Controlled Radical Polymerization - American Chemical Society

Chapter 22

Free-Radical Synthesis of Functional Polymers Involving Addition-Fragmentation Reactions

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch022

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P. Chaumont , D. Colombani , L. Boiteau , J. P. Lamps , M . - O . Zink , C. P. R. Nair , and D. Charmot 3

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Université Claude Bernard, L E M P B , Bât. 305, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne, France Institut Charles Sadron, 6 rue Boussingault, 67083 Strasbourg, France Polymer & Special Chemical Division, Vikram Sarabhai Space Center, Trivendrum 69022, India Centre de Recherches Rhône-Poulenc, 52 rue de la Haie Coq, 93308, Aubervilliers, France

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The synthesis o f functionalized polymers by means o f addition– fragmentation agents in free radical polymerization has been studied in order to determine the efficiency o f the method. In such systems, a strong decrease o f the rate o f polymerization, that can be attributed to some retardation or degradative chain transfer, is generally observed. A study o f the kinetics o f these systems has been initiated in order to investigate the cause o f such retardation and its influence on the functionality o f the polymers formed. Chain transfer reactions have been extensively used in free radical polymerization in order to both control the molar mass and to functionalize polymers. The conventional chain transfer agents such as mercaptans act by hydrogen abstraction. This radical transfer is a one step process, i.e. the thiyl radical and the terminated polymer are formed simultaneously. Reactions other than hydrogen abstractions may be used in order to achieve the same goals. This is the case o f the so-called addition-fragmentation reactions, which are two step processes. A recent review (7) has been published, giving a list o f such addition-fragmentation chain transfer agents ( A F C T A ) . A s the overall process is the same as in the classical chain transfer reactions, i.e. the termination o f the polymer chain and the re-initiation o f a new one, the activity o f A F C T A s is generally analysed in the same way, by determining the chain transfer constants. However, the two step reactions o f such systems implies some differences. This article deals with the calculation o f the kinetics o f such systems, and the prediction o f the functionality o f the polymers formed, whatever the A F C T A used. In order to compare the mathematical predictions with some experimental data, the results obtained in the study o f pentadiene-like A F C T A have been used. Since the 362

© 1998 American Chemical Society

In Controlled Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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main objective o f this paper is not to describe such systems, these latter studies have been published (2a) or will be published elsewhere (2b). However, some information needed for the comprehensiveness of the text are reported below.


Experimental Polymerizations were carried out in the bulk, excepted an experiment devoted to the study o f the influence o f the concentration o f the overall polymerization medium, and at l o w conversion, i.e. below 10%, in order to maintain the ratio [AFCTA]/[Monomer] constant. A l l reactants were purified as previously described (2a). The polymerizations were carried out in sealed ampoules, the polymerization medium being degassed by freeze-vacww/w-thaw cycles. The instruments used for macromolecular and chemical characterizations are described in reference 2a. Mechanism and Kinetics of Addition-Fragmentation Reactions Addition-fragmentation reactions will now be considered in detail. Such reactions may be described as the result o f (a) the addition o f some radical species, the "primary radicals", to the addition site o f the A F C T A , which is generally an alkene structure, giving rise to the formation o f a new "intermediate radical" and (b) the subsequent intramolecular evolution o f this latter intermediate radical, the so-called fragmentation reaction, i.e. the breaking o f some "weak" bonds located elsewhere inside the molecule. This latter reaction may be (a) a β-scission or (b) an homolytic substitution (Cf. Scheme 1).

R

.

«

+

addition

^-XiR')

.

