Article pubs.acs.org/Macromolecules
Iodine Transfer Copolymerization of Fluorinated α‑Methylstyrenes with Styrene Using 1‑Iodoperfluorohexane as the Chain Transfer Agent Justyna Walkowiak-Kulikowska,† Anna Szwajca,† Frédéric Boschet,‡ Véronique Gouverneur,§ and Bruno Ameduri*,‡ †
Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland Institut Charles Gerhardt, Ingénierie et Architectures Macromoléculaires, UMR CNS 5253, Ecole Nationale Supérieure de Chimie de Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier, France § Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, United Kingdom ‡
S Supporting Information *
ABSTRACT: Bulk iodine transfer copolymerizations (ITcoPs) of styrene (ST) with either α-fluoromethylstyrene (FMST) or α-trifluoromethylstyrene (TFMST) in the presence of a 1-iodoperfluorohexane as the chain transfer agent (CTA) are presented. The resulting poly(F-ST-co-ST) copolymers were characterized by 1H, 13C, and 19F NMR spectroscopy that evidenced a satisfactory incorporation of fluorinated α-methylstyrenes (F-ST, ca. 9−16 mol % almost as that inserted in the feed). Both fluoromonomer conversions and the vanishing of the CTA were also monitored by 19F NMR spectroscopy versus time. The controlled character of these copolymerizations was studied by both supplying a linear evolution of the experimental molar masses (Mns) versus the monomer conversion and evaluating the chain extension ability of the formed copolymers. The amounts of dead chains in all copolymers were lower than 5%. The bulkier CF3 group induced a slightly lower reactivity of TFMST comonomer since in the first step, the experimental Mns of the copolymers containing TFMST reached 43 000 g mol−1 while those based on FMST displayed 65 000 g mol−1. In the second step, the produced diblock copolymers had Mns twice higher than those of the starting copolymers, still containing the iodine end-group, as evidenced by a further azidation. In addition, the exchange constants characteristic of both degenerative transfer reactions were assessed (Cex = ca. 1.8 at 70 °C). Finally, the thermal properties of the resulting copolymers were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The insertion of a small amount (98% from Fluka), the chain transfer agent 1-iodoperfluorohexane (C6F13I, 99% Elf Atochem), Selectfluor (>95% from Sigma-Aldrich), 2-bromo-3,3,3-trifluoropropene (98% from Acros Organics), bis(triphenylphosphine)palladium(II) dichloride (PdCl2(PPh3)2, 99%, from Alfa Aesar), triphenylarsine (AsPh3, 98%, from Alfa Aesar), phenylboronic acid (98+%, from Acros Organics), magnesium sulfate (MgSO4, anhydrous, ≥97% from Fluka), potassium hydroxide (KOH, reagent grade, ≥90% from SigmaAldrich), octadecyltrichlorosilane (OTS, ≥96% from Sigma-Aldrich), sulfuric acid (H2SO4 96% POCH), hydrogen peroxide (H2O2 30% POCH), and common organic solvents (anhydrous tetrahydrofuran, THF; 1,2-dimethoxyethane, DME; acetonitrile, diethyl ether, Et2O, anhydrous toluene, and dichloromethane from Sigma-Aldrich) were used as received. 2.2. Measurements. 1H, 13C, and 19F NMR spectra were recorded on a Bruker 400 MHz FT NMR spectrometer with either deuterated chloroform or acetone as solvents. Chemical shifts are reported in ppm relative to tetramethylsilane (TMS) for 1H and 13C spectra and to CFCl3 for 19F NMR spectra. Letters s, d, t, q, and qi stand for singlet, doublet, triplet, quartet, and quintet, and the coupling constants are expressed in hertz. High-resolution mass spectra were recorded on Micromass GCT (CI). Fourier Transformed inf rared (FTIR) spectra were performed on a Nicolet 510P Fourier spectrometer with an accuracy of ±2 cm−1 from KBr pellets (10 wt %) using OMNIC software. Size exclusion chromatography (SEC, or gel permeation chromatography, GPC) was carried out in THF with a flow rate of 0.8 mL min−1 and two PL gel Mix columns at 30 °C. Detection was performed using a RI spectra physic detector SP8430. Analyses were achieved by injection of 20 μL of 20 μm filtered polymer solution (5 mg mL−1) in THF (polymer dissolved readily in THF leading to a clear solution). The molecular weights and polydispersity indices were assessed using the Waters Breeze software package. The system was calibrated by using commercially available monodispersed poly(styrene) standards from Agilent. Iodide ion concentrations [I−] were measured with a PHM 210 standard pH meter from Radiometer Analytical with an iodine selective electrode ISE251-9 and a reference electrode REF201 from Radiometer Analytical. Thermogravimetric analyses (TGA) were performed under air or nitrogen on a TGA Q50 apparatus from TA Instruments at a heating rate of 20 °C/min up to 550 °C. Dif ferential scanning calorimetry (DSC) analyses were carried out with a PerkinElmer Pyris 1 DSC apparatus under a nitrogen atmosphere at a heating rate of 20 °C/min. The temperature range was from −50 to 200 °C. The DSC system was first calibrated in temperature using indium and n-hexane. The second runs led to the Tg values assessed from the inflection point in the heat capacity jump. Contact angle measurements (CA). The surface drop contact angles of the covered glass surfaces were measured using the OCA 15+ contact angle measurement system. Measurements were performed at different sites on each surface. The final values were averages of at least 8635
