Fundamentals of Atom Transfer Radical Polymerization

Encyclopedia of Polymer Sceince and Technology c 2007 John Wiley & Sons, Inc. All rights reserved. Copyright


FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION Introduction The development of living polymerization (1,2) enabled the production of polymers with precisely controlled molecular weight, narrow molecular weight distribution, and well-defined architecture and composition. For the most recent compilation of controlled/living polymerization techniques, see references (3,4). There are a number of advantages of controlled/living radical polymerization (CRP) (5–10) as compared to ionic polymerization, such as applicability to a wide range of monomers and solvents, tolerance to impurities and functional groups, and ease of experimental set-up. The most widely used CRP techniques include atom transfer radical polymerization (ATRP) (11–14), nitroxide-mediated polymerization (NMP) (15–17), organometallic-mediated radical polymerization (OMRP), (18–20), and degenerative transfer polymerization (21–25). In each case, control is maintained via fast dynamic equilibrium between dormant species and propagating chains (26,27).

Fundamentals of ATRP The dynamic equilibrium that mediates control during ATRP is established between a low oxidation-state transition metal complex (Mtn Lm ) and its higher oxidation-state complex (X-Mtn+1 Lm ). The mechanism involves reversible reaction of Mtn Lm with an alkyl halide initiator RX by a one-electron redox process with concurrent halogen abstraction from the dormant species. This occurs via inner sphere electron transfer (28) and generates X-Mtn+1 Lm and an organic radical R• , with a rate constant of activation kact . The radical can add to vinyl monomer with a rate constant of propagation kp , terminate by coupling or disproportionation (kt ), or be reversibly deactivated by X-Mtn+1 Lm (kdeact ) (Fig. 1). The termination of a small amount (∼ 5%) of growing polymer chains at the initial stage of polymerization prevents halogen abstraction from oxidized metal complexes that suppress further termination reactions via the persistent radical effect (29,30). The ATRP equilibrium (K ATRP = kact /kdeact ) is, thus, heavily shifted towards dormant species, and the polymerization is characterized by uniform growth of polymer chains. A broad body of evidence has confirmed the presence of intermediate radical species in this process. This support includes an abundance of similarities between 1

2

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

Mt n Lm+ Pn-X

kact kdeact

+M X-Mt n +1L m + Pn kp

kt Bimolecular termination

Fig. 1. Mechanism of transition metal complex-mediated ATRP.

conventional free-radical polymerization and ATRP, such as the lack of effect of protic solvents, radical scavengers, and transfer agents (31), the atacticity of polymers prepared by ATRP (32–34), similar reactivity ratios in copolymerization (35–39), similar rates of racemization, exchange, and trapping reactions (40,41), and indistinguishable 13 C kinetic isotope effects (42); concomitant formation of higher oxidation state metal species during the polymerization (43); and direct electron spin resonance observation of radicals (44). The rate of polymerization in ATRP is proportional to initiator concentration and the ratio of activator to deactivator concentrations, according to eq. 1. Rp = − d[M]/dt=kp [M][P• ]=kp [M]KATRP [RX]([Mtn /L]/[Mtn+1 X/L])

(1)

It is noteworthy that polymerization rate does not depend on the absolute amount of catalyst in the system, which suggests that catalyst concentration can be decreased without affecting Rp , as long as the ratio of activator to deactivator concentrations remains constant. However, the synthesis of polymers with low polydispersity requires sufficient concentration of deactivator (eq. 2) in order to reduce the number of monomer units added during each activation step and equalize probability of growth of all chains.
 
[RX]0 kp Mw 1 2 PDI= =1+ + −1 (2) Mn DPn Conv. kdeact [Mtn+1 X/L]

Components A wide range of monomers have been successfully polymerized by ATRP, including various styrenes, (meth)acrylates, (meth)acrylamides, and acrylonitrile, each of which contains substituents that can stabilize propagating radicals. The polymerization rate of each monomer is determined by its unique values of kp and K ATRP , the latter of which can be adjusted by modification of the catalytic complex. Optimal ATRP conditions, including catalyst type and concentration, solvent, and temperature, must be selected for each monomer in order to obtain a sufficiently high polymerization rate while maintaining a low concentration of radicals and, thus, a controlled polymerization. The successful ATRP of acidic monomers, vinyl acetate, and dienes remains a challenge, for a variety of reasons. Acidic monomers poison the ATRP catalyst by coordination to the metal and protonation of the Nbased ligand; poly(meth)acrylic acids are typically prepared by polymerization of

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

3

a protected monomer, such as trimethylsilyl methacrylate or tert-butyl methacrylate, followed by deprotection (45). The controlled polymerization of vinyl acetate is limited by a low value of K ATRP , which is due to the high carbon-halogen bond strength exhibited by this monomer (46,47). Solving these challenges requires a thorough understanding of the rules for rational catalyst selection (see section 4.3). A variety of commercially available alkyl halides can be employed as initiators for ATRP. A typical initiator is comprised of a transferable halogen that is activated by α-carbonyl, phenyl, vinyl, or cyano substitutents. If initiation is fast and quantitative, the relative concentrations of monomer and initiator determine the number of growing chains and therefore the degree of polymerization (DP) or molecular weight of the polymer (eq. 3). DP=[M]0 /[initiator]0 ×conversion

(3)

The transition metal complex that mediates ATRP is typically Cu-based, but a multitude of other metals have been demonstrated to successfully control the process, such as Ti (48), Mo (49–51), Re (52), Fe (53–56), Ru (12,57), Os (58), Rh (59), Co (60), Ni (61,62), and Pd (63). The characteristics that a transition metal center must possess in order to be an efficient catalyst include at least two accessible oxidation states separated by one electron, affinity towards a halogen, and an expandable coordination sphere. The complexing ligand serves to solubilize the transition metal and adjust the catalyst redox potential in order to ensure an appropriate equilibrium between dormant and propagating species. Typically employed nitrogen-based ligands include derivatives of 2,2’-bipyridine (bpy) (11,64), pyridine imine (65,66), diethylenetriamine (DETA) (67), tris[2-aminoethyl]amine (TREN) (68), and tetraazacyclotetradecane (CYCLAM) (69), among others (70,71). Phosphorous-based ligands are used in ATRP catalyzed by complexes of Re (52), Ru (12,57), Fe (53,54), Rh (59,72), Ni (62,73), and Pd (63), but not Cu.

