Radical Generation Process Studies Of The Cationic Surf Act Ants

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Ultrasonics Sonochemistry 15 (2008) 320–325 www.elsevier.com/locate/ultsonch

Radical generation process studies of the cationic surfactants in ultrasonically irradiated emulsion polymerization Ya Cao *, Yuanyuan Zheng, Guangqin Pan State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China Received 6 May 2007; received in revised form 5 July 2007; accepted 12 July 2007 Available online 26 July 2007

Abstract Without any chemical initiators added, ultrasonically irradiated emulsion copolymerization of styrene and a cationic polymerizable surfactant (methacryloxyethyl dodecydimethyl ammonium bromide, C12N+) was successfully employed to prepare copolymer nanolatexes. Compared with the conventional ionic surfactants, C12N+ has much higher initiation efficiency and C12N+ system exhibits shorter induction period, much higher styrene conversions and polymerization rate Rp in short reaction time. A radical trapping experiment and gas chromatograph–mass spectrograph analysis proved that under ultrasonic irradiation, C12N+ undergoes bond scission between the two alkyl and ionic group, where both C–N bonds are weak along the chain, thereby producing much more original radicals to initiate the emulsion polymerization.  2007 Elsevier B.V. All rights reserved. PACS: 43.35.Vz Keywords: Emulsion polymerization; Ultrasonic irradiation; Initiation mechanism; Surfactant

1. Introduction High intensity ultrasound has been used to depolymerize or enhance polymerization reactions for many years [1–3]. Most sonochemical effects can be attributed to cavitation. Ultrasonic cavitation can generate a very extreme environment, i.e. local temperatures as high as 5000 K and local pressures as high as 500 atm, with heating and cooling rates greater than 109 K/s, which produces initiating radicals through depolymerization, monomer degradation or dissociation of water. Therefore, no chemical initiators are used in the ultrasonic polymerization [4–6]. Recently, ultrasonically irradiated emulsion polymerization has attracted more attention. Compared with ultrasonically initiated bulk or solution polymerization, ultrasonically irradiated emulsion polymerization produces faster polymerization and higher conversion of monomer, *

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and can produce polymer with higher molecular weight [7–15]. Chou and Stoffer [8,9,16] had reported the emulsion polymerization of methyl methacrylate initiated by ultrasound using sodium lauryl sulfate as the surfactant. The source of the free radical for the initiation process was found to come from the degradation of the sodium lauryl sulfate. The weight-average molecular weight of poly (methyl methacrylate) obtained varied from 2,500,000 to 3,500,000 g/mol, and the conversion for polymerization was up to 70%. We also found [17] all kinds of ionic surfactants serve not only as the emulsifiers, but also as the initiators in ultrasonically irradiated emulsion polymerization. The bond between the alkyl chain and ionic group is usually weak and can be dissociated easily under ultrasonic irradiation, thereby producing radicals to initiate the emulsion polymerization. Though the investigation of initiation mechanism of ultrasonically irradiated emulsion polymerization has made a big progress, the initiation efficiency of conventional ionic surfactants (such as SDS, DTAC) is not enough high,

