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1 DOI: /NCHEM.2633 Mechanically controlled radical polymerization initiated by ultrasound Hemakesh Mohapatra, Maya Kleiman, Aaron P. Esser-Kahn Contents 1. Materials and methods 2 2. Procedure for obtaining UV-Vis-NIR spectra of the copper(ii) precursor solution before and after ultrasonic agitation 2 3. Experimental procedure for ultrasound initiated radical polymerization 3 4. Procedure for calculation of % monomer conversion for the ultrasound initiated radical polymerization using 1H-NMR 6 5. Experiment demonstrating the absence of significant polymer chain growth when the ultrasound initiated radical polymerization is carried out in the absence of piezoelectric nanoparticle 6 6. Experiment demonstrating absence of thermally initiated polymerization of butyl acrylate Experiment demonstrating the effect of lowering nanoparticle loading on the kinetics of ultrasound initiated radical polymerization Experiment demonstrating the effect of changing the duration of on and off cycles during the pulse mode operation of ultrasonic horn on the kinetics of ultrasound initiated radical polymerization Experiment demonstrating evidence of Cu on the surface of BaTiO 3 NPs Experiment demonstrating ultrasound initiated radical polymerization can be restarted after a staying dormant for few hours References 20 NATURE CHEMISTRY 1

2 Materials and methods Tetragonal Barium titanate (BaTiO 3 ) nanopowder with a particle size of 200 nm was obtained from US Research Nanomaterials, inc. and was used as received. The ligand N,N,N,N,N,N - hexamethyl-[tris(aminoethyl)amine] (Me 6 TREN) was synthesized as described previously. 1 Butyl acrylate was obtained from Sigma-Aldrich and purified by passing it through a column of basic alumina followed by distillation under reduced pressure. Dry DMF was obtained after purification using in-house solvent purification system. All other reagents were purchased from commercial sources and were used without further purification. UV-Vis-NIR spectra were obtained using a Shimadzu UV-1700 absorption spectrometer. 1H-NMR spectra were obtained using a 500 MHz Bruker Avance spectrometer and were referenced to the peak corresponding to the residual protium of the nmr solvent (7.26 for CDCl 3 ). Molecular weight of polymers were determined using a Agilent 1100 Gel Permeation Chromatography (GPC) instrument equipped with two Agilent PLGel Mixed-D (5 µm) and a refractive index detector. The GPC was calibrated using polystyrene standards. The sonochemical reactions were carried out using a Qsonica Q500 ultrasonic processor equipped with a ½ diameter probe. The ultrasonic processor was operated at 20 khz and at 75% of maximum amplitude. Procedure for obtaining UV-Vis-NIR spectra of the copper(ii) precursor solution before and after ultrasonic agitation An oven dried ultrasonic reaction vessel (250 ml) was charged with 200 nm tetragonal BaTiO 3 nanopowder (1.6 g, 4.5 wt%) under a continuous stream of dry argon. A copper precursor solution containing Cu(OTf) 2 (504 mg, 1.4 mmol), Me 6 TREN (320 mg, 1.4 mmol), Bu 4 NBr (450 mg, 1.4 mmol), and DMF (35 ml) was prepared under inert atmosphere and transferred into the reaction vessel. The resulting suspension was agitated using an ultrasonic horn. A pulse mode NATURE CHEMISTRY 2

3 operation using a 4s on 4s off cycle was used. The reaction vessel was held at 15 o C 25 o C using a water bath. At intervals, 3 ml aliquots of the suspension were filtered. 250 µl of the resulting filtrate was diluted with 2.25 ml of DMF and the UV-Vis-NIR (300 nm 1100 nm) of the resulting solution was obtained. DMF was used as the blank. Experimental procedure for ultrasound initiated radical polymerization Butyl acrylate 1 and dry DMF were degassed by bubbling dry argon for 30 minutes. An oven dried ultrasonic reaction vessel (250 ml) was charged with 200 nm tetragonal BaTiO 3 nanopowder (2.4 g, 4.5 wt%) under a continuous stream of dry argon. A copper precursor solution containing Cu(OTf) 2 (760 mg, 2.1 mmol), Me 6 TREN (480 mg, 2.1 mmol), Bu 4 NBr (670 mg, 2.1 mmol), and DMF (23 ml) was prepared under inert atmosphere and transferred into the reaction vessel. Butyl acrylate (30 ml, 210 mmol) and ethyl α-bromoisobutyrate 2 (310 µl, 2.1 mmol) were then added. The resulting suspension was agitated using a ultrasonic horn. A pulse mode operation using a 4s on 4s off cycle was used. The reaction vessel was held at 15 o C 25 o C using a water bath. At intervals, aliquots ( mg) of the reaction mixture were analyzed using 1H-NMR and GPC. NATURE CHEMISTRY 3

