Radical Polymerization

Chapter 8

Use of Phosphonylated Nitroxides and Alkoxyamines in Controlled/"Living" Radical Polymerization 1

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C. Le Mercier , J.-F. Lutz , S. Marque , F. Le Moigne , P. Tordo , P. Lacroix-Desmazes , B. Boutevin , J.-L. Couturier , O. Guerret , R. Martschke , J. Sobek , and H. Fischer 2

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Downloaded by CORNELL UNIV on June 1, 2012 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch008

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Laboratoire de Structure et Reáctivité des Espèces Paramagnétiques, UMR 6517, CNRS et Universités d'Aix-Marseille 1 et 3, Av. Esc. Normandie Niemen, 13397 Marseille Cedex 20, France UMR-CNRS 5076, ENSCM, 8 rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France Physikalisch-Chemisches Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland Elf-Atochem, CRRA, Rue Moissan, B.P. 63, 69310 Pierre-Bénite, France 2

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A new series of stable β-phosphonylated nitroxides bearing a βhydrogen and different corresponding alkoxyamines were prepared in good yields using commercially available chemicals. The N-tert -butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide 3 (SG1), and the N-tert-butyl-N-(1-diethylphosphono-2,2dimethylpropyl)-N-(1-phenylethoxy) amine 10, were used in controlled / "living" polymerization of styrene. The use of either the bicomponent system (SG1 / A I B N ) or the monocomponent system (10) resulted in reasonably fast and well controlled polymerizations. The equilibrium constant Κ for the reversible homolysis of 10 was shown to be much larger (450 times) than for its T E M P O analog 16. This difference accounts for the fast kinetic and the negligible kinetic contribution of the thermal self initiation during the polymerization of styrene in the presence of 10. The X-ray structures of 10 and 16 were determined and compared. Corresponding author (e-mail address: [email protected])

© 2000 American Chemical So ciety

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

109 Introduction 1

A lot of studies have followed the first report of Otsu et al on living radical polymerization and nowadays living / controlled radical polymerization is a major topics in free radical polymerization research. Several procedures have been detailed such as stable free radical polymerization (SFRP) using stable nitroxyl radicals , atom transfer radical polymerization (ATRP) using transition metal complexes and radical addition fragmentation chain transfer (RAFT) using dithioester derivatives. In the case of a free radical polymerization mediated by a stable nitroxide, the control relies on the reversible trapping of the growing polymer radical by a stable nitroxide to form the corresponding N-alkoxyamine (Scheme 1). The living / controlled character was shown to be closely related to a general phenomenon which appears in reactions where transient and persistent radicals are formed simultaneously, the Persistent Radical Effect. The control and the rate of the polymerization depend on the value of Κ (Κ = 1^ / k j and particularly on the value of the dissociation constant kj. Hence, the bond dissociation energy (BDE) of the N O - C bond appears as a key parameter in these nitroxide controlled free radical polymerizations. 2

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4

5

RI

ι

X

JL+

N—O—C—P R2 J

*K

~* kc

y

dormant species

I

.

Ν—Ο

+

'C—Ρ J

R2'

K = kd/kc

polymer radical

Scheme 1 Among the few commercially available nitroxides, 2,2,6,6-tetramethyl piperidinoxyl (TEMPO) was shown to be the most convenient for SFRP and it has been widely used. However, the use of T E M P O suffers from two main limitations : (i) the rates of polymerization in the presence of T E M P O are dramatically decreased ; (ii) the use of T E M P O is almost limited to styrenic monomers. In order to overcome these limitations, research has focused on the search of additives which could change the equilibrium constant Κ and on the design of new effective stable nitroxides. A s shown by Moad and Rizzardo both the steric size and the electronic effects of the R and R groups of the nitroxide moiety influence the B D E of the N O - C bond in the corresponding alkoxyamines. Puts and Sogah reported that 2,5-dimethyl-2,5diarylpyrrolidin-l-oxyl provides a significantly faster polymerization reaction, compared to the T E M P O system. Georges et al observed that the polymerization of styrene in the presence of di-te^butylnitroxide is faster than the TEMPO-mediated polymerization reaction. More recently, it has been shown that morpholone and piperazinone based nitroxides and related alkoxyamines also allowed faster polymerizations than T E M P O . The increase of the bulkiness of R or (and) R results on the weakening of the B D E of the N O - C bond. However, a too large steric repulsion between R and R could result in the rapid decomposition of the nitroxide. The preferred conformation adopted by sterically crowded aliphatic tertbutyl alkyl nitroxides is shown in scheme 2 (S = small, L = large). 6