Mechanistic Considerations Measuring K ATRP . Successful polymerization of new or challenging monomers will require a thorough understanding of the factors that affect the ATRP equilibrium. The value of K ATRP for any particular catalyst and initiator system must be determined experimentally, which can be easily accomplished by reacting alkyl halide with transition metal activator and monitoring the increase in deactivator concentration over time. A plot of F([CuII Ln X]) versus time is then constructed, and K ATRP is calculated from the slope of the linear dependence (eq. 4) (30). Typical values for various initiators and Cu(I) complexes are between 10 − 10 – 10 − 4 (30,74–76). A large value of K ATRP is characteristic of an active catalyst (eq. 4). F([Cu(II)Ln X])≡

[Cu(I)Ln ]20 [Cu(I)Ln ]0 1 − + [Cu(I)Ln ]0 −[Cu(II)L n X] 3([Cu(I)Ln ]0 −[Cu(II)Ln X])3 ([Cu(I)Ln ]0 −[Cu(II)Ln X])2 1 2 =2kt KATRP t+ 3[Cu(I)L n ]0

(4)

4

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION O H3C

H3C

63. 3 (4 10-9)

H3C

H3C

F

62.1 ( 3 10-8)

Br

H3C

OCH3

61.7 (6 10-8)

60.1 (9 10-7)

N(CH3)2

H3C

Cl

CH3

55.9 (1 10-3)

H3C

54.2 (2 10-2)

53.3 (8 10-2)

59.5 (2 10-6)

52.5 (3 10-1)

OCH3 O

H3C

H3C

CN

H3C

O 50.3 ( 14)

O

CH3 OCH3

49.4 (60)

CH3

O

G o298=51.8 kcal/mol (KATRP=1) H 3C

CH3

O

CH3

H3C

O 57.5 (7 10-5)

CH3 CH3

63.2 (5 10-9)

H3C

H3C

47.2 (2.5 103)

C H

CH2

O Cl

46.9 (4 103)

S

Cl Cl

43.3 (2 106)

H3C 39.6 (9 108)

Fig. 2. Free energy change and relative K ATRP values for homolytic bond cleavage of alkyl bromides at 25 ◦ C relative to methyl 2-bromopropionate, as determined by DFT (47).

ATRP Subequilibria. In order to critically evaluate the factors that affect the ATRP equilibrium, it is convenient to express this equilibrium as the product of four reversible reactions: oxidation of the transition metal activator, or electron transfer (K ET ); formation of halide anion, or electron affinity (K EA ); bond homolysis of the alkyl halide initiator (K BH ); and association of halide anion with deactivator, or “halidophilicity” (K X ) (eq.5–9) (77). Bond homolysis is the only of the four reactions that does not depend on the nature of the catalyst. Alkyl halide bond dissociation energies have been reported to correlate well with measured values of K ATRP (Fig. 2) (47). For systems employing the same catalyst and conditions, the rate of polymerization can therefore be predicted from the calculated bond dissociation energies. KET

Mtn/L

X

+

KEA e

KBH R-X

Mtn+1X/L + e

R

+

(5)

X

(6)

X

(7)

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

X

+ Mtn +1/L

KATRP =

KX

Mtn +1X/L

kact =K K K K kdeact BH EA X ET

5

(8)

(9)

The concentration of deactivator present in the system and, thus, the extent of control over the polymerization, depends on the value of halidophilicity, K X . This value is strongly solvent dependent, and is significantly higher in nonprotic solvents than in protic solvents where the halide anion is efficiently solvated (78). Conducting ATRP in aqueous solvents typically leads to fast polymerization and loss of control as the majority of the halogen is dissociated from the deactivating species. This can be partially suppressed by the addition a large initial amount of X-Mtz+1 Lm or halide salts (79). ATRP is a redox process, and therefore catalyst activity depends on the redox potential of the transition metal/ligand complex. A linear correlation has been established between K ATRP and E1/2 values for Cu complexes with a variety of Nbased ligands (80,81). Similar correlations between redox potential and ATRP catalytic activity have been demonstrated for Fe (55) and Ru (82,83) complexes. The relative activities of catalysts derived from different transition metals cannot be predicted solely by examining redox potentials due to the differences in halidophilicity of each metal center.

Rational Catalyst Selection. Predicting Catalytic Activity. In order to obtain a sufficiently fast polymerization while maintaining control, a catalyst must be selected that exhibits high activity and stability. The rules for rational selection of ATRP catalysts have been thoroughly described (84,85) and is briefly in this article. Although these rules have been developed using Cu-based ATRP, they are applicable to all transition metals that catalyze this process. The activity of an ATRP catalyst is related to its reducing power, which in turn depends on the relative stabilities of the Cu(I) and Cu(II) oxidation states (quantified by the stability constants β(I) and β(II), see eq. 10). A ligand that strongly stabilizes the Cu(II) state will generate a corresponding Cu(I) complex with high activity. In addition, stabilization of both oxidation states (ie, large values of β(II) and β(I)) will yield a catalyst that is significantly less susceptible to ligand substitution reactions with monomer, polymer, or solvent, even at low catalyst concentration. Therefore, knowledge of the readily measurable stability constants of each oxidation state allows for prediction of the activity and stability of a catalytic complex. The most active Cu-based ATRP catalyst known to date is the complex with 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (dimethyl cross-bridged cyclam, DMCBCy) (76), which is also exceptionally stable. tot E≈E0 + RT ln [Cu(II)] − RT ln β(II) F [Cu(I)]tot F β(I)

(10)

6

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

N N N bpy (0.066)

N N dNbpy (0.6)

N

N N

PMDETA (2.7)

N

N

TPMA (62)

N

N

N

N

N

N N

N

N

Me6TREN (450)

DMCBCy (710)

Fig. 3. Rate constants of activation (M − 1 s − 1 ) for nitrogen-based ligands with ethyl 2bromoisobutyrate and CuBr in acetonitrile at 35 ◦ C.(89).

