Living Radical Polymerization - American Chemical Society

Chapter 41 Living Radical Ab Initio Emulsion Polymerization of n-Butyl Acrylate by Reverse Iodine Transfer Polymerization *

Downloaded by UNIV OF GUELPH LIBRARY on August 6, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch041

J. Tonnar, P. Lacroix-Desmazes , and B. Boutevin Laboratoire de Chimie Macromoléculaire, UMR 5076 CNRS-ENSCM, Ecole Nationale Supérieure de Chimie de Montpellier, 8 Rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France

The use of elemental iodine I in living radical polymerization, called reverse iodine transfer polymerization (RITP), represents a new straightforward way to prepare smart macromolecular architectures. Herein, ab initio emulsion polymerization of n-butyl acrylate in the presence of molecular iodine has been successfully performed. The polymerization was initiated by 4,4'-azobis(4-cyanopentanoic acid) with dodecyl sulfate sodium salt as surfactant, yielding a stable and uncolored latex. The molecular weight of the polymer chains could be modulated by the concentration of iodine. Lastly, a block copolymer poly(butyl acrylate)-b-poly(styrene-co-butyl acrylate) was synthesized by seeded emulsion polymerization, proving the living characteristics of the polymerization. A simplified mechanism of RITP of n-butyl acrylate in emulsion has been proposed. 2

604

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

605


Introduction

Living free-radical polymerization (LFRP) affords the possibility to prepare polymers with well-defined architectures such as block copolymers, gradient copolymers, and star copolymers to name a few (1,2). Several radical polymerization methods have been developed so far to achieve this goal, among which nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), iodine transfer polymerization (ITP), and reversible addition-fragmentation chain transfer polymerization (RAFT/MADIX) are the most popular (3). However, most studies on living radical polymerization dealt with solution polymerization and the successful implementation of LFRP in dispersed media, especially in aqueous medium which is largely used in industry, remains a challenge (4,5). Some attempts were reported in aqueous suspension, miniemulsion or emulsion polymerization by NMP, ATRP, ITP, RAFT, and MADIX, but only a few systems were effective in ab initio emulsion polymerization (4,6,7). Considering ITP with iodo-alkyl transfer agents (8,9), promising results were obtained in ab initio emulsion polymerization of styrene (10), miniemulsion polymerization of styrene (10-12), seeded emulsion polymerization of w-butyl acrylate (12), and microemulsion copolymerization of vinylidene fluoride with hexafluoropropylene (75). Recently, a new living radical polymerization technique based on the use of elemental iodine I , called reverse iodine transfer polymerization (RITP), was proposed by us (14,15), and patented (16-18). The basic mechanism of RITP is shown in Scheme 1. The radicals coming from the initiator can react with iodine or propagate before reacting with iodine to give iodinated compounds. Propagating radicals can also reversibly react with the previously formed A-I adduct to give dormant chains. Then, the core equilibrium of degenerative chain transfer between active and dormant chains takes place. The fundamental difference with ITP is that the transfer agents are now synthesized in situ during the process. From this scheme, one can conclude that one molecule of iodine will control two polymer chains. So, in this process, the molecular weight of the polymer is controlled by the ratio between the mass of monomer and twice the number of moles of iodine: M f=(mass of monomQr)IC2xn ,mMat) + M .j. The validity of RITP was first checked in the case of solution polymerization. For instance, in the case of the solution polymerization of methyl acrylate in benzene initiated by AIBN at 65°C in the presence of iodine (75), the experimental molecular weight determined by SEC was in very good agreement with the theoretical value: M = 10 900 g.mol" and PDI= 1.91 (M ti i = 10 500 g.mol" ). The molecular weight distribution is not narrow, which is consistent with the low degenerative chain transfer constant C =2.2 (minimal PDI=l + 1/0^=1.46 in an ideal case) (19). Furthermore, proton NMR and MALDI-TOF 2

niargete(

I2

A

1

n>SEC

n>iheore

Ca

1

ejc



606 analyses confirmed that the polymer chains were initiated by AIBN and endcapped by an iodine atom. Therefore, RITP is a very simple and powerful method to control the molecular weight and the structure of the polymers. Last but not least, RITP is very cheap compared to other living radical polymerization methods (estimation for 1 mole of polymer, based on common catalogue prices): 15€ for RITP, 400€-2800€ for ATRP with CuCl/2bipy or CuBr/2HMTETA, 10006 for NMP with TEMPO, and 760€ for RAFT with S(thiobenzoyl)thioglycolic acid which is the only commercial (but very inefficient) RAFT agent so far. Herein, some results on the ab initio emulsion polymerization of w-butyl acrylate by reverse iodine transfer polymerization (RITP) are reported.

