Living Radical Polymerization - American

Chapter 29

Downloaded by OHIO STATE UNIV LIBRARIES on May 11, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch029

Influence of Solvent and Polymer Chain Length on the Hemolysis of SG1-Based Alkoxyamines 1,2

1,3

1

Olivier Guerret , Jean-Luc Couturier , Florence Chauvin , Hafid El-Bouazzy , Denis Bertin , Didier Gigmes , Sylvain Marque , Hanns Fischer , and Paul Tordo 1

1,*

1

1

1,4

1

1

U M R 6517 case 521, Université de Provence, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France ATOFINA, Groupement de Recherche de Lacq, 64170 Lacq, France ATOFINA, Centre de Recherche de Rhônes Alpes, rue Henri Moissan, 69493 Pierre Bénite Cedex, France 4Institute of Physical Chemistry, University of Zuerich, Winterthurerstrasse 190, C H 8059 Zuerich, Switzerland 2

3

In nitroxide mediated radical polymerizations (NMP) the polymerization times and the polymer polydispersities decrease with the increasing homolysis rate of the C - O N bond between the polymer chain and the nitroxide end group. Therefore, the factors influencing the rate constants k of alkoxyamine cleavage are of considerable interest. Here, we describe the influence of the medium polarity and viscosity on k for the alkoxyamine 2-[N-tertiobutyl-N-(1-diethoxy-phosphoryl-2,2-dimethylpropyl) aminoxy] methyl propionate, and the effect of the polymer chain length on k for polystyryl-SG1 and poly(n-butyl acrylate)-SG1 macro-alkoxyamines. In contrast to other alkoxyamines, k o f 2-[N-tertiobutyl-N-(1-diethoxyphosphoryl-2,2-dimethylpropyl) aminoxy] methyl propionate does not depend on the medium polarity. This points to the absence of polar contribution to the transition state of homolysis. The solvent viscosity does not influence the rate constant k , which means that cage effects are unimportant. Finally, the influence of the chain length depends on the type of polymer, it is very weak for polystyryl-SG1 but significant for poly(n-butyl acrylate)-SG1. d

d

d

d

d

412

© 2003 American Chemical Society

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

413

Introduction A decade ago, Rizzardof/J and Georges(2J first prepared well-defined polymers using nitroxyl radicals as controlling species. Nitroxide Mediated Polymerization (NMP) emerged/5) and numerous studies were undertaken to elucidate the mechanism^) and the polymerization kinetics/J) These studies led to new polymersfi,^ and to more efficient initiators/controllers^. Scheme 1 displays the N M P mechanism, with A the rate constant for the homolysis of the C - O N bond of the alkoxyamine (the so-called dormant species), k the rate constant for its reformation, and Ap and At the rate constants for the propagation and the termination of the polymerization, respectively.


d

c

Scheme 1

R

R

N ^N-OR

R

_ J d

^

3 3

k

^

2

c

i \

Jt\

Β

% ί - Ο + R*

Rf

"PJM

ι

dormant species

termination products In principle, successful living and controlled polymerizations require that the rate constants k and k of the initiating and polymeric alkoxyamines ( R i R N O R ) fall into specific and rather narrow ranges which depend on the monomer through Ap and At, the initial concentrations and the experimental conditions such as the temperature. In particular, a large ratio A /A often allows to reach high conversion in short time. However, in order to achieve a living and controlled polymerization, A /A must not exceed an upper limiting value. Furthermore, a low polydispersity requires that 1/A must be very small compared to the polymerization time.(5) Since A and A depend on the system, the choice of the nitroxide is crucial to obtain living and controlled polymerizations of a given monomer. In particular, Ad varies by many orders of magnitude with the substitution of the nitroxide moiety (Ri, R ) and with the alkyl group R of the dormant alkoxyamine. In earlier work, we(8a 9) and others(10,ll) have presented rate constants A and Arrhenius parameters for the cleavage of various alkoxyamines in order to find predictive rationalizations. The rate constants span eight orders of magnitude from 10" s" to 0.1 s" at 120 °C.(8a) This is mainly due to variations of the activation energy which is close to the bond dissociation energy, BDE, of the C - O N bond(7a,8). This, in turn, depends on the stabilization of the leaving a

2

c

3

d

d

c

c

d

d

c

2

3

t

9

1

1


d


414

alkyl radical/7aA#a,P,/ la-d,g) that is, more stabilized alkyl radicals yield a faster homolysis, and of the nitroxide moiety(llg) which, for instance, can be stabilized by intramolecular(7/g) and intermolecular(72) hydrogen bonding. Polar ground state effectsf9,//g) and steric hmdrmce(7a,h,8a,9,l la,f,g) in both fragments also affect the BDE, and, hence, the activation energies and rate constants. Although the effects of the alkoxyamine structure on the C - O N bond homolysis are now rather well established, data related to the effects of solvent polarity, solvent viscosity and chain length are scarce. Several years ago Rizzardo and Moad(7a) showed that k of alkoxyamine 1 (Scheme 2) increased by a factor of roughly two with increasing solvent polarity going from hexane (μ = 0.08 D , ε = 1.89) to methanol (μ = 2.87 D , ε = 32.66)(13). This means that the transition state (TS) for the homolysis is more polar than the alkoxyamine ground state. Marque et al.(llg) observed the same effect for alkoxyamines 2 and 3 which carry the phenethyl group, and Ananchenko et û.(10b) confirmed this for 2. On the other hand, Ananchenko et al.fi0c) using ί-BuPh (μ = 0.36 D , ε = 231)(13) and PhCl (μ = 1.62 D , ε = 5.62)(13) as solvents, did not observe a solvent polarity effect for the alkoxyamines 4 and 5 (Schemes 2 and 3). Effects of chain length of the leaving groups have mainly been studied for polystyryl-TEMPO (PS-TEMPO). Thus, Bon et Λ.(14) showed that P S - T E M P O ( M i = 7600 g-mol" ) cleaves 9 times faster than the low molecular weight Anal.og 2, while Fukuda et &l.(15) found only a factor of 2 between P S - T E M P O (M = 1700 g-mol" ) and 2. Fukuda et û.(16) reported a two times larger k for polystyryl-SGl 6 (Scheme 3, M = 1960 g-mol" ) than for the Anal.og 4, and a slightly higher value for 6 ( M = 3300 g-mol" ) was also observed by Benoit et a.l.(17) We(18) recently reported that in a series of styryl based alkoxyamines 6 with 1400 < M < 51300 g-mol* chain length effects are absent. For the acrylate based alkoxyamines 7 (Scheme 3), Ananchenko et al.(lOc) and Benoit et ύ.(17) observed rates of homolysis that were by factors between 2 and 6 (M = 4500 g-mol" ), respectively, larger as compared to the Anal.og 5. To the best of our knowledge, there are no studies on the influence of the solvent viscosity on k and the reported effects of the solvent polarity and of the polymer chain length on k are in part controversial. This prompted this study on the influence of these factors using alkoxyamines 4-8 (Schemes 2 and 3). a