Y. Cao et al. / Ultrasonics Sonochemistry 15 (2008) 320–325

causing obvious retardation and the residuary of monomers [16,17]. Recently, we employed a cationic surfactant methacryloxyethyl dodecydimethyl ammonium bromide (C12N+) in ultrasonically irradiated emulsion polymerization to prepare high purity copolymer nanolatex [18]. C12N+ has excellent initiation efficiency and reactivity. The rate of copolymerization was high and styrene conversion achieved 95% in an hour. A detailed study of the initiation mechanism of ultrasonically irradiated emulsion polymerization using C12N+ as an emulsifier is reported here, proving the structure of ionic surfactants plays a great role in achieving high initiation efficiency and reactivity, and indicating a potential way to improve the reaction efficiency of ultrasonically irradiated emulsion polymerization. 2. Experimental 2.1. Materials Styrene (St; Xilong Chemical Plant, Chengdu, China, CP) was washed with 10 wt% aqueous NaOH solution to remove the inhibitor and then with deionized water, dried over anhydrous MgSO4, and distilled under reduced pressure prior to use. Dodecyltrimethylammonium chloride (DTAC, AR), sodium dodecyl sulfate (SDS, AR), and Bromoform (P97%) were purchased from Chengdu Kelong Chemical Reagents Factory of China. The surfmer, methacryloxyethyl dodecydimethyl ammonium bromide (C12N+), shown in Scheme 1, was synthesized by reaction of N,N-dimethylaminoethyl methacrylate (DM; Yantai Chemical Reagent Corp, Shandong, China, CP) with lauryl bromide (Chengdu Kelong Chemical Plant, Sichuan, China, CP) according to a similar procedure reported by Chern et al. [19,20]. 2.2. Apparatus Ultrasound, with a frequency of 20 kHz, was produced with a Sonics and Materials 1500 Ultrasonic Generator (USA); the power output was adjustable. The ultrasonically irradiated polymerization reactor had been reported [5,21]. 2.3. Ultrasonically irradiated emulsion polymerization Typically, 8.0 mL St, 2.4 g C12N+ (or SDS, DTAC), and 70 mL H2O were introduced into the reaction vessel. The probe of the ultrasonic horn was immersed directly in the emulsion. The system was deoxygenated by bubbling nitrogen while held at constant temperature. The polymerization was then initiated by subjecting this emulsion to ultrasonic irradiation with acoustic intensity 7.54 W/cm2 for 1 h. No extra stirring was required due to the rapid agiH2C CCOOCH2CH2 CH3

CH3 + N (CH2)11 CH3 Br CH3

Scheme 1. The molecular structure of C12N+.

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tation induced by ultrasound. Periodically, samples were withdrawn from reaction mixture for analyses. Some of the resulting latexes were dried in vacuum at 50 C and then washed with ethanol and toluene, respectively, until it was free of unreacted monomer and styrene homopolymers. The purified products were dried in vacuum at 50 C for 24 h before used for characterization. 2.4. Radical trapping experiment The radical trapping experiment was performed by ultrasonically irradiating a mixture of 3 mL of a radical scavenger, bromoform, 20 mL water and 60 mL of a 4 wt% C12N+ aqueous solution at an acoustic intensity of 9.54 W/cm2 under a nitrogen gas flow rate of 40 mL/min for 30 min. The irradiated solution was immediately removed from the 30 C cooling bath. Then, 100 mL of 1-propanol was added to the solution. The final mixed solution was subjected to GC–MS analysis. 2.5. Measurements Monomer conversions were determined gravimetrically. The molecular weights and the polydispersity index (PDI) were obtained by size exclusion chromatography (SEC) on a HPLC/GPC Agilent 1100 series instrument (HP Co., USA). THF was used as the mobile phase (1.0 mL/min), and the system was calibrated with polystyrene standards. Particle size of polymer latex was examined by transmission electron microscopy (JEM100X, Japan). A drop of a diluted polymer emulsion was put on a carbon film supported by a copper grid and observed by the electron microscope after air-drying. Gas chromatograph–mass spectrograph (GC–MS) analysis was performed on a Hewlett–Packard 5972 Mass Selective Detector interfaced to a Hewlett–Packard 5890A Gas Chromatograph. Helium was used as a carrier gas at a linear flow velocity of 20 cm/s at room temperature. A crosslinked methyl silicone capillary column (Hewlett–Packard) was used in all GC–MS analyses. An injector temperature of 280 C and a transfer line temperature of 270 C were used. The column temperature was programmed as follows: 100 C, 2.0 min isothermal, 10 C/min to 270 C. 3. Results and discussion 3.1. Ultrasonically irradiated emulsion polymerization It has been proved that all kinds of ionic emulsifiers can be broken down into radicals under ultrasonic irradiation and initiate monomers polymerization [8]. Similarly, the cationic surfmer C12N+ plays the roles of an emulsifier, an initiator, and a comonomer at the same time. We have reported that high purity copolymer nanolatex was successfully prepared by ultrasonically irradiated emulsion polymerization of styrene and C12N+ [18]. Fig. 1 shows the SEC trace of copolymer P(St–C12N+) prepared by ultra-

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0.03 g/mL SDS 0.03 g/mL DTAC

10

15

20

25

Monomer conversion (%)

100

30

Elution Volume (mL)

0.03 g/mL C12 N

80

+

60

40

20

Fig. 1. SEC trace for P(St–C12N+) polymerized by ultrasonically irradiated emulsion polymerization.