X

"

n

Y

^

+

(

R

^

v

^-X(R')

homolytic χ ^ — X - X ( R ' ) substitution

A addrtion ^—X—X(R')

Λ

Ë Z =

Y RHC (R')X L^X +

Scheme 1. Addition-fragmentation Reaction If R ° represents the primary radical, M X the A F C T A , R X ° the intermediate radical, Ζ the product and X ° the radical expelled in the reaction medium after the fragmentation reaction respectively, such processes can be written as: Addition

R* + M X -> R X *

Fragmentation

R X * -> Ζ + X

e

If ka is the rate constant o f the addition reaction, kf is the rate constant o f the fragmentation process, and i f [R°] represents the concentration o f the primary radical and [RX°] the concentration o f the intermediate radical respectively, then the rates o f these two reactions can be written as: Addition

R,=kJR-][MX]

(1)

Fragmentation

R

(2)

f

= k [RX*] f


364


However, these equations do not take into account the origin and the rate o f formation of the primary radicals R°. Whatever the value of this constant and whatever the evolution o f the expelled radical X°, due to the steady state hypothesis regarding the radical [RX°] it can be considered that Ra is equal to Rf, i f side reactions are ignored. Thus, the ratio o f the two radical concentrations can be written as: [RX*]_k [MX] a

[R-]

k

(

3

)

f

Hence, at low values o f kf compared to ka, a high concentration o f the intermediate radical R X ° is expected. Due to this high value o f [RX°], the rate o f side reactions involving the intermediate radical may be increased. The same behavior is expected by increasing the concentration of the A F C T A . Addition-Fragmentation Reactions in Free Radical Polymerisation Processes When the A F C T A is added to a free radical polymerization medium, the growing macroradicals (P°) replace the primary radical R° described in the preceding paragraph, but the reaction sequence remains the same (Cf. Scheme 2 ). A s a result o f the addition-fragmentation chain transfer reaction, a functionalized polymer is formed, and the expelled X ° radical re-initiates a new polymer chain.

Scheme 2. Addition-fragmentation chain transfer reaction In such experiments, the concentration o f the A F C T A is generally much lower than the concentration o f the monomer (M). However, in many cases (7), even low concentrations o f A F C T A may provoke a strong decrease o f the molar mass o f the formed polymer, this is expected for a chain transfer agent, but also a strong decrease of the overall rate o f polymerization (Rp). In conventional chain transfer studies, such behavior is generally described as the consequence o f a low rate o f re-initiation. Two cases are observed (3): (a) I f the rate constant o f the chain transfer reaction ktr is o f the same order o f magnitude as the rate constant o f propagation kp, the behavior is described as "retardation", (b) If ktr is much greater than k a "degradative chain transfer" is obtained. Thus, a conventional explanation for the decrease o f the rate o f polymerization in the presence o f A F C T A is the primary radical termination (2d). However, in many studies o f A F C T A s , such retardations or degradative chain transfers may be observed, p



365

even though the rate o f re-initiation is not expected to be particularly slow, such an example is the case o f the polymerization o f methyl methacrylate ( M M A ) in the presence o f 5-i-butylthiopentadiene ( T B T P ) as A F C T A (Cf. Figure 1). In fact, the radical expelled in the polymerization medium after the addition fragmentation process, X ° , is a /-butylthiyl radical, which does not provoke considerable retardation o f the polymerization o f M M A , as is observed when the classical 2-methyl-2-propanethiol is used instead o f the pentadienic A F C T A (Cf. Scheme 3 and Figure 1).

(a)

+

(b)