1

n

1

d

1

n

1

n

1

n

n

1

d9

d

Experimental Section

A l l chemicals and solvents (/-butylbenzene: f-BuPh, chlorobenzene: PhCl, cw-decaline and methyl caproate) were purchased from Aldrich and used as received. Alkoxyamine 5 was provided by Atofina as a mixture of two diastereoisomers and was used as received. Alkoxyamine 8 was prepared following known procedures/79) N M R experiments were performed with CDC1 3



415


416 l

1 3

as solvent with a 300 M H z Bruker spectrometer ( H 300MHz, C 75.48 M H z and P 121.59 M H z ) in the Centre Regional de R M N at Marseilles. 3 1

Preparation of the styryl and n-butyl acrylate based polyalkoxyamines 6 and 7. A typical procedure is as follows: A solution of monomer (100 g), alkoxyamine 5 (0.379 g, for w-butyl acrylate) and free S G I (7.6 mg) was degassed by three freeze-pump thaw cycles. Bulk polymerizations were then carried out in a round-bottom flask under nitrogen atmosphere at 115°C. At regular intervals samples were withdrawn and immediately cooled in an ice bath. Conversion was determined by H N M R . M„ and PDI were determined by SEC chromatography. For kinetic measurements samples were purified by precipitation for polystyrene and stripping for poly(w-butyl acrylate) based macro-alkoxyamines.


l

Kinetic measurements. Measurements of k were carried out as descibed previously using C W - E S R Brvksr(8a,18,20) and 300 M H z N M R Bmker(21) spectrometers. Viscosities were measured with a capillary viscometer. d

Results and Discussion

Influence of the solvent polarity. A s already mentioned, Ananchenko et al. did not observe an influence of the solvent polarity (ί-BuPh vs PhCl) on the homolysis rate constant for the SGI based alkoxyamines 4 and 5. However, v/e(2J) recently observed that k for 4 increased by a factor of 1.5 going from i BuPh to the mixture f-BuPh/PhSH (1:1; PhSH : μ = 1.23 D and ε - 4.38)(75j. Here, we determined k for 5 in solvents with different polarities. The results are collected in Table 1 and show that k of 5 does not depend on the solvent polarity. This also holds for the solvent methyl caproate which mimics the polarity of w-butyl acrylate or methyl methacrylate monomers. This result agrees with that of Ananchenko et dX.(lOc) The increase of k with increasing solvent polarity found for 4 could result from the contribution of a polar structure d

d

d

d

[(SGI) (MeCHPh)*] to the transition state of the homolysis. In the case of 5, the +

+

contributions of [(SGl)"(MeCHCOOMe) ] or [(SGl) (MeCHCOOMe)"] are very unlikely. This explanation should be checked with other low molecular weight and polymeric alkoxyamines with polar and unpolar residues.



417


418 Table 1. Values of homolysis rate constants kd of both diastereoisomers of 5 in solvents of various polarity and viscosity. E (kJ-mor'f a

Entry

Solvent cis-decaline

1

t-BuPh


2

M(Df 1.08 0.5

e/

3

kMo-cOOr *')"

RR/SS RS/SR RR/SS RSA

0.0

131.8

127.8

0.7

2.3

0.36

131.3

127.5

1.0

3.0

1.0

3.2

3

PhCl

0.54*

1.62

130.6

127.0

4

caproate

0.4

1.74

130.8

127.5

1.0

2.7

5

ί-BuPh + PS

128.4"·*

-

131.3

127.0

0.8

2.6

a

1

i4

1

E ± 2 IcJ-mol" . E was estimated using a mean frequency factor A = 2.4· 10 s" , see references (8a) and (llg). Statistical error less than 10%. Each value is at least the average of two measurements. The wholeSci.of rate constants was measured by P NMR. See reference (22) unless otherwise mentioned. Values given at 90°C. See reference 13. 'Measured at 90 °C using a capillary viscosimetery η ^ ρ π ) 0.56 cp at 50°C from reference (22). Τ = 55°C, See reference (23) 1.5g of polystyrene (Mn = 182000 g-mol" , Mw = 340000 g mol* , PDI = 1.86) was solved in gauged flask filled up to 10 mL. a

a

b

3I

c

d

=

8

1

h

1

Influence of solvent viscosity. As it is well known, the rate constants of self-termination and propagation in radical polymerizations strongly depend on the solvent viscosity.f2

Living Radical Polymerization - American

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