4 Figure 1. 1H-NMR spectra of ultrasound mediated polymerization mixture at (a) t = 0 h and (b) t = 1h. At intervals, aliquots ( mg) of the reaction mixture were diluted with CDCl 3 (0.6 ml), filtered through a syringe filtered, and the 1H-NMR was obtained. NATURE CHEMISTRY 4

5 Figure 2. (a) Representative GPC traces depicting the evolution of molecular weight during the ultrasound mediated polymerization of butyl acrylate; and (b) Portion of the GPC traces showing peaks corresponding to poly(butyl acrylate). The polymerization mixture contained n-butyl acrylate 1 (4 M in DMF), ethyl α-bromoisobutyrate 2 (40 mm), Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm), and BaTiO 3 nanoparticles (4.5 wt%). The mixture was sonicated using an ultrasonic horn with a 4s on 4s off cycle. The sonication was stopped at t = 2h. At intervals, aliquots ( mg) of the reaction mixture were diluted with 1 ml toluene, filtered, and the filtrate was injected into the GPC instrument. The chromatographic peaks with retention time (r.t.) 21 minutes and 23 minutes correspond to butyl acrylate and DMF. The peaks at r.t minutes correspond to poly(butyl acrylate). NATURE CHEMISTRY 5

6 Procedure for calculation of % monomer conversion for the ultrasound initiated radical polymerization using 1H-NMR The monomer % conversion for the polymerization was calculated as: Conversion % = (I b - 2I a )/I b 100%, Where, I a = 1H-NMR integrals corresponding to the protons labeled as c, I b = combined 1H-NMR integrals corresponding protons labeled d and d in 1H-NMR. The integrals c, d, and d are highlighted in 1H-NMR spectra in Figure 2. Experiment demonstrating the absence of significant polymer chain growth when the ultrasound initiated radical polymerization is carried out in the absence of piezoelectric nanoparticle Butyl acrylate and dry DMF were degassed by bubbling dry argon for 30 minutes. An oven dried ultrasonic reaction vessel (250 ml) was charged with 200 nm carbon black powder (2.4 g or 0 g, 4.5 wt% or 0 wt%) under a continuous stream of dry argon. A copper precursor solution containing Cu(OTf) 2 (760 mg, 2.1 mmol), Me 6 TREN (480 mg, 2.1 mmol), Bu 4 NBr (670 mg, 2.1 mmol), and DMF (23 ml) was prepared under inert atmosphere and transferred into an oven dried ultrasonic reaction vessel (250 ml). Butyl acrylate (30 ml, 210 mmol) and 2 (310 µl, 2.1 mmol) were then added. The resulting solution was agitated using a ultrasonic horn. A pulse mode operation using a 4s on 4s off cycle was used. The reaction vessel was held at 15 o C 25 o C using a water bath. At intervals, aliquots ( mg) of the reaction mixture were analyzed using GPC. NATURE CHEMISTRY 6

7 Figure 3. GPC chromatographs of polymerization reaction mixture sonicated in either the presence of nanoparticles (BaTiO 3 or carbon black) or the absence of nanoparticles. The polymerization mixture contained n-butyl acrylate (4 M in DMF), 2 (40 mm), and Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm). The mixture was sonicated using an ultrasonic horn with a 4s on 4s off cycle. The sonication was stopped at t = 2h. Aliquots ( mg) of the reaction mixture were diluted with 1 ml toluene, filtered, and the filtrate was injected into the GPC instrument. NATURE CHEMISTRY 7

8 Figure 4. 1H-NMR spectra of polymerization reaction mixture sonicated in the absence of added BaTiO 3 nanoparticles obtained at (a) t = 0 h and (b) t = 2h. The polymerization mixture contained n-butyl acrylate (4 M in DMF), 2 (40 mm), and Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm). The mixture was sonicated using an ultrasonic horn with a 4s on 4s off cycle. At intervals, aliquots ( mg) of the reaction mixture were diluted with CDCl 3 (0.6 ml), filtered through a syringe filtered, and the 1H-NMR was obtained. NATURE CHEMISTRY 8