Transfer agents are" synthesized in situ

Initiator

PnrI

-

Pirl

+

Degenerative chain transfert

Pnf

ktr,P(n)I

Scheme 1. Simplified mechanism of reverse iodine transfer polymerization (RITP) in solution (A* : radicalfromthe initiator; I : molecular iodine; M : monomer unit; η : mean number degree ofpolymerization). 2

Experimental

Materials Η-Butyl acrylate (BuA, Aldrich, 99%) was purified by vacuum distillation before use. Iodine (I , Aldrich, 99.8%), 4,4'-azobis(4-cyanopentanoic acid) (ACPA, Fluka, 98%), dodecyl sulfate sodium salt (SDS, Aldrich, 98%), and 2


607 sodium hydroxide (NaOH, Carlo Erba, 97%) were used as received. Water was de-ionized by passing through columns packed with ion exchange resins.

General procedure for emulsion polymerization of BuA Typically, 120 g of water were placed in a 250 mL glass reactor and thoroughly purged with argon for 30 minutes. Then, the reactor was thermostated at 85°C under stirring at 250 rpm. A solution of SDS (58.5 mg) in water (10 g) was added in the reactor under argon flow, followed by a solution of I (192 mg) in BuA (15.05 g). Lastly, a solution of ACPA (336.6 mg) neutralized with aqueous sodium hydroxide (NaOH, 96 mg; water, 20 g) was added, and the polymerization proceeded under argon atmosphere for 8 hours. Monomer conversion was determined by gravimetric analysis.


2

Characterizations Size Exclusion Chromatography (SEC) was performed on dried samples dissolved in tetrahydrofuran, with a Spectra Physics Instruments SP8810 pump equipped with a Shodex RIse-61 refractometer detector, a Milton Roy UltraViolet spectrometer detector, and two 300 mm columns thermostated at 30°C (columns mixed-C PL-gel 5μιη from Polymer Laboratories : 2xl0 - 2xl0 g.mol' molecular weight range). Tetrahydrofuran was used as eluent at a flow rate of 1.0 mL.min" . Calibration was performed with polystyrene standards from Polymer Laboratories and Mark Houwink coefficients for polystyrene (£=11.4xl0' dL.g* , a=0.716) (20) and poly(w-BuA) (A:=12.2X10" dL.g , a=0.700) (20) were used for the calculations (except for the block copolymerization in Table III, runs 1-2). Particle size of the latex particles was determined with a Nanotrac 250 particle analyzer (Microtrac Inc.). The measurements of pH were performed with a Consort P500 apparatusfromBioblock Scientific. 2

6

1

1

5

1

5

1

Results and Discussion RITP in emulsion This work has focused on ab initio emulsion polymerization because this is the most often used process in industry. The monomer is butyl acrylate, the


608

control agent is iodine, the initiator is 4,4'-azobis(4-cyanopentanoic acid) (pKa»5A) (t =65 min at 7MJ5 °C) (27), the temperature of the polymerization was kept constant at r=85°C, and sodium dodecyl sulfate was used as the surfactant (critical micelle concentration, CMC=2.6 g.L' ). The main parameters which have been investigated are the concentration of surfactant and the concentration of iodine. Lastly, the living properties of the latex have been checked by performing a block copolymerization. I/2

1


Effect of the concentration of surfactant For the series shown in Table I, the targeted molecular weight is kept constant at 10000 g.mol" and an excess of initiator over iodine of 1.6 has been used. The concentration of the surfactant was varied and it was equal or lower than its critical micelle concentration. In all cases, a high monomer conversion was obtained withoutflocculation,and the latex was uncolored, indicating that iodine has been consumed. The final pH of the latex was always around the pKa of the initiator which acts as a buffer. In contrast to results of RITP in solution, the molecular weight of the latex is much higher than the targeted molecular weight. This issue will be discussed later. More importantly, the deviation between the experimental molecular weight and the targeted molecular weight increases when the concentration of surfactant increases. Moreover, as shown in Figure la, the molecular weight distribution significantly broadens and a slight bimodality appears when the concentration of surfactant increases. As expected, the particle size decreases when the surfactant concentration increases (Figure lb). In the absence of SDS, much larger particles are produced but the latex is still stable: the formation of stable particles without SDS indicates that ionizable chain-ends from the ACPA initiator do participate in the electrostatic stabilization of the latex. In conclusion to this part, it appears that the best results in terms of molecular weight, molecular weight distribution, particle size and particle size distribution are obtained at low SDS concentration, where homogeneous nucleation is favored compared to micellar nucleation. The reasons for the better results at low SDS concentration are still unclear. 1