Although values of K ATRP for Cu complexes can be predicted using stability constants and redox potentials, knowledge of K ATRP is not sufficient to determine whether a polymerization will be controlled. Fast activation and deactivation (with kact  kdeact ) are required to obtain polymers with predetermined molecular weight and narrow molecular weight distribution. The rate constant kact can be determined by spectroscopically or chromatographically monitoring the consumption of alkyl halide upon activation by a Cu(I) complex, which generates radicals that are trapped by an excess of scavenging agents such as nitroxides (86). The value of kdeact can be measured by a type of clock reaction in which radicals are simultaneously trapped by a nitroxide and transition metal deactivator (87). It can also be estimated from initial molecular weight and polydispersity index values (88), as well as measured values of K ATRP and kact (41). The rate of activation in Cu-based ATRP has been shown to strongly depend on the structure of the complexing nitrogen-based ligand. The values of kact span more than six orders of magnitude and generally obey several trends, with activity depending on: linking unit between nitrogen atoms (C4  C3 < C2) and/or coordination angle; ligand topology (cyclic ∼ linear < branched); nature of the ligand (aryl amine < aryl imine < alkyl imine < alkyl amine ∼ pyridine); and steric bulk around the metal (Fig. 3) (89) A seemingly minor change in ligand structure can have a pronounced effect on catalytic activity; for example, the Cu(I) complex of DMCBCy is ∼1000 more active than that of Me4 Cyclam. A similar correlation between ligand structure and kdeact has not been observed, although it has been proposed that the ease of structural reorganization of the Cu(II) complex upon halogen abstraction should be a determining factor in the observed rate of deactivation (90). In addition to rational selection of the catalyst, consideration must be given to employing the most appropriate initiator for a particular polymerization. As mentioned earlier, fast and quantitative initiation is necessary to obtain polymers of predetermined molecular weight. An initiator that is characterized by sufficiently fast activation must be selected according to the following rules: activity depends on the degree of substitution (primary < secondary < tertiary), transferable group (Cl < Br < I), and radical stabilizing group (–Ph ∼ –C(O)OR  –CN) (Fig. 4). Avoiding and Exploiting Side Reactions. Choosing the appropriate catalyst for a particular ATRP system requires consideration of the side reactions that may take place during polymerization (Fig. 5). These reactions have been thoroughly discussed (10,85), and are briefly outlined in this article. Outer sphere electron transfer involves the catalytic reduction of propagating radicals to

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

7

Br O

O Cl

O

O Br

O 0.015

O 0.030

0.17

Br

I O

O 0.33

O

O Br

0.53

CN

Br 2.6

23

Fig. 4. Values of kact (in M − 1 s − 1 ) for various initiators with CuX/PMDETA in acetonitrile at 35 ◦ C (91).

carbanions or oxidation to carbocations (28). For example, the former can occur when a highly active (ie. reducing) catalyst is employed in the polymerization of an electrophilic monomer, such as acrylonitrile (92–95). Coordination of vinyl monomer to the catalyst (96,97) does not significantly affect polymerization under normal conditions, but may become an issue at low catalyst concentration. Conducting ATRP in protic media can be accompanied by several side reactions (79,98,99), including loss of deactivator via solvation of halide anion (as discussed earlier) and disproportionation of the metal center. The latter reaction can be avoided by choice of a ligand that sufficiently stabilizes the Cu(I) oxidation state relative to the Cu(II) state in protic media. Finally, it should be noted that certain “side reactions” can be exploited as efficient techniques for materials synthesis and the investigation of new catalysts. These include atom transfer radical coupling (a manipulation of bimolecular termination) as a route to telechelic polystyrene, (100,101) β-H abstraction from growing polymer chains for the generation of oligomers (49,102), and the formation of organometallic species that may be able to simultaneously mediate ATRP and OMRP (18,49,103).

Initiation Systems. Normal/Reverse/Simultaneous Reverse and Normal ATRP. Normal ATRP consists of an alkyl halide initiator and transition metal catalyst initially in its lower oxidation state. Although this technique is easily applicable in an academic setting, the oxidative instability of the catalyst and relatively large amounts typically employed may pose difficulties on an industrial scale. Reverse ATRP was developed in order to circumvent oxidation problems. This method utilizes a conventional radical initiator (such as AIBN) to generate propagating radicals that are reversibly deactivated by a higher oxidation state metal, in order to generate the ATRP activator in situ (104–106). Although reverse ATRP provides a convenient alternative to handling airsensitive catalysts, it cannot be used in chain extension reactions for the preparation of block copolymers because the radical source does not contain a transferable atom. Simultaneous reverse and normal initiation (SR&NI) is conducted in the presence of an alkyl halide initiator, and AIBN is used to generate the ATRP activator from its higher oxidation state complex (107) In this way, handling problems can be avoided while the ability to prepare block copolymers is maintained. This technique has been utilized in bulk and miniemulsion (108,109). Activators Generated by Electron Transfer (AGET). Due to the presence of a radical source that can initiate new chains, SR&NI is limited in its ability to prepare clean block copolymers. AGET ATRP circumvents this difficulty by

8

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION Outer Sphere Electron Transfer R + Mtn+1/Lm

R+ + Mtn/Lm

R + Mtn/Lm

R- + Mtn+1/Lm

Monomer Coordination Mtn/Lm

+ Mtn/Lm R R

Halide Dissociation X-Mtn+1/Lm

Mtn+1/Lm + X-

Disproportionation 2Mtn/Lm

Mtn+1/Lm + Mtn-1/Lm

(Mt = Cu, Os)

Formation of Organometallic Species R + Mtn/Lm

R-Mtn+1/Lm

-H Abstraction H + Mtn/Lm

n R

H-Mtn+1/Lm +

R

n R

R

Fig. 5. Possible side reactions during ATRP.

generation of the lower oxidation state activator via reducing agents that cannot produce new radicals. A variety of reducing agents can be used for this process, including zero valent Cu, (110,111) tin(II) 2-ethylhexanoate, (112) ascorbic acid, (113,114) and triethylamine (115). This technique is particularly applicable to aqueous and miniemulsion systems (116–118).

