Living Radical Polymerization - American

Chapter 40

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RAFT Polymerization in Homogeneous Aqueous Media 2

2

Andrew B.Lowe1,*,Brent S. Sumerlin , Michael S. Donovan , David B. Thomas , Pierre Hennaux , and Charles L. McCormick * 2

2

1,2,

2

Departments of1Chemistryand Biochemistry and Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406

Controlled radical polymerization has been the focus of intense research during the last decade. However, to date, research has focused primarily on polymerizations conducted in homogeneous organic media or bulk with common monomers such as styrene, methyl methacrylate or butyl acrylate. The ability to conduct polymerizations in homogeneous aqueous solution is advantageous from both environmental and commercial viewpoints. In this chapter we will present a summary of our research efforts in the area of aqueous Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization. We will demonstrate the broad versatility of R A F T , showing its applicability to neutral, anionic, cationic, and zwitterionic monomers from a range of monomer classes, including styrenics, acrylamides and (meth)acrylates.

§86

© 2003 American Chemical Society

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


587 Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization is a controlled free radical polymerization (CRP) technique based on the principle of degenerative chain transfer as a means of conferring 'living' characteristics (7). The key additive in R A F T polymerizations is a suitable thiocarbonylthio compound (chain transfer agent or CTA), which is typically a dithioester (7), or other suitable species including dithiocarbamates (2,3), xanthates (4,5), trithiocarbonates (6), and phosphoryl/(thiophosphoryl)dithioformates (7), The generally accepted mechanism for R A F T is shown in Scheme 1. Step I is initiation, with the generation of radicals from a suitable source such as an azo compound and subsequent addition of the radicals to monomer. Steps II and Π Ι constitute the so-called R A F T preequilibrium, during which all the R A F T C T A is consumed/activated with some propagation. Step IV is the main R A F T equilibrium involving chain equilibration and propagation. Step V , like any of the CRP methods, represents possible termination pathways. Ideally, the degenerative transfer of the thiocarbonylthio species is fast relative to the rate of propagation and thus living behavior is observed. A s with the traditional living polymerization techniques, as well as other CRP methods, R A F T facilitates the synthesis of (co)polymers with complex structures such as block (AB, A B A , A B C ) , graft, statistical, comb, gradient and star architectures. One advantage of R A F T is its versatility with respect to monomer choice. In principle, any monomer that is susceptible to polymerization by traditional free radical methods may be polymerized by R A F T , although judicious choice of the R A F T C T A is often required. Many examples already detail the Appl.ication of R A F T polymerizations in organic media (8-11), heterogeneous aqueous media (12-17), and bulk (18-20). We have a long-standing interest in water-soluble polymers and recently have been examining R A F T as a technique for the synthesis of novel amphiphilic polymers directly in aqueous media. A t present there are very few reports detailing homogeneous aqueous polymerizations by R A F T , or utilizing other CRP methods for that matter (21).

Neutral Monomers/Polymers Ν,Ν-Dimethylacrylamide ( D M A ) is a commercially important hydrophilic monomer used in Appl.ications such as pharmaceutics and personal care products. The CRP of D M A has proven difficult by both stable free radical and atom transfer radical polymerization techniques. Recently we disclosed the CRP, via R A F T , of D M A in organic media utilizing novel CTAs that had been designed such that the R-fragments were structurally and electronically similar to D M A and thus were expected to be highly efficient mediating species (these were synthesized by a novel thioacylation reaction) (22). Indeed, the Appl.ication of N,N-dimethyl-s-thiobenzoylthiopropionamide (TBP - B) as the R A F T C T A



Monomer

Ο

Pm

+

Monomer

Piî +

IV.

Monomer

R* +

3

2

S = Ç - S - P n

,

-

P

*•

m

5

z

- S - Ç - S - P „

η

ρ

m- P,m

2

1

Pfi-S-Ç—S—R Z

_

»» 2 Ι ·

Z

S—Ç—S—R

Monomer

m.

+

—

Ρ" +

I·

Initiator

Π.

t

,

.

'

P

n

m

2 6

- S - Ç > = S

3

Z

P —S—Ç=S


-f

+

U n

P *

Monomer

4

R-


V.

Ι - , Κ · , P j , P i , 2, 5

Dead polymer

Scheme L The RAFT Mechanism

•


in 00

590 yielded polyDMA with good molecular weight control and narrow molecular weight distributions.


