hand, an electron-withdrawing group (EWG) such as a chloro ligand (e.g., borate-4) tends to decrease the rate constant of reaction; consequently, low values for the rate constant of electron transfer are also estimated. See Table 1 for 3 kq values. The CW-EPR spectrum was obtained upon UV-irradiation of tetraarylborate (Figure 2), in the presence of the 3,3,5,5tetramethylpirrolidone N-oxide (TMPO) spin-trap agent. It

Figure 3. (a) Spatial distributions of spin density on ligand−phenyl fragment (ligand = phenyl) in the SOMO. (b) Molecular electrostatic surface potential (MESP).

In Figure 3b, the molecular electrostatic potential (MESP) map displays the regions to find the unpaired electron (regions in red color) and also represents the most probable region to react with monomer molecules. An interesting observation of the nature of the para-substituent comes from the charge distribution, where the negative charge density located in the ring increases as the ligand move from electron-withdrawing substituents to electron-donating ones (chloro < H < phenyl < methyl). A similar resonance effect has been suggested by Baciocchi51 for aryl methyl sulfoxides with electron-withdrawing substituents. Polymerization Reaction for the Monocomponent System. Polymerization measurements in the presence of different tetraarylborate were performed by isothermal photocalorimetric analysis at 303 K under irradiation in the UVspectra range (λ < 390 nm). The polymerization rate (Rp) was calculated using

Figure 2. EPR spectra of ethanol solution of riboflavin (1 × 10−3 mol L−1) and tetraphenylborate (5 × 10−3 mol L−1) system after 5 min of irradiation at 410 ± 5 nm. Red line: simulated EPR spectra of phenylradical adduct (phenyl−TMPO•), using aN = 14.50 G and aH = 22.6 G. Blue line: simulated borane−radical adduct (triarylborane− TMPO•), using aN = 11.40 G, aB = 2.90, and aH = 4.00 G as coupling constants. Microwave power of 2 mW.

Rp = D

dHt /dt × 100 ΔH0

(2) DOI: 10.1021/acs.macromol.8b01361 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. Rate constant of polymerization photoinitiated by the monocomponent system, borate-1 (1 × 10−3 mol L−1), using UV-light as irradiation source: (a) kinetic of polymerization; (b) Rp vs conversion % plot. Graphic insert: Hammett correlation at polymer conversion of 0.25% for different tetraarylborate structures at 303 K. Power intensity = 5 W cm−2.

Figure 5. Polymerization rate of HEMA photoinitiated by Rib (1 × 10−3 mol L−1) and borate-1 system using visible light (λ > 390 nm). (a) Kinetic of polymerization performed at 303 K and different borate-1 contents. (b) Polymerization rate vs conversion plot. (c) Kinetic of polymerization at different temperatures. Graphic insert: Arrhenius plot.

where dHt is the reaction heat evolved at dt and ΔH0 is the heat evolved assuming total conversion (ΔH0 = 60 kJ mol−1).35,43 The polymerization rates at various initial conversion stage were determined for the different tetraarylborate structures. The corresponding polymerization rates at different times of irradiation are shown in Figure 4. See Table 1 for RP values at 1% of conversion degree. From the polymerization data presented in Table 1 (monocomponent system), all tetraarylborate structures can initiate efficiently the polymerization under UV-light irradi-

ation. The mechanism of polymerization may be traced to the reaction of the tetraarylborate with light, yielding a reactive aryl radical (aryl•) that initiates efficiently the polymerization of HEMA monomer. hυ

ISC

borate → 1 borate* ⎯⎯→ 3 borate* 3

kd

borate* → borane• + aryl•

aryl• + HEMA → aryl−HEMA• E

(3) (4) (5) DOI: 10.1021/acs.macromol.8b01361 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. Deconvolution of GPC elution curve for (a) the monocomponent system (borate-1) and (b) two-component system (riboflavin + borate-1). (c) Hammett correlation with Mw/Mn.