Decreasing Catalyst Concentration by New Initiation Processes (ICAR and ARGET). As described earlier, the total catalyst concentration in ATRP can be reduced without affecting polymerization rate because Rp depends on the ratio of activator to deactivator concentrations. However, the amount of catalyst cannot be decreased indefinitely in normal ATRP due to unavoidable radical termination reactions that occur at the initial stages of polymerization. If catalyst concentration is less than the concentration of terminated chains, all of the activator will eventually be present as a persistent radical and the reaction will stop at low conversion. A new initiation process known as initiators for continuous activator regeneration (ICAR) allows ATRP to be conducted in the presence of ppm amounts of catalyst. The technique works by the continuous generation of radicals by decomposition of a conventional radical initiator (such as AIBN),

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION R-X + Mtn / L

kact kdeact

Mtn+1X / L + R

9

kp +M

kt

R-R ICAR ARGET

I-X Oxidized form of RA + HX

I

1/2 AIBN (or thermal) Excess Reducing Agent (RA)

Fig. 6. Mechanism of ICAR and ARGET ATRP.

which reduces Cu that is present as a persistent radical to the corresponding lower oxidation state activator (Fig. 6) (119) In the polymerization of styrene, thermal initiation generates a sufficient amount of radicals without the need for an additional radical source. Polymerization rate during ICAR ATRP depends on the concentration of the radical source, and not on the nature of the catalyst. A dramatic reduction in catalyst concentration can also be achieved using an excess of reducing agent instead of a radical source (Fig. 6). This technique, which is known as activators regenerated by electron transfer (ARGET) (120), has been utilized to prepare homopolymers and clean block copolymers in the presence of < 50 ppm of Cu catalyst (121) The reducing agents employed in this process include hydrazine, phenol, glucose, ascorbic acid, Sn(II) species, and Cu(0). Careful consideration must be given to choice of catalyst for this process (119), since a number of side reactions that can occur during ATRP (such as halide dissociation and monomer coordination) are exacerbated during polymerization in the presence of low catalyst concentration (85). However, it is noteworthy that a diminished concentration of catalyst can reduce the occurrence of certain side reactions, such as outer sphere electron transfer and β-H elimination. This has recently allowed the preparation of high molecular weight poly(styrene-co-acrylonitrile) (122) and polyacrylonitrile (95) by ATRP.

Conducting ATRP ATRP can be conducted in bulk, solution, or a variety of heterogeneous media, (123) including miniemulsion (113,124–126), microemulsion (127), emulsion (128), suspension (129–132), dispersion (133), and inverse miniemulsion (118,134,135) The choice of polymerization media depends primarily on solubility considerations. A solvent is necessary in certain instances, such as during polymerization of acrylonitrile (the formed polymer is not soluble in its monomer) or room temperature polymerization of methyl methacrylate (the system will vitrify at high conversion). It should be noted that the chosen solvent should be suitable not only for monomer and obtained polymer, but also for the catalytic complex. Judiciously selected additives can enhance the capabilities of ATRP. Lewis acid complexing agents have been shown to increase polymerization rate and in some cases decrease molecular weight distribution (136,137). They can also increase the extent of syndiotacticity and isotacticity in a polymer (138,139).

10

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

Fig. 7. Illustration of polymers with controlled functionality, composition, and topology.

Reducing agents, such as tin(II) octanoate (119), ascorbic acid (114), and Cu(0) (114), can reduce handling difficulties and allow ATRP to be conducted in the presence of significantly lower amounts of catalyst.

Materials ATRP has been used to prepare polymers with various functionalities, compositions, and topologies (Fig. 7) (140). Highlights from each of these categories are discussed in the following sections. Functionality. The presence of functional groups within a polymer are important in fine-tuning many properties, such as solubility, polarity, biocompatibility, melting/glass transition temperatures, elasticity, tensile strength, crystallinity, electrical conductivity, etc. Functionality can be incorporated within a polymer via modified monomers, initiators, or chain ends (Fig. 7). The use of a functional monomer will have the greatest effect over bulk properties, while chain end functionality can lead to materials good for blend compatibilzation. ATRP is tolerant to several types of functional groups, although certain groups may interfere with the ATRP mechanism (eg, acidic groups). These can be easily incorporated through post polymerization modification, as discussed earlier. Functional monomers. The various monomers polymerizable by ATRP include styrene derivatives, (meth)acrylates, (meth)acrylamides, and acrylonitrile. These monomers can be modified accordingly in order to incorporate more sophisticated functional groups (Fig. 8). Other classes of functional monomers include macromonomers (which consist of a polymer chain with a polymerizable group at its terminus), monomers containing an ATRP initiator (leading to hyperbranched polymers), and monomers displaying two or more polymerizable groups (leading to a cross-linked network).

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

-

SO3 Na

+

-

COO Na

O O

+

N

N

O O

F

F

F

F

+

F

Cl-

O

O

O

11

O

O O

H N +

N

Na-O3S

O OH

O O

HO

H

H H O N

O HN

O

N3

H

OH

OH

HN

SO3H

Fig. 8. Various classes of functional monomers.

