Living Radical Polymerization - American Chemical Society

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 15, 2016 | http://pubs.acs.org Publication Date: August 13, 2009 | doi: 10.1021/bk-2009-1024.ch022

Chapter 22

Borane-mediated radical polymerization; Preparation of Fluoropolymers for High Energy Density Capacitors Zhi-Cheng Zhang and T. C. Mike Chung* Department of Materials Science and Engineering The Pennsylvania State University University Park, PA 16802

This paper discusses two closely relative areas, including the borane-mediated radical polymerization mechanism and the formed fluoropolymers used as the dielectrics thin films in high energy density capacitors for energy storage. The system of borane/oxygen control radical initiators are surprisingly effective for initiating polymerization of fluoromonomers, such as vinylidene fluoride (VDF), trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), etc., at ambient temperature. Mechanistic study indicates that the in situ formed alkyl radical (C*) is responsible for the initiation, and the borinate radical (B-O*) forms a reversible bond with the propagating radical to prolong the polymerization. The control polymerization is characterized by predictable molecular weight, narrow molecular weight distribution, formation of diblock copolymer and chain end functionalized polymers, and tolerance to many functional groups that usually cause chain transfer reactions in regular free radical polymerization. The other aspect of this paper is to apply this control polymerization to prepare a whole family of PVDF copolymers. By systematically tuning the polymer chain conformation and crystal structure, we have identified the most suitable fluoropolymer for thin film capacitor that exhibits high energy density and low energy loss.

© 2009 American Chemical Society

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Matyjaszewski; Controlled/Living Radical Polymerization: Progress in RAFT, DT, NMP & OMRP ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction Despite the known autoxidation reaction of trialkylborane (BR3) by oxygen which results in quantitative formation of the three corresponding alcohols (ROH) after hydrolysis, (1,2) the detailed oxidation mechanism is complicated. Following the initial formation of a B-O-O-C moiety, one suggested mechanism is asymmetric cleavage of the C-O bond to produce B-O-O* and C* radicals. (3-5) Some reports suggest an intramolecular rearrangement of the R2B-O-O-R intermediate to form a RB(O-R)2 molecule. (6-8) Others point to homolytic cleavage of the O-O bond to form B-O* and R-O* radicals. (9-10) The problem relates to the complexity of the oxidation products from three reactive B-C bonds and the formation of two possible moieties (B-O-R and B-O-O-R). The reaction intermediates are too unstable to permit isolation. In this paper, we compare two oxidation adducts of B(C2H5)3 and B(OCH3)(C2H5)2, and carry out the oxidation reactions in the presence of VDF monomers to form PVDF polymers. The idea is to capture the unstable intermediate radicals that in situ initiate the polymerization and are incorporated in the beginning of polymer chains. The distinctive chemical shifts of PVDF, separated from those of borane oxidation fragments, greatly help the end group analysis by the NMR technique. One of the interests in this borane/O2 initiator is the preparation of fluoro copolymers with well-controlled molecular structures that can be fabricated into the dielectric thin films in capacitors for energy storage applications. Opposite to battery technology, which has high energy density and low power density, capacitors (11-13) usually exhibit high powder density but very low energy density that limits their applications. The capacitor energy density is directly governed by the dielectric material that separates the opposite static charges. The energy density can be estimated by the equation (J/cm3) = ½ ε E2 = ½ ε (V/d)2, wherein ε is the dielectric constant of the dielectric and E is the applied electric field. The ideal dielectric material should be able to form uniform (defect-free, impurity-free, and mechanic-strong) thin film with a small thickness (d) and the morphology that allows the application of high voltage (V) across the film

Experimental Oxidation of B(C2H5)3 and B(OCH3)(C2H5)2 with O2 The oxidation reactions were conducted in a 100 mL flask equipped with a magnetic stirrer. After adding 2 mmol of borane, either B(C2H5)3 or B(OCH3)(C2H5)2, and 20 mL ethylether under nitrogen, about 8 mmole of O2 was then injected into the flask to start the oxidation reaction. The reaction was carried out at room temperature under vigorous agitation for 4 hours. The solution was directly subjected to 11B NMR measurements. Both spectra are very similar, containing 3 chemical shifts between 35-30 ppm range, which correspond to three di-oxidized species (Et)(OR)B-O-O-B(OR)(Et), (Et)(OR)BO-O-R, and (Et)(OR)B-O-R, respectively. Only a very low content of fully


333 oxidized (tri-oxidized) species (near 19 ppm) is observed under this oxidation condition at room temperature with [oxygen]/[borane] = 4/1.