borane• + HEMA → borane−HEMA• nHEMA

aryl−HEMA• ⎯⎯⎯⎯⎯⎯⎯→ polymer

where kd(borate) and kd(borate‑1) are the dissociation constants of a specific borate and borate-1 systems, respectively. See Table 1 for polymerization rate and the ratio of these rate values. In agreement with Figure 4b, inset, the linear correlation of the logarithm function of Rp2 and the Hammett constant parameter (σp) proves the significative contribution of parasubstituent ligand in the first stage of polymerization, where the rate constant for bond dissociation (kd) to generate reactive radicals is evident. Polymerization Reaction for the Two-Component System. By use of visible light as the irradiation source, the mechanism of photopolymerization initiated by two components, riboflavin and tetraarylborate, is depicted by the following equations:

(6) (7)

Figure 4b inset shows a linear plot between Rp at 1% of conversion and Hammett constant parameter (σp), confirming that the polymerization rate is also affected by the parasubstituent ligand. In fact, the polymerization rate is affected in the initiation step where initiator concentration (or aryl radical concentration) in initiating the polymerization is higher, as depicted in the usual expression ji fk [borate] zyz R p = k pjjj d zz j z 2kt k {

1/2

[M] (8)

hυ

2

(R p) = k′ × kd[borate]

3

(11)

ket

Rib* + borate → Rib•− + borate•+ kd

borate•+ → borane + aryl• •

nHEMA

aryl−HEMA• ⎯⎯⎯⎯⎯⎯⎯→ polymer

(12) (13)

•

aryl + HEMA → aryl−HEMA

(9)

where k′ is the global rate constant. Rewriting eq 9 as a function of two tetraarylborate systems, the ratio square of two polymerization rate may be expressed as k jij R p(borate) zyz jj zz = d(borate) jj R zz kd(borate‐1) k p(borate‐1) {

ISC

Rib → 1Rib* ⎯⎯→ 3 Rib*

where the factor 1/2 takes into account that only a reactive radical (e.g., aryl radical) is derived from each initiator fragmentation (eq 4). Assuming that the kp, kt, and f are equal for all tetraarylborate systems, rewriting eq 8, the square of the polymerization rate will be

(14) (15)

Figures 5a and 5b show the direct contribution of tetraarylborate content to increase the conversion degree and polymerization rates. As indicated in eq 9, the (Rp)2 vs [borate1] plot (Figure 5b: inset) has a slope of which is proportional to the dissociation constant (kd) such as demonstrated in eq 9,

2

(10) F


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Macromolecules Table 2. Polymer Weight Distribution for the Monocomponent System (Borate-1) and (b) Two-Component System (Riboflavin + Borate-1) system

peak no.

Mn × 106 (g mol−1)

Mw × 106 (g mol−1)

Mw/Mn

percentage (%)

monocomponent

1 2 3 4 5 totala 1 2 3 4 5 6 totala

7.80 2.35 0.28 0.06 0.03 0.27 7.60 2.50 0.28 0.06 0.04 0.03 0.27

9.11 2.63 0.51 0.06 0.03 2.58 8.70 2.70 0.50 0.06 0.04 0.03 2.40

1.2 1.1 1.8 1.0 1.0 9.5 1.1 1.1 1.8 1.0 1.0 1.0 8.9

18.5 24.3 50.5 2.8 3.9 100 16.8 23.7 51.9 4.1 1.9 1.6 100

two-component

a

Molecular weight and distribution for total GPC elution curve.

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CONCLUSIONS In conclusion, the photochemical efficiency of tetraarylborates to generate aryl radicals was found dependent on the structural features of the para-substituent. This efficiency is a consequence of electron donor character of para-substituted aryl ligand on the tetraarylborate structure, where the negative charge density present in the ring increases as the ligand move from electron-withdrawing substituents to electron-donating ones (chloro < H < phenyl < methyl). Otherwise, due to the electron-transfer characteristics of the primary photochemical process, the polymerization rate at 1% of conversion is also affected. Indeed, the modulation of polymerization rate by tetraarylborate structure covers an overview of the development of innovative methodologies for radical polymerization based on photoinduced electron transfer reactions and modulation of the polymer size.