9 Figure 5. (a) Representative GPC traces depicting the evolution of molecular weight during the ultrasound mediated polymerization of butyl acrylate in absence of piezoelectric nanoparticles; and (b) Portion of the GPC with retention time min shown for visual comparison with figure 2. The polymerization mixture contained n-butyl acrylate (4 M in DMF), 2 (40 mm), and Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm). The mixture was sonicated using an ultrasonic horn with a 4s on 4s off cycle. The sonication was stopped at t = 2h. At intervals, aliquots ( mg) of the reaction mixture were diluted with 1 ml toluene, filtered, and the filtrate was injected into the GPC instrument. NATURE CHEMISTRY 9

10 Figure 6. 1H-NMR spectra of polymerization reaction mixture sonicated in the presence of added carbon black nanoparticles obtained at (a) t = 0 h and (b) t = 2h. The polymerization mixture contained n-butyl acrylate (4 M in DMF), 2 (40 mm), and Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm). The mixture was sonicated using an ultrasonic horn with a 4s on 4s off cycle. At intervals, aliquots ( mg) of the reaction mixture were diluted with CDCl 3 (0.6 ml), filtered through a syringe filtered, and the 1H-NMR was obtained. NATURE CHEMISTRY 10

11 Figure 7. (a) Representative GPC traces depicting the evolution of molecular weight during the ultrasound mediated polymerization of butyl acrylate in presence of carbon black particles; and (b) Portion of the GPC with retention time min shown for visual comparison with figure 2. The polymerization mixture contained n-butyl acrylate (4 M in DMF), 2 (40 mm), Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm), and 200 nm carbon black (4.5 wt%). The mixture was sonicated using an ultrasonic horn with a 4s on 4s off cycle. The sonication was stopped at t = 2h. At intervals, aliquots ( mg) of the reaction mixture were diluted with 1 ml toluene, filtered, and the filtrate was injected into the GPC instrument. NATURE CHEMISTRY 11

12 Experiment demonstrating absence of thermally initiated polymerization of butyl acrylate Butyl acrylate and dry DMF were degassed by bubbling dry argon for 30 minutes. An oven-dried flask (100 ml) was charged with 200 nm tetragonal BaTiO 3 nanopowder (600 mg, 4.5 wt%) under a dry argon. A copper precursor solution containing Cu(OTf) 2 (190 mg, 0.52 mmol), Me 6 TREN (120 mg, 0.52 mmol), Bu 4 NBr (170 mg, 0.52 mmol), and DMF (5.7 ml) was prepared under inert atmosphere and transferred into the reaction vessel. Butyl acrylate (7.5 ml, 52 mmol) and 2 (77 µl, 0.52 mmol) were then added. The resulting suspension was stirred at 80 o C. At intervals, aliquots ( mg) of the reaction mixture were analyzed using 1H-NMR. NATURE CHEMISTRY 12

13 Figure 8. 1H-NMR spectra of polymerization reaction mixture stirred at 80 o C obtained at (a) t = 0 h and (b) t = 40h. At intervals, aliquots ( mg) of the reaction mixture were diluted with CDCl 3 (0.6 ml), filtered through a syringe filtered, and the 1H-NMR was obtained. NATURE CHEMISTRY 13

14 Experiment demonstrating the effect of lowering nanoparticle loading on the kinetics of ultrasound initiated radical polymerization Butyl acrylate and dry DMF were degassed by bubbling dry argon for 30 minutes. An oven dried ultrasonic reaction vessel (250 ml) was charged with 200 nm tetragonal BaTiO 3 nanopowder (0.24 g, 0.47 wt%) under a continuous stream of dry argon. A copper precursor solution containing Cu(OTf) 2 (760 mg, 2.1 mmol), Me 6 TREN (480 mg, 2.1 mmol), Bu 4 NBr (670 mg, 2.1 mmol), and DMF (23 ml) was prepared under inert atmosphere and transferred into the reaction vessel. Butyl acrylate (30 ml, 210 mmol) and 2 (310 µl, 2.1 mmol) were then added. The resulting suspension was agitated using a ultrasonic horn. A pulse mode operation using a 4s on 4s off cycle was used. The reaction vessel was held at 15 o C 25 o C using a water bath. At interval, aliquots ( mg) of the reaction mixture were diluted with 0.6 ml CDCl 3, filtered, and the filtrate was analyzed by 1H-NMR. Table 1. Effect of lowering the amount of added BaTiO 3 on Mn and degree of polymerization 4.5 wt% BaTiO wt% BaTiO 3 Time (h) % Conversion Mn DP * Time (h) % Conversion Mn DP * * DP = degree of polymerization = Mn /(molecular weight of monomer = 128 g/mol) NATURE CHEMISTRY 14

