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Anomalous chain transfer behaviour of β-aminomercaptans in the radical polymerizations of acrylates
Eur. Polym. J. VoL 26, No. 7, pp. 811-815, 1990 Printed in Great Britain
0014-3057/90 $3.00 + 0.00 Pergamon Press plc
ANOMALOUS CHAIN TRANSFER BEHAVIOUR OF fl-AMINOMERCAPTANS IN THE RADICAL POLYMERIZATIONS OF ACRYLATES C. P. REGHUNADHANNAIR, M. C. RICHOU,P. CHAUMONTand G. CLOUET* Institut Charles Sadron (CRM-EAHP), 6, rue Boussingault, 67083 Strasbourg C6dex, France
(Received 5 September 1989) Abstract--In contrast to thiols in general, fl-aminomercaptans were found to have low chain transfer constants in the free radical polymerizations of acrylates. Transformation of the amines to their salts, or other modifications masking the basicity of the amines, produced dramatic improvements in their chain transfer capability. The chain transfer constants of typical primary, secondary and tertiary fl-aminomercaptans and their hydrochlorides were determined at 60 ° for MMA and in some cases also for styrene and ethyl acrylate. In the case of styrene, normal chain transfer behaviour was observed for the amines and their salts, suggesting that the retardation accompanying transfer is a special feature of the electron-deficient acrylate radicals. The observed trends in relative reactivities of the amines and their salts and complementary studies with other aminomercaptans led to the postulate that the decreased reactivity of the aminomercaptans towards the acrylate radicals may be due to perturbations from specific interaction between the non-bonding electrons on the amine and the electron-deficient attacking radical, through a six membered transition state which is sensitive to the steric crowding on the N-atom.
INTRODUCTION
EXPERIMENTAL PROCEDURES
Use of functional chain transfer agents in radical polymerization is an effective means for chain-endfunctionalization of vinyl polymers. The functionality of the resultant polymers are dependent on the chain transfer constants [1]. Generally, a very high chain transfer constant is desirable but the preferred value is unity, in which case the polymer chains have uniform length at comparatively high conversions. Mercaptans generally have excellent chain transfer properties. Amino-terminated vinly polymers are desirable for a variety of polymer modifications through reaction and modification of the chain-end groups. Their syntheses warrant utilization of aminomercaptans as chain transfer agents. All commercially available aliphatic aminomercaptans are fl-aminomercaptans probably because of ease of synthesis. A few published data cite utilization of 2-mecapto-ethylamine hydrochloride as a good chain transfer agent [2-4]. The salt was used because the amine was available quite pure in that form. We had special interest in amine-terminated vinyl polymers for use in the synthesis of block copolymers via the thermal "iniferter" technique. During our investigation, it was observed that the fl-aminomercaptans had very poor chain transfer capability and that they required to be transformed to their salts to have any meaningful transfer property in the case of alkyl acrylates. This finding prompted a detailed study of the chain transfer behaviour of various aminomercaptans with a view to elucidate the mechanism of chain transfer and retardation for these chain transfer agents.
Materials 2-Aminoethane thiol (AET) and its hydrochloride, 2-(butylamino) ethane thiol (BAET), 2-(diethylamino) ethane thiol (DAET) hydrochloride, piperazine, ethylene sulphide and 2- and 4-mercaptopyridines (Aldrich) were used as received. Methyl methacrylate (MMA), styrene and ethyl acrylate were purified by distillation under reduced pressure from finely powdered Call 2. AIBN was recrystallized from methanol. BAET hydrochloride was prepared from the amine by treating an ethereal solution with excess conc. HCI. The aqueous layer was treated with chloroform, dried thoroughly with Na2SO4 and the solvent evaporated to give a white fine powder which was dried in vacuum. DAET was liberated from its hydrochloride, by treating its aqueous solution with a slight excess of NaOH. The liberated amine was extracted with ether and dried; the ether was evaporated and the residue vacuum distilled. Purity was determined by titration of SH groups. Instruments ~H-NMR spectra were recorded on a Brucker 60 MHz spectrometer. Molecular weights were found by GPC using THF as eluent, with a refractive index detector. Synthesis of 2-[N-(l-butyl) N-trimethyl silyll aminoethane thiol (BSAET) To 0.05 mol of BAET in 50ml dry ether containing 0.06tool triethylamine was added dropwise 0.05mol trimethyl chlorosilane under argon at ca 10°. After the addition, the system was reacted for 2 hr. The amine hydrochloride was filtered. After recovering the ether from the residue, it was vacuum distilled. The fraction boiling at 73°/5mm was collected and characterized by ~H-NMR; purity determined from the SH content.