A multitude of styrene derivatives have been polymerized via ATRP, exhibiting both electron-withdrawing and weakly electron-donating substituents on the aromatic ring (141). The presence of electron withdrawing groups (such as sulfonates, carbonyls, and halogens) leads to a faster polymerization, due to decreased stability of the dormant species and thus increased propagation rates. The polymerization of 4-acetoxystyrene has been demonstrated (142), and styrene derivatives containing alkyl substituents were also successfully polymerized. As mentioned earlier, acidic derivatives (such as 4-vinylbenzenesulfonic acid and 4vinylbenzoic acid) are typically protected as a salt in order to prevent catalyst disruption (143). The ATRP of 4-vinylpyridine has also been accomplished, although the catalytic complex was carefully selected in order to prevent side reactions with this nucleophilic and basic monomer (74). (Meth)acrylates represent the broadest range of monomers polymerizable by ATRP. Some of the many examples of functional (meth)acrylates include 2-hydroxyethyl (meth)acrylate, (79) glycidyl (meth)acrylate (144), 2-trimethylsilyloxyethyl (meth)acrylate (145–147), 2-(dimethylamino)ethyl methacrylate (148), and allyl (meth)acrylate, among others. Various bioconjugates have been prepared by attachment of sugars (149) and short sequences of nucleotides (150,151) to vinyl monomers. ATRP of several (meth)acrylamides has also been demonstrated (152–155). Each of the functional monomers illustrated above yields polymers with unique properties. Particularly interesting properties are found in “smart”

12

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

materials, which are polymers that respond to environmental changes. These materials contain functionality that contributes to thermo- (154,156), light- (157), and/or chemo-responsive behaviors (158). For example, poly(2(dimethylamino)ethyl methacrylate) (PDMAEMA) exhibits thermo-responsive behavior, and is characterized by a lower critical solution temperature (LCST) of 32 ◦ C. Below this temperature, PDMAEMA is water-soluble while above its LCST, the polymer becomes hydrophobic and precipitates from aqueous solution. Lightresponsive polymers include (meth)acrylates containing azobenzene or spyropyran units, and will undergo a change in molecular structure upon ultraviolet irradiation (159,160). This leads to changes in dimension and polarity. There are other classes of polymers which respond to changes in pH or polarity of solvent. Functional Initiators. Initiators can be modified in a variety of ways, provided that the functional group does not interfere with the ATRP mechanism. The use of a functional initiator allows for direct incorporation of a functional group onto a chain end, yielding a telechelic polymer. Telechelic polymers have also been synthesized using functional initiators and atom transfer radical coupling (101). Several examples of functional initiators that have been successful for the ATRP of styrene are illustrated in Table 1. Polymers prepared by ATRP are halogenterminated and can subsequently be used as macroinitiators to form block copolymers. An initiator can also contain multiple initiating sites. Difunctional ATRP initiators have been used in the synthesis of multiblock copolymers, building from the core out (161). Other difunctional initiators can contain one ATRP site and an initiating group for another polymerization mechanism to make block copolymers through mechanistic transformation (162). Multifunctional ATRP initiators can be used to synthesize star (co)polymers and other hyperbranched materials as well. Chain End Functionality. The halogen located at the end of a polymer prepared by ATRP can be easily displaced by numerous other functionalities (Fig. 9). Some examples include initiating sites for other polymerization mechanisms, a polymerizable double bond for the generation of a macromonomer, or precursors for “click” chemistry. Currently, the most popular click reaction is the Cu(I)-catalyzed azide-alkyne cycloaddition (164,165), which has been prolifically combined with ATRP to prepare pendant-functionalized polymers (166), end-functional polymers (167,168), multisegmented block copolymers (169), stars (170), brushes (144,171), and other architectures (172). Polymer Composition and Microstructure. There are several combinations in which monomers can be arranged along a linear polymer chain, which is described by the instantaneous composition (Fig. 7). A linear polymer chain consisting of one type of monomer is known as a homopolymer. When more than one monomer is polymerized, random, periodic, block, or gradient copolymers are possible, depending both on the reactivities of each monomer and the method through which they were polymerized. Different properties can be obtained from polymers that have the same ratio of monomers and molecular weights, but differing instantaneous chain composition. Precise control over the composition of a single polymer chain is possible by ATRP, because essentially all the polymer chains grow at the same rate throughout the polymerization. Control over polymeric microstructure in terms of tacticity is not as simple, and can be developed further.

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

13

Table 1. Functional initiators used for the polymerization of styrene in bulk.a Initiator

Conv.

NC-Ph-CH2 -Br Br-Ph-CH2 -Br CH3 -CH(CN)-Br CN-CH2 -Br

Mn,SEC (g/mol) Mw /Mn

4-cyanobenzyl bromide 4-bromobenzyl bromide 2-bromopropionitrile bromoacetonitrile

0.48 0.48 0.48 0.48

5,500 4,500 5,100 4,500

1.10 1.16 1.09 1.10

glycidol 2-bromopropionate

0.62

6,800

1.12

t-butyl 2-bromopropionate

0.41

4,000

1.17

hydroxyethyl 2-bromopropionate

0.48

7,500

1.10

vinyl chloroacetate

0.94

5,800

1.12

α-bromo butyrolactone

0.41

4,000

1.17

2-chloroacetamide

0.12

4,000

1.51

O Br

O

O

O Br

O

O HO

Br

O

O Cl

O O Br

O

O Cl

H2N a At

110

◦ C,

with [M]0 /[I]0 /[CuBr]0 /[dNbpy]0 = 100/1/1/2 (163).

Random/Gradient Copolymers. Reactivity ratio is defined as the ratio of the rate constants of homopropagation to cross-propagation. If the reactivity ratios of comonomers are similar, a polymer with a random/stastical distribution of each monomer along the chain will be obtained. If one monomer has a significantly higher reactivity ratio than the other, a gradient copolymer is formed (173). Gradient copolymers cannot be prepared by conventional radical polymerization due to rapid monomer propagation and a very short chain lifetime. Copolymer chains rich in one comonomer are initially formed, while chains rich in the other comonomer form later in the polymerization. These materials typically exhibit poor properties and macroscopic phase separation. In ATRP, however, the gradient exists along each chain instead of between chains (174). Gradient copolymers can be synthesized by two methods, batch or semibatch. A batch process is used to prepare a spontaneous gradient copolymer if the reactivity ratios of the two monomers significantly differ. For comonomers with similar reactivity ratios, the semi-batch process can be used to generate a “forced” gradient by controlled addition of one comonomer to the reaction mixture throughout the reaction. Aspects of both random and block copolymers are brought together in gradient copolymers. These materials possess a broader glass transition temperature range, reduced order-disorder temperature, and can form new types of morphologies through self assembly (175). These properties can lead to