Scheme 3. Chain transfer reactions using (a) a conventional mercaptan and (b) pentadienic A F C T A Thus, the decrease o f the overall rate o f polymerization by primary radical termination which is invoked in the reference 2a seems to be not sufficient to explain the magnitude o f this decrease, in contrast to other systems like thiuramdisulfide iniferters (4). Therefore, the kinetics o f such systems have been re-examined. In order to find other reactions which might causes decrease of Rp, it must be first remarked that the sulfur content o f the poly(methyl methacrylate) obtained by using T B T P is higher than expected. According to the scheme o f addition-fragmentation chain transfer reactions (Cf. Scheme 2), the number o f sulfur atoms per polymer chain (n ) must be less than unity. This expected value < 1 comes from the fact that the polymer chains may be initiated both by the radical formed by the thermolysis o f the initiator and by the radical X ° expelled in the medium after the fragmentation reaction. Generally, due to multiple transfer reaction steps during the lifetime o f each radical generated by the initiator, the number o f chains initiated by X ° , i.e. the thiyl radical, is much higher than the number o f chains initiated by the initiator. Thus, the number o f sulfur atoms per chain, due to the presence o f the thiyl fragment at the "beginning" o f the chain, must be close to, but less than, one. Nevertheless, in the case o f the polymerization o f M M A with T B T P as chain transfer agent in bulk conditions, the experimental value is close to 2.5 (2). This abnormal value can be explained by the occurrence o f some side reactions, namely the cross propagation o f the intermediate radical (Cf. Scheme 4 ). This latter hypothesis is difficult to prove directly but some experimental facts are compatible only by this explanation, such as the decrease o f n when the overall concentration o f the polymerization medium is decreased (see paragraph dealing with the interconversion). s

s



366

One very important consequence o f the occurrence o f this cross-propagation reaction, is the fact that the lifetime o f the intermediate radicals, i.e. the macroradical species present in the polymerization medium between the addition and the fragmentation reactions, appears to be high enough to allow side reactions, which are in competition with the fragmentation process. This "long" lifetime may be interpreted as the consequence o f a slow fragmentation process because o f the stability o f the intermediate radical and/or the stability o f the weak carbon-sulfur bond. For example, by using bromo-pentadiene instead o f T B T P , ns becomes equal to 0.9 (2b).

X Scheme 4. Cross-propagation o f the intermediate macroradical. Whatever may be the explanation o f the long lifetime o f the intermediate radicals, it affords more opportunities for other side reactions involving these macroradical species to occur, including an increase in occurrence o f termination processes. Such termination processes, like polymer radical homo- and cross-terminations, are generally invoked in conventional free radical copolymerizations involving two different monomers, to explain the decrease o f the rate o f polymerization, especially the cross-termination reaction. Thus, two questions arise from this analysis: (a) Is it possible to explain the decrease o f the rate o f polymerization by the occurrence o f polymer radical terminations ? (b) What is the consequence o f such termination processes on the functionality o f the polymers formed ? Kinetic Studies In order to answer these two preceding questions, an equation for the rate o f polymerization must first be established. The chemical reactions taken into account are listed in Table I. According to the proposed hypothesis, i.e. the absence o f retardation and/or degradative chain transfer reactions, the only termination reactions considered are the reactions involving macroradical species. Due to the low relative concentration o f the A F C T A with respect to the concentration o f the monomer, the influence o f the penultimate effect can be neglected. Lastly, it can be assumed that i f experiments are carried out at l o w conversions o f the monomer the following are avoided : (a) the effect o f the viscosity o f the polymerization medium on the kinetics o f the


367 '

ι

%;

Α


0.6

,— ι

ι

ι

ι

.

ι

Α

Α

•

•

• *

ι

•

•

ΤΒΤΡ

• Α

Pentadiene

•

•

t-BuSH

-

0

5

10

15

20

25

Concentration (mol/L) χ ΙΟ

30

3

Figure 1. Variation o f the relative rate o f polymerization o f M M A in the presence o f T B T P , Pentadiene and t-Butyl-mercaptan.

Table I. Reactions Involved in the Free Radical Polymerization of Vinylic Monomers in the Presence of Addition-Fragmentation Chain Transfer Agents Constants

Products

ka

1°

ki kjd k'i k'

IM° IX° XM° XX°

k„

po

Notations

Rate

Remarks

Initiator h

Ra

Initiations

P + M I° + M X X° + M X° + M X

P° PX° P° PX°

R» Rxi R'i R'xi

Initiation Re-initiation

Macroradicals Reactions

P° + M P° + M X PX° + M PX° + M X PX°

k k' k

PX° Pf+X°

Rh Ra R R'h Rf

Ρ Ρ Ρ

Rt Ret Rtx

PX° po

x

h

f

x

Addition Cross propag. Fragmentation

Terminations

2P° P° + PX° PX°

kt ket ktx

Cross termin.