and the increase in the amount of borate-1 results in an increase of radical content with concomitant increase of the polymerization rate. In Table 1, for both component systems, similar values for the ratio square of the polymerization rate and the ratio square of the quenching rate constants are found, which indicates that the para-substituent effect on the tetraarylborate structure has a strong contribution in the polymerization process. From Figure 5c, plotting Rp(max) vs 104/RT in an Arrhenius plot (Figure 5c: inset), the angular coefficient for the tetraphenylborate system indicates an overall activation energy around 27.5 kJ mol−1 for the global reaction. This activation was similar for three other tetraarylborate systems (Ea ≈ 27− 30 kJ mol−1) used in this work. Other conventional photoinitiation systems such as riboflavin/amine12,30,31 or safranine/borinate35,43−45 have been observed with low values of activation energy (Ea < 10 kJ mol−1). In this sense, the activation energy found for the tetraarylborate systems reveals a moderate stability against thermal process, also indicating that the use of the tetraarylborate as initiator or co-initiator systems may be useful in polymerization studies where the major polymerization pathway is the photochemical process. Polymer Weights and Dispersity Characterization. After 25 min of irradiation, PHEMA obtained at 40 °C had multimodal molecular weight distribution, thus giving a large Mw/Mn. No correlation between σp (Hammett parameter) and Mw/Mn was found for all systems here investigated. See Figure 6 and Table 2 for GPC elution curve and polymer weight distribution, respectively. The linear dependence between σp (Hammett parameter) and Rp at 1% of the conversion (Ri) was not found in the termination step (polymer conversion >80%), where the diffusion-controlled reactions directly affect the rate of polymerization and the control of polymer molecular weight. Indeed, in the termination step, other radicals than aryl radicals are involved in the termination step. For both systems (see Figure 6c), low values of Mw/Mn were found for tetraarylborates structure with donor character ligands, showing a linear dependence with Hammett parameter (σp) and also indicating that the aryl radical may some contribute in the termination reactions. See the Supporting Information for the polymer weight distribution of the six other borate systems used in this work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01361. Polymerization kinetics, DFT calculations, and GPC data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (W.G.S.). *E-mail [email protected] (S.J.L.R.). ORCID

Willy G. Santos: 0000-0002-4935-2010 Author Contributions

The manuscript was written through contributions of W. G. Santos and S. J. L. Ribeiro. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support and grants from FAPESP (Proc. 15/22828-6) and CNPq (150080/2018G