A

Β

Figure 1. Chemical structures ofN,N-dimethyl-s-thiobenzoylthioacetamide (TBA -A)

andN,N-diethyl-s-thiobemoylthiopropionamide (TBP-B)

Given the encouraging results obtained for the polymerization of D M A in benzene using TBP, we decided to explore the possibility of polymerizing D M A in aqueous media employing T B P and 4-cyanopentanoic acid dithiobenzoate (CTP) as the R A F T CTAs (23). While C T P proved effective at 60, 70 and 80 °C, T B P yielded little-to-no polymer at 60 and 70 °C and exhibited an induction period of ~ 80 min at 80 °C prior to displaying kinetics similar to those observed for the CTP-mediated polymerization. The addition of D M F as a cosolvent to the TBP-mediated polymerizations reduced the induction period to ~ 35 min, but it was not eliminated. The difference in behavior between the organic-based and aqueous-based polymerizations of D M A with T B P is thought to be related to the relative hydrophobicity of the C T A .

Anionic Monomers/Polymers Our initial interest in anionic monomers focused on two styrenic species, namely sodium 4-styrenesulfonate (NaSS0 ) and 4-vinylbenzoic acid ( V B Z ) (24). V B Z is particularly interesting because of its tunable hydrophilicity/hydrophobicity as a function of solution pH. N a S S 0 and V B Z homopolymers were synthesized directly in water employing 4,4'-azobis(4cyanopentanoic acid) (V-501) as the radical source and C T P as the R A F T C T A . The combination of V-501 and C T P ensures that all the initiating fragments (whether initiator- or CTA-derived) are identical, see Scheme 2. These conditions led to rapid rates of polymerization with near-quantitative conversion observed for N a S S 0 after only - 100 min at 70 °C. The resulting experimentally determined molecular weights (aqueous size exclusion chromatography in 80% 0.05 M NaNO /0.01 M N a H P 0 / 20% C H C N , calibrated with P N a S S 0 standards) were in excellent agreement with theory: M o r y = 21,000. M ^ c = 19,800. M = 22,200. MJM = 1.12 for example. We also demonstrated the ability to use the N a S S 0 homopolymers as macroC T A s for the block copolymerization with V B Z . The resulting N a S S Q - V B Z 3

3

3

3

2

4

3

3

n t h e

w s e c

n

3

3


591 s

H C=ÇH 2

R-j-H C-ÇH-]-S-C— 2

V-501 H0 70 °C 2

X

X = S0 " orC0 " 3

2

Scheme 2. RAFT homopolymerization ofNaSS0 and VBZ Downloaded by STANFORD UNIV GREEN LIBR on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch040

3

A B diblock copolymer was shown to undergo reversible pH-induced micellization due to the 'smart' properties of the polyV B Z block. Subsequently we turned our attention to the acrylamido family of monomers and investigated the R A F T polymerization of sodium 2-acrylamido2-methyIpropanesulfonate (AMPS) and sodium 3-acrylamido-3methylbutanoate ( A M B A ) (25). As with the N a S S 0 and V B Z homopolymers, A M P S and A M B A were homopolymerized in water employing V-501 as the radical source and C T P as the R A F T C T A . Figure 2 shows the linear increase in molecular weight with conversion for an A M P S homopolymer, indicative of a controlled polymerization. Polydispersity increases with conversion but remains relatively low at < 1.30. Figure 3 shows the kinetic plot for the same polymerization. After an initial induction period of ~ 65 min, die polymerization exhibits pseudo-first order kinetics, implying a constant concentration of radicals. The observed induction period is not an uncommon feature of R A F T polymerizations and has been observed previously. The ability to form block copolymers employing either A M P S or A M B A macro-CTAs for the subsequent block copolymerization of the second monomer, was also demonstrated, see Figure 4. We are currently in the process of examining the aqueous solution properties of a series of A M P S - A M B A A B diblock copolymers, but we expect them to show similar pH-induced aggregation as the Anal.ogous styrenic-based diblocks since the A M B A blocks are tunably hydrophilic/hydrophobic while the A M P S blocks are permanently hydrophilic. 3

Cationic Monomers/Polymers During our early research involving the anionic styrenic monomers, we also examined two cationic styrenic derivatives: N,N-dimethylvinylbenzylamine ( D M V B A C ) and (ar-vinylbenzyl)trimethylammonium chloride ( V B T A C ) (24), see Figure 5.



592

0.2

0.0

0.4 Conversion

Figure 2. Molecular weight versus conversion plotfor an AMPS homopolymerization. Reproduced with permission from Macromolecules 2001, 34,6561-6564. Copyright 2001 A m . Chem. Soc.

1.6

2

0.8

±=

0.4 0.0

0

50 100 150 200 250 300 Time

(min)

Figure 3. Pseudo-first order rate plotfor an AMPS homopolymerization. Reproduced with permission from Macromolecules 2001, 34,6561-6564. Copyright 2001 A m . Chem. Soc.