368


macroradical reactions and (b) the change o f the relative concentration o f the A F C T A versus the concentration o f the monomer expected i f the chain transfer constant is not equal to one. Applying the steady state conditions to all radical species, four equations are obtained: [P] [X°] [P°] [PX°]

=> =^> => =>

Ri + Rxi = Ra R'i + R'xi = Rf Ri + R'i + Rh+Rx - Ra+Rh + Rt + Rct Rxi + R'xi + Ra = R + R + Ret + Rtx x

(5) (6) (7) (8)

f

Adding equation 7 and 8 and subtracting equation 6 gives: Ri + R i = R ^ R c t + R*

(9)

X

Equation 9 reflects the fact that, in absence o f some degradative processes, the overall rate o f initiation is equal to the overall rate o f termination o f the macroradical species. A s the rate o f initiation (equation 5) is equal to Ra, i.e. the rate o f thermolysis of the initiator moderated by the efficiency factor, it can be assumed that, i f the only parameter modified in these experiments is the concentration o f A F C T A , then the overall rate o f termination remains constant. In particular, the overall rate o f termination is equal to the rate o f termination o f the experiment carried out in absence o f A F C T A , Rto ; such a condition can be written as: R

t 0

=R +2R t

c t

+R

(=R )

t x

(10)

d

Equation 10 may be developed as: 2

k [P-]o = k [ P - ] 4 - 2 k [ p - ] [ P X - ] + k J P X - ] t

t

2

ct

(11)

Introducing α as the ratio o f the two macroradical species ( α = [ΡΧ°]/[Ρ°] ), the above equation may be written as:

[Ρ·]

f

k

[Ρ Ί ο

Κ

Κ

c t

k

t e

2

V

1 / 2

(12)

Κ J

The overall rate o f polymerization is calculated as the sum o f all the reactions giving rise to the increase o f any macroradical species: R =R +R p

h

h

+R +R a

x

(13)

The fact to consider the addition step o f the addition-fragmentation process as a propagation step is not conventional. In the case o f mercaptans, such consideration is


369


not founded because o f the termination o f the chain through hydrogen abstraction. However, in the case o f addition-fragmentation, the addition step provokes the increase of the degree o f conversion value by one. Such influence may be neglected in conventional experimental conditions, i.e. low concentration o f A F C T A , as discussed in the paragraph dealing with the study of the chain transfer constant. Equation 13 can be written as: R

p

= k [ P - ] [ M ] + k J P r ] [ M X ] + k [P-][MX] + k [ P r ] [ M ] h

a

x

4

O )

In absence o f A F C T A , the rate of polymerization becomes: 15

R o=Mn [M] P

( >

0

Dividing equation 14 by equation 15 expresses the relative rate of polymerization:

x h a [MX] 1 + τ ^ α + | ^ α + τ- -, [ M ] k

k

k

(16)

1

Rpo

[P*]

KJ

0

Finally, replacing the ratio [P°]/[P°] in the equation 16 by the value written in equation 12, the equation of the relative rate o f polymerization is obtained: 0

R

[MX]

1+~ α +| τ^α + τ^

R, pO

1+2 ^ α + ^ α k, k

[M]

(17)