dx.doi.org/10.1021/ma501828w | Macromolecules 2014, 47, 8634−8644
Macromolecules
Article
five CA measurements on the same sample. A drop (0.2 μL) of water was deposited using a microsyringe. The angles were determined using the Laplace−Young equation approximately in 30 s after deposition of a water drop. 2.3. Synthesis of Fluoromonomers. Noncommercially available fluorinated monomers, α-fluoromethylstyrene (FMST) and αtrifluoromethylstyrene (TFMST), were synthesized according to literature procedures (Supporting Information, section 1).40,41 2.4. Synthesis of Copolymers. Bulk Copolymerization of Styrene with Fluorinated α-Methylstyrenes (F-ST). Bulk controlled radical copolymerizations of styrene (90 mol %) with fluorinated αmethylstyrenes (10 mol % of FMST or TFMST) in the presence of C6F13I were carried out at 70 °C in sealed Schlenk tubes degassed by five freeze−thaw cycles. AIBN was used as the radical initiator. The transfer constants of C6F13I were determined at low conversion ( 1]. This radical, Rf•, adds onto a monomeric unit (step 2c) and propagates (step 2d). In the last step 3b, the exchange of iodide from the transfer agent, Rf−I, to the propagating radical, Rf Mn•, leads to the formation of the dormant chain, Rf Mn−I, and a new radical Rf•, which initiates another chain. The process described in step 4 is the transfer of iodine atom from an end-functionalized chain, Rf Mn−I, to a propagating macroradical Rf Mm•. This step does not create any new chains but contributes to the extension of existing chains. Consequently, this so-called degenerative transfer, DT, reaction is thermodynamically neutral [corresponding equilibrium constant K(4) is worth 1]. If the initial concentration of radical initiator A2 is small with respect to initial concentration of transfer agent Rf I, a large majority of macromolecules exhibit similar structures. As in any radical process, termination occurs in ITP polymerization (step 5). Minimizing the termination step remains essential to keep a good control of the polymerization. Ideally in ITP, the rate of degenerative transfer should be higher than that of propagation to obtain polymers with narrow molar mass distribution. Since in the ITP mechanism two different reactions occur, it is essential to distinguish both these reactions. The first one (3b) 8636
dx.doi.org/10.1021/ma501828w | Macromolecules 2014, 47, 8634−8644
Macromolecules
Article
Scheme 2. Radical Copolymerization of Styrene with Fluorinated α-Methylstyrene in the Presence of 1-Iodoperfluorohexane Initiated by AIBN at 70 °C
S1−S5 and S19−S23), 13C (Figures S6−S10 and S24−S28), and 19F (Figures S11−S15 and S29−S33) NMR spectroscopy, detailed in the Supporting Information. For the last series, the spectra display signals centered at −81.3, −113.1, −122.4, −123.5, −124.2, and −126.7 ppm, assigned to trifluoromethyl and five difluoromethylene groups, respectively, as well as the vanishing of the signal of CF2I end-group, centered at −58 ppm. The broad multiplet centered at ca. −226 ppm (Figures S11−S15) is attributed to the fluoromethyl group of FMST units incorporated in the copolymer, in contrast to the singlet centered at −212.7 ppm characteristic of CH2F group in the monomer. The integrals of both signals observable only in crude NMR allowed assessing the FMST monomer conversion. It was worth evidencing if these radical copolymerizations behaved in a controlled manner. 1H (Figures S16 and S34) and 19 F (Figures S17 and S35) NMR spectroscopy enabled us to monitor such ITP reactions versus time (detailed in the Supporting Information) and to assess the conversion and molar percentages of both comonomers (ST and FMST or TFMST) versus time. It is noted that the evolution of ln[M]0/ [M] versus time for the bulk iodine transfer copolymerization of fluorinated α-methylstyrenes with styrene displayed linear first-order kinetic plots (Figure 1), indicating a constant
is based on the transfer agent itself and is defined by the transfer constant CT1 (CT1 = kT1/kp). CT1 actually measures the activity of the iodinated transfer agent. The second one (3) occurs between two polymeric chains (DT) and is defined by exchange constant Cex (Cex = kex/kp). Unlike CT1, Cex is characteristic of the DT (i.e., the evolution of the molecular weight distribution versus monomer conversion). In addition, Cex should be large enough to favor the control, since it influences the molar mass distribution: Mw/Mn = 1 + 1/Cex at final conversion.43 The evolution of the number-average degree of polymerization, DPn, versus time can be assessed by the equation DPn =
[M]0 − [M]t ([C6F13I]0 − [C6F13I]t ) + f ([I]0 − [I]t )
(1)
where [M]0, [M]t, [I]0, [I]t, f, [C6F13I]0, and [C6F13I]t stand for the monomer and initiator concentrations, the initiator efficiency, and the chain transfer agent concentration at time 0 and at t time, respectively. At low monomer conversion (