Effect of the concentration of iodine For die series shown in Table II, the excess of initiator over iodine was kept constant at 1.6 and the surfactant was used at low concentration, much below its critical micelle concentration. The concentration of iodine was varied in order to target molecular weights of about 5000, 10000, and 20000 g.mol" . In addition, in each case, a reference experiment in the absence of iodine has been 1


Table I. Effect of the concentration of surfactant*

Run 1 2 3 4 5

[SDSJ/CMC Time (h) Conv. pH (%)" 0 15 83 5.1 0.1 8 95 5.1 0.15 7.7 90 5.2 0.25 17 90 4.8 1.0 7 96 4.9

PDI

dp

1.88 2.21 1.97 4.32 5.18

(nm) 443 119 87 59 44

1

(R-mot ) 22000 28200 21200 45500 37400

"Polymerization of BuA in ab initio emulsion polymerization at T=%5°C in the presence of ACPA as initiator: targeted molecular weight M = 10 000 g.mol' , [ACPA]/[I ]=1.6. 1


w

2

^Conversion determined by gravimetry.

performed. Again, one can observe that the experimental molecular weight of the latex is always higher than the targeted molecular weight. Nevertheless, these results clearly show that the molecular weight of the polymer increases when the concentration of iodine decreases. Furthermore, even if tailing at low molecular weight is observed, the chromatograms show that the whole molecular weight distribution is shifted toward higher values when the quantity of iodine is decreased (Figure 2). Thus, this set of experiments indicates that the molecular weight of the poly(BuA) latex is efficiently tuned by varying the concentration of iodine. By focusing on a RITP experiment and its reference counterpart in the absence of iodine (Table II, runs 2a and 2b), a first observation is that the final pH is more acidic in the RITP experiment than in the reference experiment. This low pH might be the result of the formation of hydriodic acid HI as discussed later in this paper. As previously noticed, the value around pH=5 corresponds to the pKa of the initiator which acts as a buffer. Another remarkable effect of iodine is that the molecular weight distribution is much narrower and monomodal in RITP than for the reference experiment (Figure 3a). In the case of emulsion polymerization of butyl acrylate, chain transfer to polymer usually occurs and leads to the formation of microgel (22). Thus, multimodality and broadening at high molecular weight in the reference experiment could be ascribed to chain transfer to polymer. Interestingly, because RITP modulates the molecular weight of the polymer chains, multiple intermolecular chain branching is minimized. Therefore, RITP prevents the polymer chainsfromforming poly(BuA) microgel. Besides, RITP leads to much smaller particles than the reference experiment (Figure 3b). This is an indirect proof of the production in situ of surface active



Figure 1. Molecular weight distributions (figure la) and particle size distributions (figure lb) of poly(BuA) samples prepared by RITP in emulsion at different emulsifier concentrations : (+) no SDS ; (A)[SDSJ=0.1xCMC; [SDS]=0.15 xCMC ; (•) [SDS]=0.25xCMC ; (O) [SDS]=lxCMC.


611

Table II. Effect of the concentration of iodine" Run

[IJ/[BuA]

M ,targeled n


1

la lb 2a 2b 3a 3b

(s-mof )

Time (h) 15 5.25 7.7 6.5 7 5.5

Conv. pH

PDI

dp (nm) 79 124 87 134 69 128

5200 53 4.7 12000 1.69 99 7.1 47400 8.90 10100 90 5.2 21200 1.97 96 7.1 48600 7.74 19500 93 5.6 47000 2.78 94 7.1 40300 6.80 "Polymerization of BuA in ab initio emulsion polymerization at 7^=85°C in the presence of ACPA as initiator : [ACPA]/[I ]=1.6, [SDS]=0.15xCMC. ^Conducted without iodine but with the same concentration of initiator as in the corresponding RITP experiment. 3

12.9xl0' No iodine* 6.5xl0No iodine* 3.3xl0No iodine* 3

3

2

300

1.0E + 03

1.0E + 04

1.0E + 05

1.0E + 06

1.0E + 07

M o le eu la r weight (g .m ο I") 1

Figure 2. Molecular weight distributions ofpoly (BuA) samples prepared by RITP in emulsion. Targeted molecular weight : (•) 5 200 g.mot\ (O) 10 100 g.mol , ( Δ ) 19 500 g.mol . 1

1


In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006. 1.0E+05 1

1.0 Ε+06

Molecular weight (g.mol' )

1.0E4Q4 1.0E+07

0.00 1 .0E+01

2.00 -

4.00 -

6.00 -

8.00 -

10.00 -

12.00 -

3b

0.