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(18) Shanmugam, S.; Xu, S.; Adnan, N. N. M.; Boyer, C. Heterogeneous Photocatalysis as a Means for Improving Recyclability of Organocatalyst in “Living” Radical Polymerization. Macromolecules 2018, 51 (3), 779−790. (19) Zhang, J.; Lalevée, J.; Zhao, J.; Graff, B.; Stenzel, M. H.; Xiao, P. Dihydroxyanthraquinone Derivatives: Natural Dyes as Blue-LightSensitive Versatile Photoinitiators of Photopolymerization. Polym. Chem. 2016, 7 (47), 7316−7324. (20) Wang, X.; Shi, Y.; Graff, R. W.; Cao, X.; Gao, H. Synthesis of Hyperbranched Polymers with High Molecular Weight in the Homopolymerization of Polymerizable Trithiocarbonate Transfer Agent without Thermal Initiator. Macromolecules 2016, 49 (17), 6471−6479. (21) Figg, C. A.; Hickman, J. D.; Scheutz, G. M.; Shanmugam, S.; Carmean, R. N.; Tucker, B. S.; Boyer, C.; Sumerlin, B. S. ColorCoding Visible Light Polymerizations to Elucidate the Activation of Trithiocarbonates Using Eosin Y. Macromolecules 2018, 51 (4), 1370−1376. (22) Shanmugam, S.; Xu, J.; Boyer, C. Aqueous RAFT Photopolymerization with Oxygen Tolerance. Macromolecules 2016, 49 (24), 9345−9357. (23) Jiang, Z. Q.; Zhao, S. Q.; Su, Y. X.; Liu, N.; Wu, Z. Q. Combination of RAFT and Pd(II)-Initiated Isocyanide Polymerizations: A Versatile Method for Facile Synthesis of Helical Poly(Phenyl Isocyanide) Block and Star Copolymers. Macromolecules 2018, 51 (3), 737−745. (24) Métafiot, A.; Kanawati, Y.; Gérard, J. F.; Defoort, B.; Marić, M. Synthesis of β-Myrcene-Based Polymers and Styrene Block and Statistical Copolymers by SG1 Nitroxide-Mediated Controlled Radical Polymerization. Macromolecules 2017, 50 (8), 3101−3120. (25) Pan, X.; Tasdelen, M. A.; Laun, J.; Junkers, T.; Yagci, Y.; Matyjaszewski, K. Photomediated Controlled Radical Polymerization. Prog. Polym. Sci. 2016, 62, 73−125. (26) Guillaneuf, Y.; Bertin, D.; Gigmes, D.; Versace, D. L.; Lalevée, J.; Fouassier, J. P. Toward Nitroxide-Mediated Photopolymerization. Macromolecules 2010, 43 (5), 2204−2212. (27) Santos, W. G.; Scurachio, R. S.; Cardoso, D. R. Photochemical Behavior of Safranine-Riboflavin Complex in the Degradation of Folic Acid. J. Photochem. Photobiol., A 2014, 293, 32−39. (28) Bertolotti, S. G.; Previtali, C. M.; Rufs, A. M.; Encinas, M. V. Riboflavin Triethanolamine as Photoinitiator System of Vinyl Polymerization. A Mechanistic Study by Laser Flash Photolysis. Macromolecules 1999, 32 (9), 2920−2924. (29) Ahmad, I.; Iqbal, K.; Sheraz, M. A.; Ahmed, S.; Mirza, T.; Kazi, S. H.; Aminuddin, M. Photoinitiated Polymerization of 2-Hydroxyethyl Methacrylate by Riboflavin/Triethanolamine in Aqueous Solution: A Kinetic Study. ISRN Pharm. 2013, 2013, 1. (30) Encinas, M. V.; Rufs, A. M.; Bertolotti, S.; Previtali, C. M. Free Radical Polymerization Photoinitiated by Riboflavin/Amines. Effect of the Amine Structure. Macromolecules 2001, 34 (9), 2845−2847. (31) Bertolotti, S. G.; Previtali, C. M.; Rufs, A. M.; Encinas, M. V. Riboflavin/Triethanolamine as Photoinitiator System of Vinyl Polymerization. A Mechanistic Study by Laser Flash Photolysis. Macromolecules 1999, 32 (9), 2920−2924. (32) Gelfi, C.; De Besi, P.; Alloni, A.; Righetti, P. G.; Lyubimova, T.; Briskman, V. A. Kinetics of Acrylamide Photopolymerization as Investigated by Capillary Zone Electrophoresis. J. Chromatogr. A 1992, 598 (2), 277−285. (33) Sheraz, M. A.; Kazi, S. H.; Ahmed, S.; Mirza, T.; Ahmad, I.; Evstigneev, M. P. Effect of Phosphate Buffer on the Complexation and Photochemical Interaction of Riboflavin and Caffeine in Aqueous Solution: A Kinetic Study. J. Photochem. Photobiol., A 2014, 273, 17− 22. (34) Jedrzejewska, B.; Marcin, T.; Paczkowski, J. Styrylquinolinium Borates as Donor-Acceptor Initiators for Sensitized Photopolymerization of TMPTA. Mater. Chem. Phys. 2009, 117 (2−3), 448−454. (35) Santos, W. G.; Schmitt, C. C.; Neumann, M. G. Polymerization of HEMA Photoinitiated by the Safranine/Diphenylborinate System. J. Photochem. Photobiol., A 2013, 252, 124−130.

2). W.G.S. is grateful to Prof. D. R. Cardoso and Celso Prof. V. Santilli for EPR/LFP and Photo-DSC facilities, respectively (FAPESP 2017/01189-0 and 2011/51555-7).