2

t

Generally, few values o f the rate constants used in the above equation are known. Five parameters (k ,k' ,k , kc , ktx ) in equation 17 are to be determined. However, it can be remarked that this equation is compatible with both the increase and decrease of the relative rate o f polymerization, depending upon the values o f α and o f the rate constants. x

h

a

t

Interconversion In order to determine a , an attempt to apply the same type o f calculations as in the case o f classical free radical copolymerizations has been made, i.e. the fact that the rate o f interconversion o f the two macroradical species must be equal. The reactions involved in the interconversion o f P ° and P X ° are shown in the Scheme 5. It can be seen that, according to our hypothesis o f the absence o f side reactions involving the radical X ° , the conversion o f P ° into P X ° is achieved by the first step o f the addition-fragmentation process, and the conversion o f P X ° into P ° is achieved by both (a) the cross-propagation and (b) the fragmentation o f the P X °



370

radical. In fact, this latter reaction does not directly provoke the formation o f a P ° radical, but a X ° radical. However, as it is shown in the Scheme 5, the only evolution o f this radical is (a) the re-initiation o f a new polymer chain, i.e. the formation o f a new P ° radical, and (b) the addition onto the A F C T A . This latter reaction (a) gives rise to the formation o f a new X ° radical and/or (b) is slower than the re-initiation because o f the greater concentration of the monomer compared to the concentration of the A F C T A . Thus, it can be assumed that the rate o f fragmentation o f the macroradical P X ° is actually approximately the same as that o f the rate o f reinitiation o f P ° The fragmentation process appears to act as an interconversion process o f P X ° into P°.

P

#

• P X *

P X

#

P

#

cross propagation +M

(R ) x

Scheme 5. Reactions involved in the interconversion process.

Commenting on this conclusion, it should be remarked that the expression "slow rate o f re-initiation" seems to be inappropriate to explain the behavior o f retardation. In fact, the rate o f any chemical reaction results from (a) the value o f the rate constant, (b) the concentrations o f the reagents and (c) the order o f the reaction for each reagents. In the case o f the re-initiation process involving the X ° radical, the rate constant may be low, i.e. the reactivity o f the radical may be low. However, as the rate of formation of this radical species is governed by the fragmentation process, the only direct consequence o f a low value o f the re-initiation constant is the increase o f the concentration o f the X ° radical into the polymerization medium. In the absence o f any



371

side reaction, [X°] increases rapidly until the rate o f re-initiation becomes equal to the rate o f fragmentation, which is generally expected i f the steady state hypothesis is verified. A consequence o f this increase o f [X°] may be the simultaneous increase o f any side reaction involving this radical, such as termination reactions. However, i f the rate constant o f re-initiation is not equal to zero, i.e. the radical X ° is able to react with the monomer, the low value o f the rate constant o f re-initiation does not affect directly the overall rate o f polymerization. A s a consequence o f the above discussion, it can be assumed that: R =R +R a

x

(18)

f

The above equation may be developed as: k [ P ' ] [ M X ] = k [ P r ][M] + k [PX* ] a

x

(19)

f

The parameter α may be calculated as: JçJMX]_ k [M] + k x

(20)

f

Actually, this expression represents both (a) free radical copolymerization i f k [ M ] » k and (b) the addition-fragmentation processes, i f k [ M ] « k . In this latter case, the expression is identical to the equation 3. In intermediate cases, such as the polymerization o f M M A in the presence o f T B T P , a decrease o f the concentration o f the monomer must also decrease the yield o f the copolymerization compared to the yield o f fragmentation. This conclusion has been confirmed. Thus, i f n is equal to 2.3 in bulk conditions, this value becomes equal to 1.2 when the system, i.e. with the same relative concentrations o f the reagents, is polymerized at 20% in toluene(2£). Replacing α in equation 17 gives the final expression o f the relative rate o f x

f

x

f

s

polymerization as: k R

p

RpO

kJMX]

k k [M] + k h

f

x

k k

ct

t

fk f

h

kJMX] k [M] + k x

kJMX]

h

U k [M] + k x

k J f

kVMX] f

kJ

[M] V

k [MX]

V

/

(

2

1

)