0

sonically irradiated emulsion polymerization in 60 min. The number-average molecular weight is about 1.22 · 106 g/mol, weight-average molecular weight 2.5 · 106 g/ mol and polydispersity 2.1. The typical TEM photograph of prepared latex with the use of 0.030 g/mL C12N+ as the emulsifier, is shown in Fig. 2. The number-average diameter of latex particles is about 26 nm, which is much smaller than that prepared by conventional emulsion polymerization. Due to the intense dispersion, emulsifying, and disrupting effects of ultrasound waves, nanoscale latex particles can be produced easily [7,11]. As shown in Fig. 3, upon addition of a little C12N+, ultrasonically irradiated emulsion polymerization of styrene can be initiated and St conversions increase significantly with the reaction time. The St conversion reaches more than 95% in 60 min. Compared with the anionic surfactant SDS and cationic surfactant DTAC, C12N+ system exhibits shorter induction period, much higher St conversions and polymerization rate Rp in short reaction time. The maximum Rp of C12N+ system is nearly triple of SDS or DTAC systems. Thus, C12N+ has very high initiation efficiency. The results indicate that the surfactant C12N+ plays a very important role in obtaining a high polymer yield in ultrasonically irradiated emulsion polymerization.

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

4

-1 -1

Rp*10 (mol L s )

3

-5

2

1

0

Reaction time (min) Fig. 3. (a) Monomer conversions of St and (b) Rp versus reaction time with different surfactants C12N+, SDS and DTAC.

3.2. Initiation mechanism We have reported that ionic surfactants serve as the initiators in ultrasonically irradiated emulsion polymerization, because the bond between the alkyl chain and ionic group is weak and can be dissociated easily under

35 30

Percent (%)

25 20 15 10 5 0 0

20

40

60

80

Diameter of microspheres (nm) Fig. 2. TEM photograph and particle size distribution of P(St–C12N+) latex particles prepared by ultrasonically irradiated emulsion polymerization.

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ultrasonic irradiation, thereby producing radicals to initiate the emulsion polymerization [17]. For example, DTAC is a quaternary ammonium salt. The C–N bond between the dodecyl and cationic group (the bond energy of C–N is 293 kJ/mol) is weak and can be dissociated easily under the additional energy field. The decomposition processes are shown as follows: CH3 C12H25 N+ CH 3 Cl

CH3

_

Ultrasound

C12H25

N

+

CH3

+

_

CH3 Cl

CH3 +

While, C12N is a similar quaternary ammonium salt as DTAC. But it has much higher initiation efficiency than that of DTAC, which may relate to its unique molecular structure (Scheme 1). The ionic group of C12N+ locates in the middle of the molecular chain. Both bonds between the alkyl chains and ionic group may be dissociated under ultrasonic irradiation (shown in Scheme 2), thereby producing more radicals to initiate polymerization. Through radical trapping experiment and GC–MS analysis, we can find out the decomposition processes of C12N+. The GC–MS analysis was carried out for the emulsion mixture of C12N+, bromoform, and water irradiated by ultrasound for 30 min. The gas chromatogram of the emulsion is shown in Fig. 4. The strongest peak is observed at 14.737 min and the corresponding mass spectrum is shown in Fig. 5. This peak is assigned to the fragment ions of C12N+ unreacted. The peaks at the mass to charge ratio (m/z) of 212, 113, 156 and 58 are the fragment ions listed below, respectively: O

CH3 N CH 2 CH3

CH2

CH3

11

(2) m/z =113 CH3

CH3

N

N CH 2

CH3

CH3

135 and 149 are assigned to the fragment ions [C4H8Br]+ and [C5H10Br]+, respectively. The Br81 isotopic peaks are also seen at m/z 137 and 151. The library search gives 96% fit with the standard mass spectrum. Obviously, 1-bromododecane comes from the radical reaction of C12 H25 , which is produced by the decomposition processes b of C12N+ (Scheme 2, b-2), and bromoform during ultrasonic irradiation. Fig. 7 is the mass spectrum of peak 2.987 min. It is attributed to methacryloxyethyl dimethyl ammonium ion (Scheme 2, b-1). The peaks at m/z of 113 and 58 are the fragment ions of [CH2@C(CH3)COOCH2CH2]+ and

CH3

O CC O CH 2CH2

Fig. 5. Mass spectrum of GC peak at 14.737 min.