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REFERENCES

(1) Le Quéméner, F.; Subervie, D.; Morlet-Savary, F.; Lalevée, J.; Lansalot, M.; Bourgeat-Lami, E.; Lacôte, E. Visible-Light Emulsion Photopolymerization of Styrene. Angew. Chem., Int. Ed. 2018, 57 (4), 957−961. (2) Zhang, J.; Zivic, N.; Dumur, F.; Xiao, P.; Graff, B.; Fouassier, J. P.; Gigmes, D.; Lalevée, J. N-[2-(Dimethylamino)Ethyl]-1,8-Naphthalimide Derivatives as Photoinitiators under LEDs. Polym. Chem. 2018, 9, 994. (3) Yang, J.; Liao, W.; Xiong, Y.; Wu, Q.; Wang, X.; Li, Z.; Tang, H. Naphthalimide Dyes: Polymerizable One-Component Visible Light Initiators. Dyes Pigm. 2018, 148, 16−24. (4) Zhang, T.; Jiang, B.; Huang, Y. UV-Curable Photosensitive Silicone Resins Based on a Novel Polymerizable Photoinitiator and GO-Modified TiO2nanoparticles. Composites, Part B 2018, 140, 214− 222. (5) Ma, X.; Gu, R.; Yu, L.; Han, W.; Li, J.; Li, X.; Wang, T. Conjugated Phenothiazine Oxime Esters as Free Radical Photoinitiators. Polym. Chem. 2017, 8 (39), 6134−6142. (6) Steindl, J.; Koch, T.; Moszner, N.; Gorsche, C. Silane-Acrylate Chemistry for Regulating Network Formation in Radical Photopolymerization. Macromolecules 2017, 50 (19), 7448−7457. (7) Hardis, R. Cure Kinetics Characterization and Monitoring of an Epoxy Resin for Thick Composite Structures. A thesis 2012, 55. (8) Corrigan, N.; Almasri, A.; Taillades, W.; Xu, J.; Boyer, C. Controlling Molecular Weight Distributions through Photoinduced Flow Polymerization. Macromolecules 2017, 50 (21), 8438−8448. (9) Borská, K.; Moravčíková, D.; Mosnácě k, J. Photochemically Induced ATRP of (Meth)Acrylates in the Presence of Air: The Effect of Light Intensity, Ligand, and Oxygen Concentration. Macromol. Rapid Commun. 2017, 38, 1600639. (10) Aydogan, C.; Yilmaz, G.; Yagci, Y. Synthesis of Hyperbranched Polymers by Photoinduced Metal-Free ATRP. Macromolecules 2017, 50 (23), 9115−9120. (11) Styan, K. E.; Easton, C. D.; Weaver, L. G.; Meagher, L. OneReactant Photografting of ATRP Initiators for Surface-Initiated Polymerization. Macromol. Rapid Commun. 2016, 37 (13), 1079− 1086. (12) Dadashi-Silab, S.; Doran, S.; Yagci, Y. Photoinduced Electron Transfer Reactions for Macromolecular Syntheses. Chem. Rev. 2016, 116 (17), 10212−10275. (13) Al Mousawi, A.; Dumur, F.; Garra, P.; Toufaily, J.; Hamieh, T.; Graff, B.; Gigmes, D.; Fouassier, J. P.; Lalevée, J. Carbazole Scaffold Based Photoinitiator/Photoredox Catalysts: Toward New High Performance Photoinitiating Systems and Application in LED Projector 3D Printing Resins. Macromolecules 2017, 50 (7), 2747− 2758. (14) Yang, Q.; Zhang, X.; Ma, W.; Ma, Y.; Chen, D.; Wang, L.; Zhao, C.; Yang, W. Visible Light-Induced RAFT Polymerization of Methacrylates with Benzaldehyde Derivatives as Organophotoredox Catalysts. J. Polym. Sci., Part A: Polym. Chem. 2018, 56 (2), 229−236. (15) Alejo, T.; Prieto, M.; García-Juan, H.; Andreu, V.; Mendoza, G.; Sebastián, V.; Arruebo, M. A Facile Method for the Controlled Polymerization of Biocompatible and Thermoresponsive Oligo(Ethylene Glycol) Methyl Ether Methacrylate Copolymers. Polym. J. 2018, 50 (2), 203−211. (16) Tehfe, M.; Louradour, F.; Lalevée, J.; Fouassier, J.-P. Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry. Appl. Sci. 2013, 3 (2), 490−514. (17) Chen, F. H.; Pathreeker, S.; Biria, S.; Hosein, I. D. Synthesis of Micropillar Arrays via Photopolymerization: An in Situ Study of Light-Induced Formation, Growth Kinetics, and the Influence of Oxygen Inhibition. Macromolecules 2017, 50 (15), 5767−5778. H