Synthesis of Fluoro Co- and Ter-polymers Using Borane/Oxygen Initiator The polymerization reactions were carried out in a 75 mL stainless steel autoclave equipped with a magnetic stirrer. In a typical reaction, 0.3 mmol of triethylboron initiator and 30 mL of acetonitrile solvent were added into the autoclave under an inert atmosphere. The autoclave was then cooled by liquid nitrogen before vacuum distilling in certain amounts of monomers. About 0.4 mmol of oxygen was introduced into the autoclave in order to oxidize organoborane and initiate polymerization before the mixture was warmed to the ambient temperature. Then the mixture was stirred constantly for a certain period of time at the ambient temperature before the reaction was terminated via venting the unreacted monomers. The resulting polymer was obtained by removing the solvent under a vacuum, subsequently purified by precipitating from the polymer solution in acetone with excess hexane. It was washed three more times with hexane, and finally dried in a vacuum oven at 70 °C for 8 h. To obtain high quality co- and ter-polymers, with high purity and narrow molecular weight and composition distributions, the polymerization reactions were usually terminated at a low monomer conversion (< 30%) to assure a constant monomer feed ratio during the polymerization. Note that CTFE shows similar (slightly higher) reactivity with VDF, but TrFE exhibits a significantly lower reactivity than VDF. Thin Film Preparation and Measurements The polymer thin films were prepared by two steps. First, the polymer film (thickness = 30-40 μm) was obtained by a solution casting method, in which the copolymer or terpolymer (8-10 wt%) was dissolved in acetone, and cased on a glass slide. After evaporating the solvent at the ambient temperature overnight, the resulting film was annealed in a vacuum oven at melting temperature for 2-3 hours to obtain uniform thin film (thickness = 10-15 μm). Gold electrodes (thickness = 50 nm) were sputtered on both surfaces of the polymer film for the measurements of the dielectric constant and D-E polarization-depolarization cycles. The dielectric constant was measured by a HP multifrequency LCR meter, equipped with a temperature chamber. The electric displacement-electric field was measured by the displacement of the polymer film thickness, using a modified Sawyer-Tower circuit and a linear variable differential transducer (LVDT), driven by a lock-in amplifier (Stanford Research Systems, Model SR830). Electric fields ranging from 50 to 600 MV/m were applied across the polymer film using an amplified ramp waveform at 10 Hz.


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Results and Discussion


Borane Oxidation/Polymerization Mechansim (14) In this paper, we compare two oxidation adducts of B(C2H5)3 and B(OCH3)(C2H5)2, and carry out the oxidation reactions in the presence of VDF monomers to form PVDF polymers under various conditions. The idea is to capture the unstable intermediate radicals that in situ initiate the polymerization and are incorporated in the beginning of polymer chains. The distinctive chemical shifts of PVDF, separated from those of borane oxidation fragments, greatly help the end group analysis by the NMR technique. Figure 1 compares two sets of the 11B NMR spectra of the oxidation adducts of B(Et)3 and B(OMe)(Et)2 by varying amounts of oxygen at ambient temperature. E tO B

OEt

EtO

Et

B OMe Et

Et B OEt Et

Et B Et Et

Et B OMe Et

Figure 1. 11B NMR spectra of the oxidation adducts of (left) B(Et)3 and (right) B(OMe)(Et)2 by varying amount of oxygen at ambient temperature. [Reproduced with permission from Macromolecules 2006, 39, 5187-5189. Copyright 2006 Am. Chem. Soc.] The oxidation adducts are quite complicated in B(Et)3 yet are better controlled in B(OMe)(Et)2. With an incremental amount of oxygen, two new chemical shifts are located at 56 and 32 ppm, corresponding to B(OEt)(Et)2 and B(OEt)2Et, and a minor peak at 36 ppm associated with B-O-B. Increasing [O2]/[B(Et)3]>1, the B(Et)3 completely disappears, and the main oxidation products become di-oxidation adducts with a very small amount of trioxidization one at around 19 ppm. On the other hand, the incremental oxidation of B(OMe)(Et)2 with oxygen ([O2]/[B(OMe)(Et)2] 100). The unstable intermediates containing free radicals during the intermolecular reaction should in situ initiate the radical polymerization to form poly(vinylidene fluoride) (PVDF) and deposit the adduct in the beginning of the PVDF chain. Table 1 summarizes the experimental results. The polymerization is effective at ambient temperature to obtain PVDF with an almost quantitative yield in five hours. Comparing a set (A-1 to A-6), the molecular weight increases with the monomer conversion. However, plots of the polymer molecular weight vs. the monomer conversion, the line is above the theoretical line. Some uncontrolled radical polymerization may take place in the beginning of polymerization. The protecting borinate radical (B) (Scheme 1), may not have a sufficient quantity to mediate all propagating radicals. Usually, excess mediates are needed to have a successful control radical polymerization. (15-17) The polymer molecular weight reaches Mv >63,000 g/mole in about 5 hours. The relatively slow propagating rate, compared to that of the regular free radical polymerization mechanism, removes the safety concern for heat transport and temperature control usually associated with bulk or solution polymerization of fluoromonomers. In addition, this process produces fluoropolymers with high purities, without any contaminants from surfactants or suspension agents in most of commercial fluoropolymers, which are difficult to remove but detrimental in electric applications.

Figure 2, (left) 1H and (right) 19F NMR spectra of a PVDF polymer (run A-6) prepared by a B(Et)3/O2 initiator (mole ratio= 3/2). [Reproduced with permission from Macromolecules 2006, 39, 5187-5189. Copyright 2006 Am. Chem. Soc.] Figure 2 shows the 1H and 19F NMR spectra of a typical PVDF polymer (run A-6). In addition to two major chemical shifts at 2.9 and 2.3 ppm, corresponding to (CF2-CH2-CF2-CH2) and (CF2-CH2-CH2-CF2), respectively,



338 three minor chemical shifts at 1, 1.5, and 2.0 ppm are associated with three types of protons at the CH3-CH2-CH2-CF2- end group, which is originated from the oxidation adduct (CH3-CH2*) of B(Et)3. The same chain end assignments were observed in the 19F NMR spectrum. In addition to several main chain peaks at near 91, 95, 113, and 116 ppm, there were several weak chain end peaks near 93 ppm (CF2-CH2-CH2-CH3) and 100 ppm (CF2-CH2-CH3). In fact, all reaction runs with [O2]/[B(Et)3]

Living Radical Polymerization - American Chemical Society

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