15 Figure 9. Effect of BaTiO 3 nanoparticles loading on the kinetics of ultrasound mediated polymerization of butyl acrylate. The polymerization mixture contained n-butyl acrylate (4 M in DMF), 2 (40 mm), Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm), and BaTiO 3 nanoparticles: (a) 4.5 wt% and (b) 0.47 wt%. The mixture was agitated using an ultrasonic horn with a 4s on 4s off cycle. The time in x-axis refers to clock time. At interval, aliquots ( mg) of the reaction mixture were diluted with 0.6 ml CDCl 3, filtered, and the filtrate was analyzed by 1H-NMR. Experiment demonstrating the effect of changing the duration of on and off cycles during the pulse mode operation of ultrasonic horn on the kinetics of ultrasound initiated radical polymerization Butyl acrylate and dry DMF were degassed by bubbling dry argon for 30 minutes. An oven dried ultrasonic reaction vessel (250 ml) was charged with 200 nm tetragonal BaTiO 3 nanopowder (2. 0 g, 4.5 wt%) under a continuous stream of dry argon. A copper precursor solution containing Cu(OTf) 2 (630 mg, 1.7 mmol), Me 6 TREN (400 mg, 1.7 mmol), Bu 4 NBr (560 mg, 1.7 mmol), and DMF (19 ml) was prepared under inert atmosphere and transferred into the reaction vessel. Butyl NATURE CHEMISTRY 15

16 acrylate (25 ml, 170 mmol) and 2 (250 µl, 1.7 mmol) were then added. The resulting suspension was agitated using an ultrasonic horn. A pulse mode operation using 2s on 8s off cycle was used. The reaction vessel was held at 15 o C 25 o C using a water bath. At interval, aliquots ( mg) of the reaction mixture were diluted with 0.6 ml CDCl 3, filtered, and the filtrate was analyzed by 1H-NMR. The conversion was calculated as: conversion % = (I b - 2I a )/I b 100%, where I a is the 1H-NMR integrals corresponding to the protons labeled as c, and I b is the combined 1H-NMR integrals corresponding protons labeled d and d in 1H-NMR spectra similar to one shown in Figure 2. Figure 10. Effect of changing the duration of on and off cycles on the kinetics of ultrasound mediated polymerization of butyl acrylate. The polymerization mixture contained n-butyl acrylate (4 M in DMF), 2 (40 mm), Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm), and BaTiO 3 nanoparticles (4.5 wt%). The mixture was sonicated at 20 khz using an ultrasonic horn with (a) 4s on 4s off cycle or (b) 2s on 8s off cycle. The time in x-axis refers to clock time. At interval, aliquots ( mg) of the reaction mixture were diluted with 0.6 ml CDCl 3, filtered, and the filtrate was analyzed by 1H-NMR. NATURE CHEMISTRY 16

17 Experiment demonstrating evidence of Cu on the surface of BaTiO 3 NPs Butyl acrylate and dry DMF were degassed by bubbling dry argon for 30 minutes. An oven dried ultrasonic reaction vessel (250 ml) was charged with 200 nm tetragonal BaTiO 3 nanopowder (2.4 g, 4.5 wt%) under a continuous stream of dry argon. A copper precursor solution containing Cu(OTf) 2 (760 mg, 2.1 mmol), Me 6 TREN (480 mg, 2.1 mmol), Bu 4 NBr (670 mg, 2.1 mmol), and DMF (23 ml) was prepared under inert atmosphere and transferred into the reaction vessel. Butyl acrylate (30 ml, 210 mmol) and 2 (310 µl, 2.1 mmol) were then added and mixed. A small aliquot ( 0.2 ml) of the resulting suspension was saved for SEM analysis. The polymerization suspension was agitated using a ultrasonic horn for 2h. A pulse mode operation using a 4s on 4s off cycle was used. The reaction vessel was held at 15 o C 25 o C using a water bath. 1µl of the test samples (i.e, reaction mixtures with and without sonication) was dropped on a carbon tape glued to a SEM pin stub. The solvent was dried in a fumehood over night. The samples were then imaged using FEI Magellan 400 XHR SEM at 2kV and 25pA using a TLD detector at immersion mode. The samples were further subjected to an Energy Dispersive Spectroscopy (EDS) using an 80mm 2 detector with Aztec software from Oxford Instruments. These measurements were performed at 20kV and 1.6nA. NATURE CHEMISTRY 17