593

75

- pdy(ANBA) macro-CTA

50

I Downloaded by STANFORD UNIV GREEN LIBR on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch040

Ε

25

400

600

800

1000

1200

ButionTime(s)

Figure 4. ASEC chromatograms demonstrating diblock copolymer formation. Reproduced with permission from Macromolecules 2001,34,6561-6564. Copyright 2001 A m . Chem. Soc.

As with N a S S 0 and V B Z , the D M V B A C (A) is tunably hydrophilic/hydrophobic via reversible protonation of the tertiary amine residues, while V B T A C is permanently hydrophilic. Homopolymers of V B T A C were prepared in aqueous media employing the V-501/CTP initiator/CTA pair at 70 °C. The P V B T A C was then used as a macro-CTA for the block polymerization of T^A^-dimethylvinylbenzylammonium chloride. The resulting A B diblock copolymer had a unimodal molecular weight distribution (M & ; 11,700. M„sEc = 11,400. M c = 12,500. M /M„ = 1.10 - based on poly(2vinylpyridine) standards). Also the observed block copolymer composition, as determined by *H N M R spectroscopy (49:51 mol basis), was in excellent agreement with the theoretical composition (50:50 mol basis). It was also shown that these diamine A B diblock copolymers were capable of reversible pH-induced micellization, with aggregates of - 38 nm being observed by dynamic light scattering under high p H conditions (here the D M V B A C block is deprotonated and hydrophobic and thus the block copolymer self assembles with the D M V B A C block in the core, surrounded by the hydrophilic V B T A C coronal chains). More recently we have been examining the synthesis of hydrophilichydrophilic A B diblock copolymers comprised of D M A with either D M V B A C or V B T A C (2(5). It is known that when synthesizing A B diblock copolymers by sequential monomer addition, especially for blocks comprised of monomers from two different families, that the order of polymerization can be extremely important (/). 3

n

w S E

w


eoiy


594

Figure 5. Chemical structures ofN,N-dimethylvinylbenzylamine (A) and (arvinylbenzyl)trimethylammonium chloride (B)

Figure 6 shows the observed A S E C chromatograms for the synthesis of a P ( D M V B A C - b l o c k - D M A ) copolymer employing P D M V B A C as a macro-CTA. While block copolymer formation is observed, there is also clearly residual homopolymer and significant high molecular weight impurity. Indeed, even less well-defined block copolymers were obtained when the permanently charged P V B T A C homopolymer was employed as the macro-CTA. On the other hand, Figure 7 shows the A S E C chromatograms obtained for the block polymerization of D M V B A C employing P D M A as the macro-CTA. Under these conditions quantitative blocking efficiency is observed with the resulting A B diblock copolymers having unimodal and narrow molecular weight distributions, see Table I. These results further emphasize that the order of polymerization for the synthesis of A B diblock copolymers can be extremely important and indicates for the preparation of acrylamido/styrenic block copolymers that the acrylamido monomer should be polymerized first and used as the macro-CTA.

Zwitterionic Monomers/Polymers Polymeric betaines are interesting for numerous reasons that include their anti-polyelectrolyte behavior (27) and their bio/hemocompatibility (28). A t present very few examples exist of controlled structure polybetaines (29-36) and most were prepared via group transfer polymerization followed by postpolymerization modification (29-32). Recently, we reported the ability to polymerize styrenic, methacrylate and acrylamido-sulfopropylbetaines directly in aqueous salt employing C T P as the R A F T C T A (37). The order for the rate of polymerization decreases in the order methacrylate > styrenic > acrylamido


595


Poly(DMVBAC) macro-CTA - - - PoIy(DMVBAC-d-DMA)

I

1

1300

1400

1

>

1

'

1

·

1500 1600 1700 Elution Time (s)

*—

1

1800

Figure 6. SEC Chromatograms for the preparation ofAB diblock copolymers employing PDMVBAC as a macro-CTA

f\

PDMA macro-CTA - - - PolyCDM^g-fr-DMVBACgo) • PdyiDMA^-d-DMVBACgo) -— PolyiDMA^-d-DMVBAC^)

ι ·* i

1400

1S30

" 1Θ0Ο

1700

ΐώθ

'

1900

elution time (s)

Figure 7. Observed ASEC chromatograms for the synthesis of a P(DMA-blockDVBAC) copolymer employing PDMA as a macro-CTA


596


Table I. Molecular Weight and Polydispersity Data for the P(OMA-blockDMVBAC) Copolymers Expt. M DMVBAC Block*

Copolymer M„*

PDI*

5,900

7,100

12,000

1.20

50

9,900

9,400

14,300

1.17

70

13,800

10,000

14,900

1.17

Theoretical DP DMVBAC Block

Theoretical M DMVBAC Block

30

n

n

P2VP standards) The slower rate of polymerization for M A E D A P S is interesting considering that the rate constant for propagation is typically higher for acrylamido-based monomers than either styrenics or methacrylates under classical free radical polymerization conditions. The slower kinetics exhibited by M A E D A P S may be indicative of a higher rate constant of addition for the propagating M A E D A P S macro-radical towards a macro-CTA or, possibly, a lower rate constant for fragmentation of the macro-RAFT intermediate radical (38,39). Considering the higher reactivity and lower bulk of the M A E D A P S acrylamido radical compared to the D M A P S methacrylate radical, contributions from both possibilities are likely. The results are listed in Table II.