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10

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110 Usually, nitroxides with hydrogen atoms attached to an a_o ™ Ν—Ο Ν — ο carbon are not stable and cannot be isolated. However, Volodarsky reported that nitroxides bearing one hydrogen atom attached to an 1 (stable) 2 (persistent) α-carbon (referred as β-hydrogen Scheme 2 according to the nomenclature of ESR couplings), such as 1 (S = H , scheme 2), can be stable on condition that the hydrogen is locked close to the nodal plane of the nitroxyl fonction. However, even for this kind of nitroxides the increase of the steric strain is limited and i f the bulkiness of the L groups is too large the nitroxides undergo unimolecular decay and their half-life can be very short (nitroxide 2, Scheme 2). In preliminary communications we reported that nitroxide 1 was more efficient than T E M P O and di-tert-butyl nitroxide (DTBN) for the SFRP of styrene. Recently, these preliminary results have been confirmed. Moreover, 1 was shown to be also efficient in the control of the free radical polymerization of various acrylic monomers. In the course of our program on the search of stable nitroxides, we have found a new series of stable β-phosphonylated nitroxides bearing a β-hydrogen (nitroxides 3 - 9, figure 1) which are able to efficiently control the free radical polymerization of different monomers. ' This paper describes briefly the synthesis of these nitroxides and their corresponding alkoxyamines. Then we compare the kinetic parameters of the reversible homolysis of model alkoxyamines with those of the T E M P O analogs, and finally we report on the use of these compounds to control the free radical polymerization of styrene. N


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R

4

P(0)(OR6)

R3—NI .0

R5

2

3 (SGI): 4: 5: 6: 7: 8: 9:

Figure 1 : Stable β-Phosphonylated

R R R R R R R

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= R = i - B u , R = H , R = Et = R = r-Bu,R = H , R = C H P h = i - B u , R = /-Pr,R = H , R = Et = i-Bu, R = çy-Hex, R = H , R = Et = P h C H ( M e ) , R , R = ( C H ) , R = Et = / - P r , R , R = ( C H ) , R = Et = c y - H e x , R , R = ( C H ) , R = Et 2

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Nitroxides 3-9.

Experimental section Reagents for polymerizations. Dicumyl peroxide, D C P , (98%, Aldrich), benzoic anhydride (98%, Lancaster) were used as received. A I B N (98%, Fluka) was recrystallized in ethanol and styrene (Aldrich) was distilled over CaH . 2


Ill Polymerizations. Typically, a mixture of alkoxyamine and styrene in a Schlenk flask was thoroughly purged with argon. Then, bulk polymerization was conducted at 123°C and samples were withdrawn under positive argon purge and analyzed by Size Exclusion Chromatography (SEC) and H N M R . Analyses. Molecular weights and polydispersities were determined by SEC calibrated with polystyrene standards. Monomer conversion was determined by *H N M R analysis on crude samples.


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General procedure for the preparation of a-aminophosphonates 3' - 9 \ A mixture of the amine (50 mmol) and the aldehyde or ketone (50 mmol) was stirred at 40°C for 1 h under nitrogen atmosphere. After addition of diethyl phosphite (75 mmol) at room temperature, the solution was stirred at 40°C for 24 h. The reaction mixture was then diluted with diethyl ether (100 ml) and washed with 5 % aqueous HC1 until p H 3. The aqueous phase was extracted with diethyl ether. Sodium hydrogencarbonate was added to the aqueous phase until p H 8, and the aqueous phase was then extracted with diethyl ether (2 χ 30 mL). The combined organic extracts were dried over anhydrous N a S 0 . Removal of the solvent under reduced pressure afforded the a-aminophosphonates. 2