Article

Macromolecules (36) Ermoshkin, A. A.; Nikolaeva, E. S.; Neckers, D. C.; Fedorov, A. V. Macromolecules 2008, 41, 9063−9066. (37) Jdrzejewska, B.; Kabatc, J.; Pietrzak, M.; Pczkowski, J. Styrylpyridinium Borate Salts as Dye Photoinitiators of Free-Radical Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2002, 40 (10), 1433−1440. (38) Jedrzejewska, B.; Pietrzak, M.; Rafinski, Z. Phenyltrialkylborates as Co-Initiators with Cyanine Dyes in Visible Light Polymerization of Acrylates. Polymer 2011, 52 (10), 2110−2119. (39) Popielarz, R.; Sarker, A. M.; Neckers, D. C. Volume 31, Number 4 February 24, 1998 ©. Macromolecules 1998, 31 (4), 951− 954. (40) Santos, W. G.; Pina, J.; Burrows, H. D.; Forbes, M.; Cardoso, D. R. New Insight into the Photophysics and Reactivity of Trigonal and Tetrahedral Arylboron Compounds. Photochem. Photobiol. Sci. 2016, 15 (9), 1124−1137. (41) Grisdale, P. J.; Williams, J. L. R.; Glogowski, M. E.; Babb, B. E. Boron Photochemistry. VI. The Possible Role of Bridged Intermediates in the Photolysis of Borate Complexes. J. Org. Chem. 1971, 36, 544−549. (42) Jedrdrzejewska, B. Factors Affecting the TMPTA Radical Polymerization Photoinitiated by Phenyltrialkylborates Paired with Tri-Cationic Hemicyanine Dye. Kinetic Studies. Colloid Polym. Sci. 2013, 291 (9), 2225−2236. (43) Santos, W. G.; Schmitt, C. C.; Neumann, M. G. Photochemistry of Tetraphenyldiboroxane and Its Use as Photopolymerization Coinitiator. Photochem. Photobiol. 2013, 89 (6), 1362−1367. (44) Santos, W. G.; Tominaga, T. T.; Nascimento, O. R.; Schmitt, C. C.; Neumann, M. G. Phototransients of 2-Ethylaminodiphenylborinate Generated by Direct Photolysis and Photosensitization. J. Photochem. Photobiol., A 2012, 236, 14−20. (45) Santos, W. G.; Cardoso, D. R.; Arrivetti, L. D. O. R.; Schmitt, C. C.; Neumann, M. G. DFT, Spectroscopic, and Photoproduct Study of 2-Aminoethyldiphenylborinate and Tetraphenyldiboroxane. J. Organomet. Chem. 2014, 755, 125−133. (46) Santos, W. G.; Budkina, D. S.; Deflon, V. M.; Tarnovsky, A. N.; Cardoso, D. R.; Forbes, M. D. E. Photoinduced Charge Shifts and Electron Transfer in Viologen-Tetraphenylborate Complexes: PushPull Character of the Exciplex. J. Am. Chem. Soc. 2017, 139 (23), 7681−7684. (47) Murphy, S. T.; Zou, C. F.; Miers, J. B.; Ballew, R. M.; Dlott, D. D.; Schuster, G. B. Tetraarylborates ([AR]4B-) - Estimation of Oxidation Potentials and Reorganization Energies from EletronTransfer Rates. J. Phys. Chem. 1993, 97 (50), 13152−13157. (48) Murphy, S.; Schuster, G. B. A Kinetic Method for Determination of Redox Potentials: Oxidation of Tetraarylborates. J. Phys. Chem. 1995, 99 (2), 511−515. (49) Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chem. Rev. 1991, 91 (2), 165−195. (50) Buettner, G. R. Spin Trapping - Electron-Spin-Resonance Parameters of Spin Adducts. Free Radical Biol. Med. 1987, 3 (4), 259− 303. (51) Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Aryl Sulfoxide Radical Cations. Generation, Spectral Properties, and Theoretical Calculations. J. Phys. Chem. A 2006, 110 (32), 9940− 9948.

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Polymerization Rate Modulated by Tetraarylborate Anion Structure

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