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Chapter 3

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Mechanisms and Kinetics of Living Radical Polymerization: Absolute Comparison of Theory and Experiment Takeshi Fukuda, Chiaki Yoshikawa, Yungwan Kwak, Atsushi Goto, and Yoshinobu Tsujii Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

Theories that have been proposed to describe the polymerization rates and polydispersities in living radical polymerization (LRP) were experimentally tested for main branches of LRP including nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), iodide-mediated polymerization, and reversible addition-fragmentation chain transfer (RAFT) polymerization. The theories were verified on an absolute scale in both the absence and presence of conventional initiation. The cause for the marked retardation in polymerization rate observed in a dithiobenzoate-mediated RAFT polymerization of styrene was eluciated.

24

© 2003 American Chemical Society

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

25 Introduction e

The recent development of living radical polymerization (LRP) has achieved the combination of the robustness of radical chemistry with the feasibility of living polymerization to finely control polymer structure (7). Main reactions in LRP include the reversible activation shown in Scheme la, where the dormant species P-X is activated to the alkyl radical P , which, in the presence of monomer, will propagate until it is deactivated back to P-X. Three main mechanisms of reversible activation are currently considered important, which are dissociation-combination (DC), atom transfer (AT), and degenerative chain transfer (DT) processes (Scheme lb-Id). In the DC process, the P-X bond is cleaved by a thermal or photochemical stimulus to produce the stable or persistent radical X and the polymer radical P \ The A T process is kinetically akin to the DC process, except that the activation is catalyzed by the activator A , and the complex A X plays the role of a persistent radical. The DT process is a chain transfer reaction in which activation and deactivation occur at the same time, and is kinetically different from the former two. The reversible additionfragmentation chain transfer (RAFT: Scheme 2a) belongs to the DT category.


e

e

(a) Reversible Activation (General Scheme) P-X

«

*

ρ·

^deact

F^—'

(Dormant)

(Active)

(b) Dissociation-Combination (DC) P-X

-

*

=

p*

+

χ·

=

^act ^id, ^deact ^ [X*] C

(c) Atom Transfer (AT) p-X

+

A

«

-

Ρ

+

AX

*da fract fra[A], *deact = *da[AXl =

(A = activator)

(d) Degenerative Chain Transfer (DT)

+

P'-X

Scheme I. Reversible activation processes.


26

(P-X) (Z^CHaetc.)

(P.(X>P*)

(P'-X)

(b) Cross-Termination

Ρ* +

ι

•

dead polymer


Ζ (Ρ-(Χ>Ρ·)

Scheme 2. (a) RAFT and (b) cross-termination. Theories have been developed describing the time evolutions of monomer consumption rate and chain length distribution in these systems. Qualitative aspects of those theories have been experimentally confirmed in part (2), but until recently, they have never been experimentally tested quantitatively. In this publication, we will review our results on absolute comparison of theory and experiment for the three main branches (Scheme lb-Id) of LRP. It is absolute comparison, because the parameters appearing in the theoretical formula have all been determined by independent experiments. We will also discuss another important (and somewhat controversial) topic, the cause for the rate retardation observed in some RAFT polymerizations, by summarizing our published and unpublished results.

Theory of LRP 3

In a typical LRP run, the living chain is active for, say, 10"" s and becomes dormant for about 10 s, which is followed by many such active-dormant cycles, thus the chain growing in an intermittent fashion. This gives characteristic kinetic features to LRP. As the number of active-dormant cycles increases, the chain length distribution becomes narrower. Thisn arrowing process and the related polymerization rate are different for two typical cases: the first case is the complete absence of conventional initiation. The existence of radicalradical termination will increase the persistent species (X* in Scheme lb or A X in Scheme lc) with time, which will make the equilibrium incline towards the dormant state, or make the transient lifetime (active time per activationdeactivation cycle) shorter and shorter with increasing polymerization time t. The second case is the presence of conventional initiation. The conventional 2


27 initiation will keep the concentration of the persistent species stationary, thus making the polymerization rate higher and the polydispersity index (PDI) smaller than in the absence of it. Putting these remarks in theory, the following relations are obtained for LRP with a DC process: when R< = 0 (3,4), υ

3

ln([M]o/[M]) = (3ί2)/θρ(Κ[Ρ^Χ]^) Ψ [YB - (1/^.B)]" = (3/8)*d> 1

(I) (2)

(small 0

and when R * 0 and is sufficiently large (5,6), Downloaded by IMPERIAL COLL LONDON on May 26, 2014 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch003

t

,/2

ln([MV[M]) = W ^ ) /

(3)