14

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

N3

PPh3 THF, RT

N

PPh3

NH

CH2

CH2

OH

>95% H2O, THF, RT >95% NaN3, DMF >95%

OH

PBu3

H2N

Et3N >95% (PSt)

NH2

P(n-Bu)3, 10 eq. X

TiCl, CH2Cl2

R

SiMe3

R

R

25 eq. O

3 eq. CH2 Bu3Sn

CH2 CH

Bu3SnH in situ >95% H

CH2

X

CH2 CH2

OH

benzene

X = Br,Cl

Cu(0), 0.5 eq./ CuBr

R

O CH2 CH

X

CH2 OH

Fig. 9. Examples of post-polymerization modification of a polymer chain prepared via ATRP.

several desirable applications, such as vibration- and noise-dampening materials, compatibilizers for immiscible blends, and surfactants in emulsion polymerizations (176,177). Alternating/Periodic Copolymers. Comonomers that exhibit a tendency to polymerize in an alternating fashion form periodic, or alternating copolymers. This occurs when both monomer reactivity ratios are much less than one, indicating cross-propagation is preferred to homopropagation. The most common example is the copolymerization of styrene (an electron-donating monomer) with maleic anhydride (a strong electron acceptor) (178). Tendency to alternate can also be strongly increased in the presence of additives, such as Lewis acids (Fig. 10) (179). Block Copolymers. Block copolymers are prepared by extension of a pure macroinitiator with another monomer (180,181). The order of monomer reactivity must be obeyed during block copolymer synthesis. If the first monomer is similarly or more ATRP active than the second monomer, a clean extension occurs to form well-defined block copolymers. However, if the first monomer is less reactive, inefficient extension of the macroinitiator will result in a product exhibiting bimodal molecular weight distribution (91). A method known as halogen exchange was developed to overcome this problem in ATRP (182,183). Halogen exchange requires sufficient amount of the catalyst to be present in order for less reactive chloride-terminated chain ends to form, and therefore it cannot be employed in polymerization techniques that utilize low catalyst concentration, such as ARGET ATRP. A recent report demonstrates that efficient chain extension of a less

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

15

MMA-LA MMA

+

St

MMA

St

MMA

LA

LA

St

MMA

LA

MMA

LA

St

MMA

LA

LA

Fig. 10. Formation of poly(methyl methacrylate-alt-styrene) in the presence of a Lewis acid.

meso

+ racemo

(a)

LA

LA

LA

+

meso

(b)

Fig. 11. Free radical propagation in the absence (a) or presence (b) of Lewis acid (LA).

reactive monomer with a more reactive one can be achieved during ARGET and ICAR by conducting the reaction in the presence of a small amount of styrene (184). An enormous variety of block copolymers have been prepared by ATRP, including diblock, triblock, and multisegmented; block copolymers by mechanistic transformation; and organic/inorganic hybrid block copolymers. Many of these materials are used commercially, for example, as thermoplastic elastomers, adhesives, and surfactants. Control of Tacticity/Stereoblocks. Control over tacticity is difficult in a radical polymerization because the propagating center is a nearly planar sp2 hybridized carbon, which results in poor stereoselectivity (Fig. 11a). However, the use of Lewis acids to prepare highly isotactic acrylic polymers via ATRP has been demonstrated (185–187). Lewis acids coordinate to the carbonyl group of acrylicbased monomers. If the complex is located between the last two segments of a growing polymer chain, they will be forced into a meso configuration (Fig. 11b). This produces an isotactic polymer that contains a high percentage of meso dyads.

16

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

+ Telechelic polymer

Functional polymer

+

Initiator

Macromonomer

+ R Monomer

Macroinitiator

Fig. 12. Methods to obtain a polymeric brush. Macromonomer ATRP

R Crosslinker

Initiator F-Br (F: functionality) [F-Br]/[MM] < 1

F Br F Br

Br F

Core-functionality (F) Low M w /Mn

Fig. 13. Preparation of star copolymers with narrow molecular weight distribution.

This method has also been utilized for the formation of stereoblock copolymers. A Lewis acid was added to the polymerization of N,N-dimethylacrylamide after a certain conversion, which yielded an atactic-b-isotactic polymer. Topology. The various topologies available by ATRP include graft copolymers, star copolymers, cyclic polymers, (hyper)branched materials, and crosslinked networks (Fig. 7). Graft/Brush Copolymers. A graft or brush copolymer consists of many polymer chains originating from a linear polymer backbone. Due to the unique structure of polymeric brushes, they can form super soft-elastomers (188,189) or can be used as templates for semiconducting or magnetic nanorods (190). The three methods to synthesize brushes are grafting from (191–196), grafting onto (166,197), and grafting through (Fig. 12) (198–204). Grafting onto involves reaction of a pre-formed polymeric side chain with a backbone polymer. This method typically cannot be used to synthesize brushes with a high grafting density due to steric crowding of the reactive sites. In the grafting through method, macromonomers are directly polymerized to form the brush copolymer. This method is often limited in the degree of polymerization that can be obtained. The most often used method is grafting from, in which monomer is grown from a linear polymer backbone that is functionalized with many ATRP initiating sites along the chain. This method allows significant control over the

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION O

S2

O Br

O

O

S2

S 2

O O

CuBr / bpy, Me 2CO, 50oC

S 2

17

S2

S2

S2 S2

S2 S2 S2

S2

S2

S2 S2

S2 Bu3P, H2O

SH

- Bu3PO

SH SH

Fig. 14. Formation of a biodegradable nanogel.