S

7n

η

14.00 -

16.00

Particle size (nm)

1.0E+02

1.0E+03

1

Figure 3. Molecular weight distributions and particle size distributions ofpoly(BuA) samples prepared by RITP in emulsion at T=85°C : (•) RITP experiment, M^^^ 10100 g.mol ; (O) RITP experiment with the same concentration of iodine but in the presence of an excess ofNal; (Δ) Reference experiment without iodine but with the same concentration of initiator.

3a

300


613 species. Indeed, the polymer chains contain a hydrophilic head (carboxylic acid) and a hydrophobic tail. The smaller particle size in the case of RITP indicates that the concentration of the oligomeric surfactants builds up during the RITP process and that these oligomers take part in the nucleation step. So, from this comparison, one can conclude that RITP creates in situ ionizable surfactants that contribute to the electrostatic stabilization of the latex.


Kinetics of RITP in emulsion Kinetically, iodine is known to be a powerful terminator. So, one can typically divide the process into two periods. A first inhibition period where the radicals essentially react with iodine to form the iodinated transfer agents (A-I and A-M -I oligomers), and a second period where the polymerization takes place (75). Figure 4 shows the evolution of monomer conversion versus time for a RITP experiment in emulsion. As in solution polymerization, an inhibition period is clearly observed, and then the monomer conversion increases up to high conversion. This inhibition period is in qualitative agreement with the expected mechanism of RITP: indeed, it means that the rate of the reaction between radicals (A* or A - M oligoradicals) and iodine is higher than the rate of propagation. So, the presence of the inhibition period confirms the favored reaction of radicals with iodine. n

e

n

100 ~

80

οc

60 4r

1c

40 4\

υ

20 *

ο

Inhibition ... period....

jPolymerizaition period!

7 h l|

•

t

t.

100 I inhibition

200 300 Time (min)

400

500

Figure 4. Evolution ofmonomer conversion versus time for RITP ofBuA in emulsion at T=85°C (fACPAJ/[I ]=1.7, [SDS]=0.15xCMQ M„, = 10 WOg.mor'). 2

larseled


614


For the same experiment, the evolution of molecular weight versus monomer conversion shows a linear trend and the polydispersity index remains in an acceptable range for iodine transfer polymerization of butyl acrylate (Figure 5a). The size exclusion chromatograms at low, medium and high monomer conversion clearly show that the whole molecular weight distribution shifts toward higher molecular weights when the monomer conversion increases (Figure 5b). Therefore, this kinetic survey indicates that the evolution of the molecular weight and molecular weight distribution is compatible with a living process dominated by degenerative chain transfer.

Block copolymerization in emulsion In order to check the living properties of the poly(BuA) chains, a latex prepared by RITP was used as a seed for block copolymerization (Table III). Styrene was chosen as the second monomer and it was added to the seed latex in a batch process. 86% monomer conversion was reached in the second stage of polymerization. The increase of the particle size is consistent with a constant number of particles (Figure 6a). It means that there is no re-nucleation which is a necessary condition for block copolymerization. The experimental molecular weight for the copolymer is 55100 g.mol" . It is close to the theoretical value of 58600 g.mol" . Furthermore, the chromatograms show that the molecular weight distribution shifts toward higher molecular weights, indicating that the polymer chains are reactivated, and the UV signal is similar to the refractive index signal, giving evidence for the formation of a diblock copolymer rather than the formation of two homopolymers (Figure 6b). So, all these results indicate that the preparation of the block copolymer was successful and it confirms the high potential of RITP to tailor polymers directly in dispersed aqueous media. 1

1

Table III. Block copolymerization in emulsion*

Run

Type

PDI Time Conv. pH M p (Z-mot ) (h) (%) n>eX

1

1 2

Seed latex Poly(BuA) Block copolymerization Poly(BuA)-èpoly(styrene-coBuA)

8 15

95 86

5.1 5.0

26700 55100

2.18 1.68

d (nm) P

119 167

1

"Seed: [ACPA]/[I ]=1.6, [SDS]=0.10xCMC, targeted Λ/„= 10 000 g.mol ; Second monomer: styrene; Monomerpeed/Monomerseed^l^ w/w. 2



1

2

r

Figure 5. RITP ofBuA in emulsion at T=85°C ([ACPA]/[I ]=L7, [SDS]=0.15xCMC, targeted M„= 10 100 g.mol ): (a) Evolution ofM„ and PDI versus monomer conversion; (b) Evolution of the molecular weight distribution with monomer conversion: (Δ) conversion=6% (O) conversion^7%, (•) conversion=99%.