2

a

k Ux[M] + k > t

f

The usefulness o f such equation is very limited, due to the large number o f unknown parameters. However, it appears that it is not necessary to invoke degradative behavior o f the chain transfer reaction to explain the strong decrease o f the rate o f polymerization. Thus, as suggested by equation 21, such decrease could be due to high values o f the cross-termination constant between the two macroradicals, kct, and/or o f the homo-termination constant o f the intermediate radical, ktx. Since such high values do not depend upon the fragmentation process, they can be checked easily by the comparison o f the decrease o f the relative rate o f polymerization



372

determined for two systems: (a) monomer A / A F C T A and (b) monomer A / monomer B , since the monomer Β presents a chemical structure analogue to that o f A F C T A , i.e. the same structure o f the addition site. Such a comparison is shown in Figure 1 (see also Table II) for the systems (a) M M A / T B T P and (b) MMA/pentadiene. It can be seen that the decrease o f the relative rate o f polymerization is o f the same order o f magnitude, the slight difference between the two curves being easily explained by the difference of the reactivity o f the pentadiene group in the case o f A F C T A compared to the case of pentadiene. Chain transfer constant The chain transfer constant o f any A F C T A may be calculated in the same way as a conventional chain transfer reaction, i.e. through the determination o f the kinetic chain length (v) o f the polymer5 formed. This kinetic chain length represents the average number o f steps o f growth per effective radical and is given by the ratio o f the propagation rate to the rates o f all processes involving the termination o f the chain. Thus, i f R stands for the rate o f macroradical terminations, ν can be written as: T

v=— ^ — R +R T

ο

- =^ ν R

f

p

+^ R

(22) p

In the absence o f A F C T A , the kinetic chain length becomes:

ν

= ^ °"R

(23)

T

Since the overall rate o f macroradical termination is the same, the variation o f the relative rate of polymerization can be taken into account and written as follows: 1

_

ν

R

1

v

P°

R

0

R +

p

f R

(24) p

Replacing R f and Rp by their values, the above equation can be written as:

1

ν

_

R

1

v

0

PO

R

p

+

Mç ( k + k a ) [ M ] + (k. + k a ) [ M X ] h

x

(25)

h

In order to simplify this equation, it can be assumed that (a) the cross propagation and (b) the homo-propagation o f the intermediate radical may be neglected. In other words, the chain transfer process is a "pure" addition-fragmentation reaction. In this case, the α parameter can be written as:

a =^

3 k.


(26)


373

Table Π. Bulk Polymerization* of M M A in the Presence of Additives M X Additive MX Concentration of MX Relative Rate of 4

(mol/L xlO )

TBTP

Pentadiene

t-BuSH

Polymerization*

0.0 0.5 2.5 5.0 10 25

1.00 0.97 0.95 0.83 0.74 0.59

0.0 0.5 2.5 5.0 10 25

1.00 0.99 0.97 0.95 0.91 0.72

0.0 0.6 2.8 5.6 11 28

1.00 0.98 1.03 1.01 1.00 1.03

experimental Conditions: [ A I B N ] = 3.05 χ 10 mol/L, temperature 60°C, polymerization time 1 hour. kRatio o f rates o f polymerization with and without A F C T A , determined from S E C data. 3


374

Therefore, the kinetic chain length becomes: 1 _ 1 R ν

v

R

0

k

p 0

(

2

7

)

[ M X ] [M]

p

*


[MX]

a

h

K

[M]

Thus, the conventional chain transfer constant, as determined by Mayo's equation, can be written as: r

^

(28a) [M]

In the above equation 28, the expression k .

comes from the fact that the [M] addition has been considered as a propagation step. This equation may be written as : 1

C*

_k k

h

a

+

[MX]

(

2

8

b

)

[M]