1

[YB - Ο/^,Β)]" = (V2)k t

(small 0

d

(4)

where R is the rate of conventional initiation, M is the monomer, the subscript "0" denotes the initial state, Κ = k /k , and kp, k, k , and k are the rate constants of propagation, termination, dissociation, and combination, respectively; Y + 1 = jc B/jc with JC ,B and χ being the weight- and number-average degrees of polymerization, respectively, of the grown portion (B) of the polymer. Given the PDI of the whole polymer (Y + 1), and that of the initiating portion (A) or the initiatingd ormant (low-mass or polymer) species, (7 1)> can be calculated with {

d

c

d

c

B

W>

ntB

η = M M ] + *i,VRiio[VRl 10]o 0


29

give the conversion and the PDI of the product polymer at time t as well as the PDI of the initiating polymer (cf. the curve for t = 0), with which we can compute the polydispersity factor Y according to eq 5. (Note that these experiments were so designed as to yield accurate kinetic data, not to give polymers with a low polydispersity.)


B

1

1

ι—•—ι— —ι— —ι—ι—ι—»—ι—»—ι—«—ι—»—ι—•—I 17 18 19 20 21 22 23 24 25 26 elution time / min

I 17

•

1 18

»

1 19

•

! 20

«

1 21

"

1

•

1

22

23

•

1 24

1

1 25

1

1 26

elution time / min

Figure 1. GPC chromatograms for the styrene/PS-DEPN/(BPO) systems (80 °C): [PS-DEPNJo = 25 mM; [BPO] = (a) 0 and (b) 4.7 mM. 0


30 Table la lists the kinetic parameters used for the theoretical equations 1-4. They were either taken from the literature or, when literature data were unavailable, determined by independent experiments. The solid curve in Figure 2a shows eq 1 calculated with these data, which reproduces the experimental data (open and filled circles) very well. The solid and broken curves in Figure 2b show eq 2 and eq 4, respectively, which also reproduce the experimental polydispersity data satisfactorily. These are the first examples to experimentally justify the power-law equation 1 and the relevant PDI equation 2 on an absolute scale. The theoretical prediction that conventional initiation makes R larger (Figure 2a) and PDI smaller (Figure 2b) than in the absence of it has been thus confirmed. The dotted line is the best-fit represendation of the plots (squares) according to eq 3 (from which we have obtained k (Table la) in this specific system (9) and used it in the above analysis of eq 1).


p

ATRP An u>polystyryl bromide (PS-Br) ( M = 1200 and MJM = 1.08) was used as an initiating polymer. Polymerization of styrene in i-butylbenzene (50/50 v/v) was carried out at 110°C with the Cu(I)Br/dHbipy complex catalysis in the presence and absence of the conventional initiator VR110, where dHbipy is 4,4'-di-ii-heptyl-2,2 -bipyridine and VR110 is 2,2'-azobis(2,4,4trimethylpentane). Particular care was taken to avoid oxidation of Cu(I)Br prior to polymerization runs. Table lb lists the kinetic parameters used in this study. Again, parameters unavailable in literature were determined by independent experiments. To test eq 1 with Κ « (^/^)[Cu(I)Br] , we examined the system with [PS-Br] =13mM and [Cu(I)Br] = lOmM. It was confirmed that thermal initiation of the diluted styrene was entirely negligible in the examined time range (< 35 min). The duplicated experimental points (circles in Figure 3a) are well reproduced by the ^-dependent linear line predicted by the theory (solid line), confirming the theory on an absolute scale. The addition of 40mM of the conventional initiator VR110 to this system increased R b y a factor up to about 3, as shown by the squares in Figure 3a. Figure 3b shows the comparison of the PDI equations 2 and 4 (k = Aa[Cu(I)Br] ) with the experiments with [VR110] = 0 (circles) and [VR110]o = 40mM (square), respectively. In both systems, satisfactory agreement of theory and experiment was found. It was confirmed for the ATRP system, too, that conventional initiation not only increases the polymerization rate but lowers the polydispersity (at least at an initial stage of polymerization where the contribution of terminated chains to polydispersity is insignificant). n

a

,

0

0

0

p

d

0

0


31 0.07

»

0.06 0.05

Τ"

1

I

·

ι

(a)

-

[BPO] >0

0.04

/

5

^

-

0.03


0.02

IBPO] = 0

i

0

0.01

Ο.ΟΟι r*^*

0.16 r—»

1

•«

50

100

—

0

1 ι

f

ι

ι

ι

ι

ι

·-

-»

*

150

ι

ι

i

ι

200

i

» I

2 3

i

!

Figwre 2. P/or of (a) Ιη([Μ]

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