Living Radical Polymerizations Mediated by Metallo-Radical and

Chapter 25

Living Radical Polymerizations Mediated by Metallo -Radical and Organo-Transition Metal Complexes

Downloaded by STANFORD UNIV GREEN LIBR on August 8, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch025

Bradford B. Wayland, Xuefeng Fu, Chi-How Peng, Zhi Lu, and Michael Fryd Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323

Abstract Cobalt porphyrin derivatives are observed to participate in the control of living radical polymerization (LRP) by two distinctly different mechanisms. Cobalt (II) porphyrin (Co(II)) metallo-radicals mediate a LRP by reacting with polymeric radical (Ρ•) to produce a quasi-equilibrium with an organometallic complex (Co-P) which gives living character through the persistent radical effect. Diamagnetic organo -cobalt complexes in combination with an additional radical source such as AIBN produces an alternate form of LRP. The rates of polymerization by the organo-cobalt mediated route are relatively fast and approach that of regular radical polymerization because the radical concentration is determined by the organic radical source. The cobalt (II) metallo-radical concentration is vanishingly small in the organo-cobalt mediated process which removes the need for ligands that use large steric requirements to suppress cobalt (II) catalyzed chain transfer. Radical polymerizations of alkyl acrylates, acrylic acid and vinyl acetate mediated by the organo-cobalt porphyrin route are used in illustrating the principal mechanistic features of this process.

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© 2006 American Chemical Society

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

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Cobalt(II) tetramesityl porphyrin ((TMP)Co *) is a prototype persistent metal-centered radical that mediates the living radical polymerization of acrylates by forming a dormant organometallic complex ((TMP)Co-P) with the acrylate polymer radical (Ρ·) (eq 1-3). The (TMP)Co - mediated polymerization is a metal-centered 1,2

11

11

1) (TMP)Co-R —

(TMP)Co * + R-

2) R« + «CH =CH(C02R') -A*, p. 3) (TMP)Co . + P. — (TMP)Co-P 2


11

radical example of a broader class of LRP processes often categorized under the name of stable free radical polymerization (SFRP). AU SFRP processes use a stable "persistent" radical to control the propagating polymer radical (Ρ·) concentration through a quasi-equilibrium with a dormant complex (P-X). The best known subcategory of SFRP is nitroxide radical polymerization (NRP) mediated by stable organo nitroxide radicals (R NO). Applications of the cobalt(II) metallo-radical mediated LRP are limited by both a small rate of polymerization that results from the required low radical concentrations for SFRP, and termination of polymer radical chain growth by cobalt (II) β-Η abstraction with subsequent catalytic chain transfer (eq 4-6). 3

4

2

5

4) (por)Co« + .CH(X)CH -P — (por)Co-H + P-CH=CH(X) 5) (por)Co-H + CH =CH(X) — * (por)Co-CH(X)CH 6) (por)Co-CH(X)CH — ^ (por)Co * + *CH(X)CH 2

2

3

n

3

3

The large steric requirements of the TMP ligand suppress the chain transfer by (TMP)Co from acrylate polymer radicals which permits formation of high molecular weight acrylate polymers with low polydispersity. ' In addition to the cobalt (II) mediated SFRP, cobalt porphyrins manifest an entirely separate category of controlled/living radical polymerization that is mediated by diamagnetic organo-cobalt (III) complexes. This organo-cobalt controlled process utilizes radical sources like alkyl azo compounds to provide a slow continuous influx of new radicals. The cobalt porphyrin system thus has the capability to mediate living radical polymerization by several different mechanistic pathways which is a feature shared with organo-complexes of tellurium, stibine, germanium, and titanium. 11

1 2

6

7

8

9

Prototype transition metal mediators for L R P Cobalt complexes of tetra mesityl porphyrin (TMP) and tetra anisyl porphyrin (TAP) are examples of large and small steric requirement complexes for applications in organic solvent media. Phenyl sulfonated porphyrin derivatives such as tetra (3, 5-disulfonato mesityl) porphyrin provide analogous species for application in aqueous media. 10



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L R P of acrylates mediated by (TMP)Co-R Solutions of methyl acrylate (2.5M), (TMP)Co" (1.2xlO M), and V-70 (((CH3)2(OCH3)C(CH2)C(CH )(CN))2N2) (0.7-2.0x10" M) in benzene were heated to 333K and the conversion to polymer was followed by *H N M R (Figure 1). The induction time periods prior to observing polymerization, decrease with increasing concentration of V-70 and correspond to the time required to inject sufficient radicals to convert effectively all of the (ΤΜΡ)Οο · into diamagnetic organometallic complexes, (TMP)Co-R. Production of a modest excess of total organic radicals compared to the initial concentration of (TMP)Co results in large increases in the rate of polymerization without loss of control (Figure 1). The acrylate polymers formed when using this procedure showed a linear increase in number average molecular weight (M ) with conversion, low polydispersity (—1.06) and molecular weights close to the theoretical values corresponding to one polymer chain per cobalt porphyrin (Figure 2). The methyl acrylate (MA) polymerizations using V-70 (t (333 K) = 11 min.) as a radical source that shown in Figure 1 illustrate both the cobalt (II) metallo-radical mediated (Figure 1A) and the organo-cobalt mediated processes (Figure 1 (B, C)). When the ratio of total moles of radicals injected into solution from the radical source to the initial moles of (TMP)Co . is less than unity the polymerization process is mediated by the excess of (TMP)Co . through a SFRP process. The polymerization is very slow because the radical is maintained at a low concentration by a quasi-equilibrium between (ΤΜΡ)Οο · and the organoCo(TMP) species (Figure 1A). The living character of this SFRP is a manifestation of the persistent radical effect. Radical polymerization of M A for the condition where the total moles of radicals from V-70 exceeds the initial moles of (TMP)Co . is illustrated in Figure 1 (B, C). The process has an induction period where only a small fraction of the polymer forms followed by a period of rapid polymerization that begins when the moles of radicals that enter solutionfromV-70 exceed the initial moles of (TMP)Co .. At that point all the (TMP)Co . has been converted to (TMP)CoP and the excess radicals change the polymerization process from a Co * mediated SFRP to an organo-cobalt mediated polymerization. The most important feature is that during this period of fast polymerization the process _3

3

3

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n

1/2

11

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11

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361 remains controlled and produces low polydispersity polymers where M increases linearly with monomer conversion. (Figure 1,2).

n


12

1

1.5 2 Time (hour)

2.5

3

Figure 1. Monomer conversion as a function of time in the radical polymerization of methyl acrylate in benzene at 333K with fMAJj = 2.5Mand [(TMP)Co ']i = L2xlOT M. The quantity of V-70 radical initiator added was varied to produce different ratios of the total

Living Radical Polymerizations Mediated by Metallo-Radical and

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