/CH

3

CHf-C

CHr-CH «=0

N-CH CH

3

ο

2

CH

2

H3C-N-CH3

H3C-N-CH3

£H

ίη

2

£H

2

2

CH

2

^H

2

so *

SO3"

MAEDAPS

DMAPS

3

DMVBAP

Figure 8. Chemical structures of 3-[2-(N-methylacrylamido)-ethyldimethyl ammonio]propane sulfonate (MAEDAPS), 3-[N-(2-methacroyloyethyl)-N,Ndimethylammoniofpropane sulfonate (DMAPS), and 3-(N,Ndimethylvinylbenzylammonio)propane sulfonate (DMVBAPS).


597 Table II. Summary of the Molecular Weights and Polydispersities for the Homopolymers of M A E D A P S , D M A P S and D M V B A P S . Reproduced with permission from Macromolecules 2002, 35, 8663-8666. Copyright 2002 A m . Chem. Soc. Conversion

Sample

dn/dc


(%)"

c

Theoretical M„

Observed M

MJM *

d

n

n

PMAEDAPS*

91

0.1533

44,100

58,250

1.08

PDMAPS"

93

0.1293

45,700

47,500

1.04

PDMVBAPS"

90

0.1578

44,900

47,200

1.06

a. b. c.

Prepared using 4-cyanopentanoic acid dithiobenzoate as the RAFT CTA As determined by the residual monomer concentration employing the RI detector Measured using Wyatt's Optilab Interferometric refractometer in 80% 0.5 M NaBr / 20%CH CN. As determined by aqueous size exclusion chromatography in 80% 0.5 M NaBr / 20% CH CN using Wyatt's DAWN EOS multi-angle laser light scattering detector. 3

d.

3

Modification of Gold Surfaces By virtue of the R A F T mechanism, (co)polymers prepared via this technique bear thiocarbonylthio end-groups. We recently demonstrated the ability to utilize RAFT-synthesized (co)polymers for the effective stabilization of gold nanoparticles (40,41). The method takes advantage of the facile in-situ reduction of the dithioester end-groups with preformed gold colloids (Scheme 3)· We successfully stabilized gold nanoparticles with neutral, anionic, cationic and zwitterionic (co)polymers. The simultaneous reduction was performed in aqueous media using N a B H . Interestingly, when a M A E D A P S - D M A block 4

Scheme 3. Preparation of (co)polymer-stabilizedAu nanoparticles. Reproduced with permission from J. Am. Chem. Soc 2002,124, 11562-11563. Copyright 2002 A m . Chem. Soc.


598 copolymer was used, the colloidal solutions were only stable in aqueous salt solution (here the D M A block was covalently attached to the A u surface and thus was stabilized by outer betaine chains). This stability is consistent with the solubility characteristics of polymeric betaines which are generally insoluble in pure water but become soluble upon the addition of salt. Transmission electron microscopy (TEM) was used to verify the presence of the attached polymers as evidenced by their stabilization of the A u nanoparticles, see Figure 9. This surface modification chemistry has been extended to flat A u surfaces as well. The dithioester end-groups of RAFT-synthesized (copolymers were reduced with aqueous N a B H in the presence of gold-coated silica slides. Successful surface modification was confirmed by atomic force microscopy ( A F M ) (Figure 10) and contact angle measurements.


4

Conclusions Herein we have summarized some of our research concerning homogeneous aqueous R A F T polymerizations. We have demonstrated the versatility of R A F T by detailing its Appl.icability to a wide range of functional monomers using a variety of R A F T CTAs. We have also shown that it is possible to covalently

Figure 9. TEM micrographs ofAu sol (a), reduced Au sol (b), Ρ AMPSstabilizedAu-NPs (c), PVBTAC-stabilized Au-NPs (d), andPDMA-stabilized Au-NPs (e). Scale bars correspond to 40 nm. Reproduced with permission from J. Am. Chem. Soc 2002,124, 11562-11563.



Figure 10. AFM phase images ofgold surfaces modified with PVBTAC (A) andPNaSSO, (B). (z range = 120°)


600 attach RAFT-synthesized polymers in a facile manner to Au surfaces by a simple in situ reduction process.

Acknowledgements


We would like to thank Energizer Battery Company, GelTex Pharmaceuticals, and the U . S. Department of Energy for financial support.

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