Figure 6. Characterizations of the seed poly(BuA) latex prepared by RITP and of the block copolymer latex prepared by seeded emulsion polymerization ofstyrene. (a) Overlay ofparticle size distributions : (•) seed latex, (O) copolymer latex; (b) Molecular weight distributions: (•) refractive index detector (seed latex), (O) UV detector at 254nm (copolymer latex), (Δ) refractive index detector (copolymer latex).


617 Features of RITP in emulsion Results presented above show some discrepancies between experimental and theoretical molecular weights for RITP in emulsion. One important feature in emulsion polymerization is the partitioning of the different species between the different phases. In the present case, one important reaction to consider is the hydrolytic disproportionation of iodine in the aqueous phase (23,24) ([I2]aq l-32xl0" mol.L' ) (25). Many equilibria are actually taking place and they are leading to the formation of hypoiodous acid (HOI), iodide (Γ), iodate (Ι0 '), and triiodide (I "). It also forms protons and thus it tends to lower the pH, as observed in Table II. Such side reactions of iodo compounds in water do occur and they complicate the mechanism of RITP in emulsion. In order to illustrate this point, the effect of iodide ions on RITP in emulsion has been investigated. The addition of an excess of sodium iodide versus iodine conducts to the formation of triiodide. It has a dramatic effect on the polymerization: the addition of excess iodide ions leads to the loss of control of the RITP in emulsion (Figure 3). The iodine is consumed in a competitive reaction to form triiodide and therefore iodine is no longer available to control the polymerization. This set of experiments clearly shows that the formation of triiodide competes with the RITP mechanism. =

3

1


3

I

2

3

+

H20

-

3HOI

-

fc

H O I + I- + H +

]

I 0 3 - + 2I- + 3 H +

J

V 312 + 3 H 2 0 *

IO3- + 51- + 6 H +

Scheme 2. Hydrolytic disproportionation of iodine and some other related reactions.

Based on our knowledge, a simplified mechanism of RITP in emulsion can be tentatively proposed. In the water phase, the ionizable radicals "A coming from the initiator can react with iodine to form "A-I or propagate before reacting with iodine to give surface active transfer agents "A-M -I. The propagating radicals "A-M * can also reversibly react with the previously formed "A-I adduct to give surface active dormant chains "A-M -I. In the same time, side reactions with water such as hydrolytic disproportionation of I consume a part of the iodine (n ejfecnve < nnjmai)- During the inhibition period, the concentration of the surface active transfer agents builds up. Then, the nucleation step occurs and the e

n

n

n

2

I2i


618 core equilibrium between active and dormant chains takes place inside the latex particles. From this description, one can conclude that the side reactions of iodo compounds with water account for the upward deviation of the molecular weight from the ratio between the mass of monomer and twice the initial number of moles of iodine in the reaction medium. By analyzing the evolution of pH for a targeted M of 10 000 g.mol" , preliminary attempts to quantify the loss of iodine through side reactions (oc=n effectivelnommai) lead to a reasonable agreement between the experimental value M„ Ec and the corrected targeted value K,targeted(corrected)= ("laSS of monom&)l(2aX nommai) + M .j. 1

n

I2l

tS


A

Conclusions A new living radical polymerization method named RITP has been invented. It is an easy-to-make method which does not require the synthesis nor storage of control agents. Furthermore, RITP can be performed in dispersed aqueous media (such as in ab initio emulsion polymerization). Work is in progress in our laboratory about side reactions in water in order to get a better control of the molecular weight of polymers prepared by RITP in emulsion. Based on our knowledge, RITP is a robust and economical method which is expected to boost the development of living/controlled radical polymerization at an industrial scale.

Acknowledgments Chantai RUFIER is acknowledged for her early contribution to this topic during her training period in our laboratory. Vincent Bodart and Christophe Fringant (SOL V AY) are acknowledged for their constant interest in the RITP process.

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Living Radical Polymerization - American Chemical Society

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