18 DOI: /NCHEM.2633 Figure 12. Structural and elemental analysis of BaTiO3 nanoparticles using SEM-EDS. The polymerization mixture contained n-butyl acrylate (4 M in DMF), 2 (40 mm), Cu(OTf)2/Me6TREN/BuN4Br (40 mm), and BaTiO3 nanoparticles (4.5 wt%). The mixture was either sonicated for 2h (Sample 2) or held without sonication at room temperature for 2h (Sample 1). The samples were imaged at lower (A) and higher (B) magnifications. EDS of the samples (C) reveal presence of copper as demonstrated by semi-quantitative EDS composition data (table in panel C) as well as the Cu Kα peak (inset in panel C) NATURE CHEMISTRY

19 Experiment demonstrating ultrasound initiated radical polymerization can be restarted after a staying dormant for few hours All of the butyl acrylate and dry DMF were degassed by bubbling dry argon for 30 minutes prior to use. An oven dried ultrasonic reaction vessel (250 ml) was charged with 200 nm tetragonal BaTiO 3 nanopowder (2.4 g, 4.5 wt%) under a continuous stream of dry argon. A copper precursor solution containing Cu(OTf) 2 (760 mg, 2.1 mmol), Me 6 TREN (480 mg, 2.1 mmol), Bu 4 NBr (670 mg, 2.1 mmol), and DMF (23 ml) was prepared under inert atmosphere and transferred into the reaction vessel. Butyl acrylate (30 ml, 210 mmol) and 2 (310 µl, 2.1 mmol) were then added. The resulting suspension was agitated using an ultrasonic horn. A pulse mode operation using a 4s on 4s off cycle was used. The reaction vessel was held at 15 o C 25 o C using a water bath. The sonication was stopped at reaction time of t = 1h. The reaction vessel was cooled over liquid nitrogen (-196 o C) under inert atmosphere. The reaction vessel was sealed using rubber septa and all sealed joints were reinforced using several layers of black electrical tape. The reaction vessel was held at -196 o C for about 3 h. The reaction mixture was degassed further using 3 cycles of freeze-pump-thaw. Previously degassed butyl acrylate (15 ml) and dry DMF (12 ml) were added to the mixture and the sonication resumed at t = 4.6 h. At intervals throughout the experiment, aliquots ( mg) of the reaction mixture were diluted with toluene (1 ml), the resulting solution was filtered and analyzed using GPC. NATURE CHEMISTRY 19

20 Figure 11. Restarting ultrasound mediated polymerization after a dormant state. The evolution of the number average molecular weight (M n ) as a function of clock time (t). The polymerization mixture contained n-butyl acrylate (30 ml, 4 M final concentration), DMF (23 ml), 2 (40 mm), Cu(OTf) 2 /Me 6 TREN/BuN 4 Br (40 mm), and BaTiO 3 nanoparticles (4.5 wt%). The mixture was sonicated at 20 khz using an ultrasonic horn with a 4s on 4s off cycle. The time in x-axis refers to clock time. Sonication was stopped at t = 1h and entire reaction mixture was held at -196 o C for 3h. The reaction was thawed, additional n-butyl acrylate (15 ml) and DMF (12 ml) were added, and the sonication resumed at t = 4.6h. Aliquots from the reaction were analyzed by gel permeation chromatography (GPC). M n were determined by GPC calibrated to polystyrene standards. Reference 1. Ciampolini, M. & Nardi, N. Five-Coordinated High-Spin Complexes of Bivalent Cobalt, Nickel, and Copper with Tris(2-dimethylaminoethyl)amine. Inorg. Chem. 166, (1966). NATURE CHEMISTRY 20