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General procedure for the oxidation of a-aminophosphonates 3% 4% 7% 8' and Ψ into the β-phosphonylated nitroxides 3, 4, 7, 8 and 9. A solution of mchloroperbenzoic acid (8 mmol) in dichloromethane (20mL) was added at 0°C to a solution of the a-aminophosphonate (8 mmol) in dichloromethane (10 mL). The mixture was stirred for 6 h at room temperature. Then a saturated aqueous solution of sodium hydrogencarbonate was added until neutral p H . The organic phase was successively washed with water, 1 M aqueous sulfuric acid, water, saturated aqueous sodium hydrogencarbonate and water. After drying, over anhydrous N a S 0 , removal of the solvent under reduced pressure afforded an oil which was purified by silica gel chromatography (pentane / ethyl acetate). 2

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General procedure for the oxidation of a-aminophosphonates 5% 6' into the βphosphonylated nitroxides 5 and 6. Oxone (40 mmol) was added at room temperature to a solution of the a-aminophosphonate (10 mmol), sodium carbonate (60 mmol) in water (20 mL) and ethanol (70 mL). The mixture was stirred for 24 h at room temperature. The reaction mixture was then filtered and the solvent was evaporated under reduced pressure. The residue was extracted with dichloromethane and the organic phase was dried over anhydrous N a S 0 . Removal of the solvent under reduced pressure afforded an oil which was purified by silica gel chromatography (pentane / ethyl acetate). General procedure for the synthesis of alkoxyamines 10 - 15. Under inert atmosphere, a solution of the nitroxide (5 mmol) and the alkyl bromide (10 mmol) in benzene (8 mL) was transferred to a mixture of CuBr (10 mmol), bipyridine (20 mmol) and Cu(0) (5 mmol i f necessary, see Table I) in benzene (8 mL). After 2 days stirring at room temperature, the mixture was filtered and washed with a 5 % w/v aqueous solution of C u S 0 . The organic phase was dried over anhydrous N a S 0 and removal of the solvent under reduced pressure afforded an oil which was purified by silica gel chromatography (pentane / diethyl ether). 2

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112 Results and discussion Synthesis of the nitroxides 3 - 9 and alkoxyamines 10 - 15 Synthesis of the nitroxides 3 - 9 . The β-phosphonylated nitroxides 3 - 9 were prepared by oxidation of the corresponding a-aminophosphonates 3' - 9 ' (Figure 2). The compounds 3 ' - 9' were obtained in reasonabte to good yields (30 - 90 %) through two major approaches : a - the one pot reaction of equimolar amounts of carbonyl compound and amine with a slight excess of dialkylphosphite (3% V) ; b the addition of a dialkylphosphite to an imine, either generated in situ (4', 5% 6% 8'), or isolated (9'). Oxidation of the a-aminophosphonates 3', 4% 7% 8' and 9' with mchloroperbenzoic acid (m-CPBA) in dichloromethane gave after purification the corresponding nitroxides S G I , 4, 7, 8 and 9 in 48, 25, 37, 41 and 46 % yields respectively. The improvement of the oxidation of 3' was investigated and allowed the preparation of S G I on a large scale in 80 % yield with a purity superior to 90 % and without any purification. Oxidation of the a-aminophosphonates 5' and 6 with m-CPBA gave very poor yields and the nitroxides 5 and 6 were isolated in respectively 26 and 18 % yields through oxidation of 5' and 6' by Oxone (2 K H S O K H S 0 - K S 0 ) in a mixture of ethanol, water and N a C 0 . Only the nitroxide 4 was obtained as a solid.


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R4

R4

P(0)(OR6)

R3- - N -

I

[O]

I

R R R R R R R

Τ 8' 9'

R5

.0

Η

3' 4' 5' 6'

-P(0)(OR%

R3- - N -

2

R5 3

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3 (SGI) 4 5 6 7 8 9

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= R = i - B u , R = H , R = Et = R = *-Bu, R = H , R = C H P h = ί-Bu, R = i-Pr, R = H , R = Et = ί-Bu, R = cy-Uox, R = H , R = Et = PhCH(Me), R , R = (CH ) , R = Et = /-Pr, R , R = (CH ) , R = Et = çy-Hex, R , R = (CH ) , R = Et 6

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Figure 2 : Oxidation of the a-Aminophosphonates 3 - 9 to the Nitroxides 3-9.