CC O CH 2CH2

(1) m/z= 212

CH2

Fig. 4. The total ion chromatogram of the ultrasonically irradiated emulsion mixture (C12N+, bromoform and water).

CH3

(3) m/z= 156

(4) m/z = 58

The peak at 8.712 min (Fig. 4) is 1-bromododecane and the corresponding mass spectrum is shown in Fig. 6. The peak at m/z 248 is the molecular ion and the peak at m/z 250 is very typical of Br81 isotopic ion. The peaks at m/z

H2C C COOCH2CH2 CH3

a +

H2C C COOCH2CH2 CH3 a

CH3 N CH3

(CH2)11CH3Br b

b

-

+

+

CH3 N (CH2)11CH3 CH3 a-2

a-1

CH3 + N C COOCH CH H2 C 2 2 CH3 CH3 b-1

+

CH2 (CH2)10CH3 b-2

Scheme 2. The possible decomposition processes of C12N+ under ultrasonic irradiation.

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From the results discussed above, we could conclude that the C12N+ undergoes bond scission between the dodecyl or ethyl, and the cationic group under ultrasonic irradiation to produce radicals a-1, a-2, b-1 and b-2. Because two sites of C–N bonds can be dissociated easily under ultrasonic irradiation, C12N+ will producing much more original radicals to initiate the emulsion polymerization. Thus, compared with conventional ionic surfactants such as SDS and DTAC, C12N+ has much higher initiation efficiency. It will provide new way to improve the reaction efficiency of ultrasonically irradiated emulsion polymerization. Fig. 6. Mass spectrum of 1-bromododecane in the emulsion sample.

4. Conclusion

Fig. 7. Mass spectrum of methacryloxyethyl dimethyl ammonium ion.

Ultrasonically initiated emulsion copolymerization of styrene and a cationic polymerizable surfactant C12N+ was successfully carried out to prepare copolymer nanolatexes. Compared with the conventional ionic surfactants, C12N+ has much higher initiation efficiency and reactivity. The rate of copolymerization was high and styrene conversion achieved 95% in an hour. Because the ionic groups of C12N+ locate in the middle of chains, both bonds between the two alkyl chain and the ionic group are weak due to the strong induction effects of the ionic groups, and can be dissociated easily under ultrasonic irradiation, thereby producing much more original radicals to initiate the emulsion polymerization. Therefore, the structure of the ionic surfactants is very important. If there is more than one weak bond in the surfactant chain, which can be dissociated under ultrasonic irradiation, high initiation efficiency, polymerization rate and high monomer conversion may be obtained in the ultrasonically irradiated emulsion polymerization. Acknowledgements The authors are grateful to National Natural Science Foundation of China (50303013) for financial support of this work.

Fig. 8. Mass spectrum of dodecyl N,N-dimethylamino ion in the emulsion sample.

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(CH)3N+@CH2, respectively. It gives 78% fit with the standard mass spectrum. Therefore, it is clearly shown that C12N+ undergoes bond scission between the dodecyl and cationic group under ultrasonic irradiation to produce radicals b-1 and b-2. Fig. 8 is the mass spectrum of peak 8.018 min. It is attributed to dodecyl N,N-dimethylamino ions (Scheme 2, a-2). The peak at m/z 213 is the molecular ion. The peaks at m/z 58 and 44 are assigned to the fragment ions (CH)3N+@CH2 and (CH2)3N+, respectively. Thus, the dodecyl N,N-dimethylamino ions (a-2) was produced by bond scission between the ethyl and cationic group under ultrasonic irradiation (the decomposition processes a).

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