grafting density, which depends on how the polymer backbone is synthesized. In order to prevent brush-brush coupling, dilute solutions are required during polymerization. Coupling can also be prevented using a miniemulsion method. A variety of molecular brushes have been prepared by ATRP, including brushes with a gradient in grafting density (205), block copolymer side chains, (206), and a block copolymer backbone with different types of polymers grafted onto each block. Polymer chains can also be grafted from surfaces or particles to create a multitude of hybrid materials (207,208). Star Copolymers. A star copolymer is a nonlinear structure with a central branch point (209). The polymer chains radiating out from this point are known as arms. It is possible for a star copolymer to have multiple arms with multiple functionalities. Star copolymers typically exhibit high molecular weights and low viscosities, and can be used as lubricants, coatings, and carriers for small molecules. There are two general methods to synthesize a star copolymer: polymerizing the arms from the core, or attaching pre-polymerized arms to the core. In the first method, organic or inorganic multifunctional cores can be used to prepare 3, 4, 6, or 8 armed stars (210–216). A hyperbranched core with many functional sites (created by the polymerization of a monomer functionalized with an ATRP initiator) was also reported to yield a star with ∼80 arms (140). To generate stars starting from pre-formed arms, the arms can be attached to a functionalized core (for example, using click chemistry) (170,217) or cross-linked in the presence of divinyl compounds (218–220). Arms that are cross-linked and still contain ATRP initiating groups at the core may be used for the generation of miktoarm stars, which contain arms of different functionality and different lengths (220). A recent technique using macromonomers has been developed to create stars with exceptionally low polydispersity (Scheme 13) (221,222). Polymeric brushes with a star structure have also been synthesized by grafting polymer branches from the arms of a star-shaped backbone (223). Hyperbranched/Cross-linked Networks. The synthesis of hyperbranched and cross-linked networks with well-defined structures has been realized through ATRP and other CRP processes. Branching is often unavoidable in radical polymerizations due to radical transfer to polymer. The amount of branching can be controlled in ATRP by polymerizing monomers containing initiator functionality (224–226), or by polymerizing a dilute solution of divinyl monomer (227).

18

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

Branching is regulated in each case by controlling the degree of polymerization. Cross-linked topologies can be obtained directly by polymerization of divinyl monomers, or monomers containing cross-linkable pendant functionalities which are then connected in a post-polymerization step (228). If the amount of crosslinker is high enough, a network is obtained. Reversibly degradable gels have been prepared using ATRP by utilizing disulfide bonds as the crosslinking points (228). The disulfide bond is broken under reductive conditions, and can be reformed under oxidizing conditions. Biodegradable nanogels were also prepared using disulfide linkages, which have potential as drug delivery systems (Fig. 14) (229). Cyclic Polymers. The formation of cyclic polymers via condensation and ionic polymerization methods has been well documented. Generating cyclic polymers through a radical polymerization mechanism, however, is difficult due to the unselective reactivity of the propagating radical. Cyclization of a telechelic polymer chain synthesized via ATRP has been reported (230,231). Heterotelechelic azide- and acetylene-terminated polystyrene was cyclized using Cu(I)-catalyzed click chemistry under high dilution. This method can be extended to other functional polymers as well.

“Green” ATRP Recent advances in catalyst design, initiation systems, and materials synthesis have expanded the potential of ATRP as an environmentally benign process (98,99). The rational selection of highly active and stable catalytic complexes and the development of ICAR and ARGET have allowed ATRP to be controlled efficiently by essentially insignificant amounts of catalyst. It has been abundantly demonstrated that ATRP is compatible with environmentally friendly reaction media, such as water (79,123,232,233) and carbon dioxide (234–236). Finally, a variety of “green” materials have been prepared by ATRP, including self-plasticized polymers (237), degradable polymers(126,238–243), materials for water purification (244), and nonionic polymeric surfactants (245).

Summary and Outlook Since the emergence of ATRP over a decade ago, this powerful process has been enthusiastically explored in order to enhance mechanistic understanding and prepare a wide variety of new materials. Exquisite process control can be attained by appropriate choice of initiator and transition metal catalyst, both of which can be selected from a multitude of commercially available compounds. Recent fundamental investigations into the factors that affect activation, deactivation, and the position of the ATRP equilibrium have provided the polymer synthesis community with rules for the rational selection of the most suitable catalyst for almost any ATRP system. This can potentially provide new approaches for the polymerization of currently challenging monomers, such as α-olefins and (meth)acrylic acids. The development of new initiation systems, such as ICAR and ARGET, has allowed ATRP to be successfully controlled in the presence of only 10 ppm of catalyst

FUNDAMENTALS OF ATOM TRANSFER RADICAL POLYMERIZATION

19

and can provide an avenue for facile industrial scale-up and the preparation of sensitive biomaterials. Indeed, a number of companies specializing in the production of polymers using ATRP are currently in operation. The materials that have been prepared by ATRP include functional polymers; random, gradient, and block copolymers; graft and brush copolymers; star polymers; surface-grafted materials; hyperbranched polymers; and cross-linked networks. In addition, due to their well-defined structure and composition, polymers prepared by controlled/living methods are ideal candidates for structure–property studies. Finally, ATRP has exhibited increasing potential to not only be conducted in an environmentally friendly manner, but also to provide materials that can help combat current environmental problems.

Acknowledgements The authors are grateful to the members of the CRP Consortium at Carnegie Mellon University and the National Science Foundation (Grant DMR 0549353) for funding. Sincere thanks to Wade Braunecker for detailed discussions and help in preparation of the manuscript.