Thus, the dependency o f C * to the ratio [ M X ] / [ M ] remains low i f [ M X ] / [ M ] « kh/ka., that is generally the case for experiments carried out in the presence o f low concentrations o f the transfer agent. However, the chain transfer constant is expected to decrease with the increase o f the concentration o f the A F C T A and the decrease o f the concentration o f the monomer. Furthermore, the calculation is much more complicated i f the cross-propagation interferes with the fragmentation process. However, in all cases, a constant may be found by the extrapolation o f the Ctr value to a null concentration o f A F C T A : C^o = ka/kh- For systems exhibiting low values o f Cuo, this extrapolated constant may be used in order to calculate the kinetic chain length. Functionality B y using the chain transfer constant Cbo, the functionality (f) o f the formed polymer may be derived as the ratio o f the number o f functionalized chains versus the overall number o f polymer chains formed during the lifetime o f an effective radical. In order to calculate this number, the decrease o f the kinetic chain length due to the decrease o f the relative rate o f polymerization must be considered. Therefore, the functionality can be written as:


375

B y replacing ν by its value, it becomes: R P

R f =

Γ

P 0

R„ 1+—-C R„

tt0

P


[MX]

v

»°

0

[M]

(30)

[MX] ν 0

[M]

From this last equation, a decrease o f the functionality can be expected with (a) a decrease o f the relative rate o f polymerisation, (b) a decrease o f the chain transfer constant, (c) a decrease o f the kinetic chain length o f the homopolymer and (d) a decrease o f the ratio [AFCTA)/[monomer]. In order to check the dependency o f the functionality to the relative rate o f polymerization, both Cuo and v values have been taken from the study o f the system M M A / T B T P (2), i.e. = 2.4, v = 4000. F o r each value o f the ratio [ T B T P ] / [ M M A ] , the experimental decrease o f the relative rate o f polymerization has been measured. A s shown in Figure 2, with 1% o f T B T P , the relative rate o f polymerization is roughly 0.3, whereas the functionality o f the formed polymer remains greater than 0.95. 0

0

Conclusion The main results o f this study are: (a) The decrease o f the relative rate o f polymerization observed in many chain transfer reaction involving additionfragmentation processes can be explained by the termination reaction involving the

-η

•

ι

ι

ι

ι

Γ 1.0

1.0

0.8

0.8

Η

Ο ••a o.6 β ο

Η

0.4

Η

0.6 "3,

0.4

J

0.2 0.2

ο.ο

Η

Η

-ι 8

•

1

10

1

1

12

ο.ο

R

14

[ Τ Β Τ Ρ ] / [ Μ Μ Α ] χ 103

Figure 2. Theoretical functionality (equation 30) o f polymer formed. Bulk polymerization o f M M A in the presence o f T B T P as chain transfer agent.



376 macroradical species. The occurrence of some degradative side reactions, like primary radical termination, is not necessary to observe a strong decrease of the rate of polymerization, even if such degradative side reactions cannot be a priori excluded, (b) For such systems, the chain transfer constant shows some dependency upon the concentration of the AFCTA that can be neglected in conventional experimental conditions, (c) The influence of the decrease of the relative rate of polymerization on the functionality may be neglected, since the chain transfer activity of the AFCTA and its concentration remain sufficiently high. Acknowledgments The authors thank the referees for suggestions and remarks. This work was kindly supported by Rhône-Poulenc Chimie. Literature Cited 1. Colombani, D.; Chaumont, P. Prog. Polym. Sci. 1996, 21, 439. 2. (a) Nair, C. P. R.; Chaumont, P.; Charmot, D. J. Polym Sci, Polym. Chem., 1995, 33, 2773. (b) Chaumont, P.; Colombani, D.; Zink, M.O.; Charmot, D. (to be published). 3. Odian, G. Principles of Polymerization, 2nd Edition, Wiley : 1981, p.227. 4. Nair, C.P.R.; Chaumont, P.; Clouet G. Macromolecular Design, Concept and Practice, Mishra, N.M. Ed.; Polymer Frontier International : 1994, p.431. 5. Rempp, P.; Merrill E.W. Polymer Synthesis, Huthig & Wepf: 1986, p.79.


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