P(0)(OR6)

2

d

β-Phosphonylated

A l l these nitroxides contain a β-hydrogen and are stable compounds which can be isolated and stored either neat or in solution. Their Electron Spin Resonance (ESR) study in solution showed that the a p coupling was either very small (a < 0.01 mT) or unresolved (for S G I , 4, 8 and 9). This observation allowed us to assume for these compounds the existence at ambient temperature of a largely predominant conformer in which the 18

H

HP

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Figure 3 ; Predominant conformation of SGI, 4, 8 and

9.


113 Hp atom is eclipsed by a bulky alkyl group and lies close to the nodal plane of the nitroxyl function (Θ « 90 °, Figure 3). In this preferred conformation, the Hp atom is sterically masked, thus impeding the decay of the nitroxide through disproportionation . The influence of the steric hindrance due to the two tert-butyl groups of the nitroxide 4 was also reflected in its X-ray structure . Η

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Synthesis of the alkoxyamines 10 - 15. The synthesis of alkoxyamines through the trapping of free radicals with nitroxides has been largely described. Matyjaszewski et al. developed a versatile and efficient method based on the A T R P system for synthesizing several alkoxyamines derived from T E M P O . Hawker et al. coupled T E M P O and its derivatives with 1-phenyl-ethyl radical generated through addition of styrene to the Jacobsen's Manganese (III) catalyst. Low temperature multi-step syntheses of a variety of alkoxyamine initiators were also reported by Braslau et al? In order to evaluate their initiator efficiency in the polymerization of styrene and acrylates, and to measure the Bond Dissociation Energy (BDE) of their N O - C bond, the alkoxyamines 10 - 15 derived from the nitroxides S G I , 8, 9 and 1 were synthesized (Figure 4). They were all prepared following the method derived from A T R P and first reported by Matyjaszewski. In this method, the radicals R* are generated via copper (I) reduction of the corresponding organic halides R X . Owing to the facile cleavage of most of the targeted alkoxyamines, their synthesis was carried out at room temperature and excess of copper (I) (2 eq.) and alkyl halides (2 eq.) relative to nitroxides was necessary in order to obtain the complete conversion of nitroxides. In our experimental conditions, we found that the addition of 1 equivalent of copper (0) slightly improved the yield and the reaction time (Table I). Even though they were not optimized, good yields of alkoxyamines were obtained. The compound 12 was characterized only in solution and could not be isolated since the cleavage of its N O - C bond occurs below room temperature. 21


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P(G)(OEt)

tBu—NR7-

A

Ph

2

tBu -P(0)(OEt)

2

-iPr

R3-

H

Ο H

-Η

Ph-

-R9

Me

Me

R8

10: R = P h , R = M e , R = H 11; R = P h , R = H , R = H 12; R = P h , R = M e , R = M e 7

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9

-H

Ph-

3

13 : R = i-Pr 1 4 : R = c)/-Hex

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Figure 4 : Alkoxyamines 10 -15. In order to roughly appreciate the thermal stability of the prepared alkoxyamines, we measured by E S R their cleavage temperature ie the temperature at which a significant signal of the nitroxide moiety is observed (Table I). It is interesting to point out that for the alkoxyamines 10 - 12 derived from the nitroxide S G I , the cleavage temperature was at least 35 °C lower than for the T E M P O analogs (data not


114 reported). Moreover, in the series 11, 10, 12, there is a significant decrease o f the cleavage temperature. This trend can be explained by an increase of the steric strain in the alkoxyamine and by an increase of the stability of the released radical.


Table I. Synthesis and cleavage temperature of the alkoxyamines 10 -15. Alkoxyamine

Amount of Cu(0) (eq.)

Yield (%)

Cleavage Temperature (°C)

10 11 12

0 0 0 1 1 1

95 40 Not isolated 79 88 90

60 90

Radical Polymerization

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