BIBLIOGRAPHY 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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195. V. Percec and F. Asgarzadeh, J. Polym. Sci., Part A: Polym. Chem. 39, 1120 (2001). 196. S. C. Hong, T. Pakula, and K. Matyjaszewski, Macromol. Chem. Phys. 202, 3392 (2001). 197. H. Gao and K. Matyjaszewski, J. Am. Chem. Soc. 129, 6633 (2007). 198. C. J. Hawker, D. Mecerreyes, E. Elce, J. Dao, J. L. Hedrick, I. Barakat, P. Dubois, R. Jerome, and I. Volksen, Macromol. Chem. Phys. 198, 155 (1997). 199. H. Shinoda and K. Matyjaszewski, Macromolecules 34, 6243 (2001). 200. S. C. Hong, S. Jia, M. Teodorescu, T. Kowalewski, K. Matyjaszewski, A. C. Gottfried, and M. Brookhart, J. Polym. Sci., Part A: Polym. Chem. 40, 2736 (2002). 201. D. Neugebauer, Y. Zhang, T. Pakula, S. S. Sheiko, and K. Matyjaszewski, Macromolecules 6746 2003. 202. H. Shinoda, K. Matyjaszewski, L. Okrasa, M. Mierzwa, and T. Pakula, Macromolecules 36, 4772 (2003). 203. H. Kaneyoshi, Y. Inoue, and K. Matyjaszewski, Macromolecules 38, 5425 (2005). 204. K. Matyjaszewski, K. L. Beers, A. Kern, and S. G. Gaynor, J. Polym. Sci., Part A: Polym. Chem. 36, 823 (1998). 205. S. J. Lord, S. S. Sheiko, I. LaRue, H.-I. Lee, and K. Matyjaszewski, Macromolecules 37, 4235 (2004). 206. H. Lee, W. Jakubowski, K. Matyjaszewski, S. Yu, and S. S. Sheiko, Macromolecules 39, 4983 (2006). 207. J. Pyun, T. Kowalewski, and K. Matyjaszewski, Macromol. Rapid Comm., 24, 1043 (2003). 208. J. Pyun and K. Matyjaszewski, Chem. Mater. 13, 3436 (2001). 209. K. Matyjaszewski, Polym. Int. 52, 1559 (2003). 210. K. Matyjaszewski, P. J. Miller, E. Fossum, and Y. Nakagawa, Appl. Organomet. Chem. 12, 667 (1998). 211. S. Angot, K. S. Murthy, D. Taton, and Y. Gnanou, Macromolecules 31, 7218 (1998). 212. A. Heise, C. Nguyen, R. Malek, J. L. Hedrick, C. W. Frank, and R. D. Miller, Macromolecules 33, 2346 (2000). 213. K. Matyjaszewski, P. J. Miller, J. Pyun, G. Kickelbick, and S. Diamanti, Macromolecules 32, 6526 (1999). 214. Y. Gnanou and D. Taton, Macromol. Symp. 174, 333 (2001). 215. O. R. Costa, W. L. Vasconcelos, R. Tamaki, and R. M. Laine, Macromolecules 34, 5398 (2001). 216. A. P. Narrainen, S. Pascual, and D. M. Haddleton, J. Polym. Sci., Part A: Polym. Chem. 40, 439 (2002). 217. H. Gao and K. Matyjaszewski, Macromolecules 39, 3154 (2006). 218. X. Zhang, J. Xia, and K. Matyjaszewski, Macromolecules 33, 2340 (2000). 219. J. Xia, X. Zhang, and K. Matyjaszewski, Macromolecules 32, 4482 (1999). 220. H. Gao, N. V. Tsarevsky, and K. Matyjaszewski, Macromolecules 38, 5995 (2005). 221. H. Gao and K. Matyjaszewski, Macromolecules 40, 399 (2007). 222. H. Gao, S. Ohno, and K. Matyjaszewski, J. Am. Chem. Soc. 128, 15111 (2006). 223. K. Matyjaszewski, S. Qin, J. R. Boyce, D. Shirvanyants, and S. S. Sheiko, Macromolecules 36, 1843 (2003). 224. C. J. Hawker, J. M. J. Frechet, R. B. Grubbs, and J. Dao, J. Am. Chem. Soc. 117, 10763 (1995). 225. S. G. Gaynor, S. Edelman, and K. Matyjaszewski, Macromolecules 29, 1079 (1996). 226. K. Matyjaszewski and S. G. Gaynor, Macromolecules 30, 7042 (1997). 227. R. Baudry and D. C. Sherrington, Macromolecules 39, 1455 (2006). 228. N. V. Tsarevsky and K. Matyjaszewski, Macromolecules 38, 3087 (2005). 229. J. K. Oh, D. J. Siegwart, H.-i. Lee, G. Sherwood, L. Peteanu, J. O. Hollinger, K. Kataoka, and K. Matyjaszewski, J. Am. Chem. Soc. 129, 5939 (2007).

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230. N. V. Tsarevsky, B. S. Sumerlin, and K. Matyjaszewski, Macromolecules 38, 3558 (2005). 231. B. A. Laurent and S. M. Grayson, J. Am. Chem. Soc. 128, 4238 (2006). 232. M. F. Cunningham, Prog. Polym. Sci. 27, 1039 (2002). 233. M. Sawamoto and M. Kamigaito, Macromol. Symp. 177, 17 (2002). 234. J. Xia, T. Johnson, S. G. Gaynor, K. Matyjaszewski, and J. DeSimone, Macromolecules 32, 4802 (1999). 235. H. Minami, Y. Kagawa, S. Kuwahara, J. Shigematsu, S. Fujii, and M. Okubo, Des. Monomers Polym. 7, 553 (2004). 236. C. J. Duxbury, W. Wang, M. de Geus, A. Heise, and S. M. Howdle, J. Am. Chem. Soc. 127, 2384 (2005). 237. H. J. Paik, S. G. Gaynor, and K. Matyjaszewski, Macromol. Rapid Commun. 19, 47 (1998). 238. J.-Y. Yuan, C.-Y. Pan, and B. Z. Tang, Macromolecules 34, 211 (2001). 239. J. Huang, R. Gil, and K. Matyjaszewski, Polymer 46, 11698 (2005). 240. N. V. Tsarevsky and K. Matyjaszewski, Macromolecules 35, 9009 (2002). 241. R. R. Shah, D. Merreceyes, M. Husseman, I. Rees, N. L. Abbott, C. J. Hawker, and J. L. Hedrick, Macromolecules 33, 597 (2000). 242. D. Bontempo, K. L. Heredia, B. A. Fish, and H. D. Maynard, J. Am. Chem. Soc. 126, 15372 (2004). 243. Y. Iwasaki and K. Akiyoshi, Macromolecules 37, 7637 (2004). 244. N. Saleh, T. Phenrat, K. Sirk, B. Dufour, J. Ok, T. Sarbu, K. Matyjaszewski, R. D. Tilton, and G. V. Lowry, Nano Lett. 5, 2489 (2005). 245. K. A. Davis and K. Matyjaszewski, Adv. Polym. Sci. 159, 1 (2004).

PATRICIA L. GOLAS LAURA A. MUELLER KRZYSZTOF MATYJASZEWSKI