*To whom all correspondence should be addressed. 811
C.P. REGHUNADHANNAIR et
812
~H-NMR (CC14): 6 ppm--0.22 (s, 3H, Si---CH3), 0.77 (t, 3H, ---CH3), 1.26 (m, 4H, N---C---CH2--CH2--), 1.62 (s, IH, --SH), 4.3-4.7 (m, broad, 4H, ---CH2---CH2--S) SH%: Calc. 16.1; Found 14.5.
al.
thiols). The transfer constants (Ct~) were determined by using Mayo's equation (1) for low thiol concentration where retardation by the thiol can be neglected.
Synthesis of N-(2-mercapto ethyl) piperazine (MEP) Piperazine (0.05 mol) was reacted overnight with 0.01 mol of ethylene sulphide in dry T H F under reflux. The T H F was evaporated and the residue vacuum distilled, collecting the fraction boiling at 73°/5 mm Hg. Yield: 45%.
C6HI4N2S Calc. Found SH% Calc. Amine*°/0 Calc.
C 49.30 48.50 22.60 19.20
H 9.60 9.60 Found Found
N 19.20 19.20 22.52 18.00
S 21.92 20.82
Its dihydrochloride was prepared by treating an alcoholic solution with HC1 and then precipitating in ether.
Polymerization The polymerizations were carried out in sealed evacuated glass tubes containing the monomer, initiator, solvent and chain transfer agent in a bath at 60 °. Ethanol was used as the solvent. After the polymerization, the contents were precipitated in a large excess of methanol or heptane. The precipitates were filtered, washed and dried. Conversions were limited to < 7 % . Molecular weights were determined from GPC, calibrated with PMMA and PS as standards, using T H F as eluent. RESULTS AND DISCUSSION T h e c h a i n t r a n s f e r c o n s t a n t s (Ctr) o f v a r i o u s m e r c a p t a n s were d e t e r m i n e d f o r M M A at 60 °. I n the cases o f B A E T a n d A E T , the studies were e x t e n d e d to styrene. B A E T w a s studied also w i t h ethyl acrylate. T h e c h a i n t r a n s f e r c o n s t a n t s f o r v a r i o u s s y s t e m s are cited in T a b l e 1 a n d typical e x p e r i m e n t a l c o n d i t i o n s a n d p o l y m e r c h a r a c t e r i s t i c s are given in T a b l e 2 (similar c o n d i t i o n s h a v e b e e n e m p l o y e d f o r the o t h e r
1
1
Ctr [~-] +
P,, and P,,o are the degrees of polymerization of polymer with and without the chain transfer agent respectively. The values of Ctr for various systems are tabulated in Table 1. Representative Mayo plots are shown in Figs 1 and 2. An examination of the Ctr values for the flaminomercaptans reveals that they are considerably lower than expected for the acrylate monomers. Transformation of the amines to the salts brings about drastic improvements in their chain transfer values, although they do not attain the normal values (e.g. that of butane thiol). Sometimes a 50-fold increase is observed. Ikada et al. [3], observed that AET does not give rise to amine functional polymers unlike its hydrochloride; no explanation was given for this observation. Palit et al. [2], attributed the lower Ctr values of 2-aminoethane thiol hydrochloride itself to the strong electron-withdrawing ammonium group which opposes the electron transfer from the hydrogen to the radical. They did not however study the free amine. This explanation does not appear valid since we have observed a reverse trend for the amines and their hydrochloride. Further, the electron-withdrawing theory cannot explain the lower Ctr of fl-aminomercaptans since, in the case of 2-mercaptoethanol, a still lower Ctr would be expected. Published values show that 2-mercaptoethanol and mercaptoacetic acid and their esters, possessing strong, electron-withdrawing groups adjacent to the mercapto groups, are as good transfer
Table 1. Chain transfer constants for the various thiols Chain transfer constant
Chain transfer agent AET AET BAET BAET BAET DAET MEP BSAET 2-Mercaptopyridine 4-Mercaptopyridine (EtO)2P(O)NH(CH2)2 SH [6] C15N3P3NH(CH2), SH [7] Dibutylamine I-Butane thiol t + Di-butylamine
l-Butanethiol
2-Mercaptoethanol Ethyl mercaptoacetate
Monomer MMA ST* MMA ST EA** MMA MMA MMA MMA MMA MMA MMA MMA
Amine C~ 3.1 x 10-5 11.09 1.6 x 10 2 22 2.6 x 10-2 7.3 × 10-5 0.3 9 x 10 5 1.56 x 10-2 0.17 0.28 0.20 7.4 x 10-4
Amine hydrochloride C~ 0.11 [3] 10.8 0.48a 8.82 0.23a 0.39 0.28b __ 1.59 x 10-2 0.28 --8 x 10 4
R = Ct+' C~ 35 1 30 0.4 9 54 1 __ 1 1.7 --1.1
MMA
0.6
--
--
MMA MMA MMA
0.655 0.605 0.625
----
----
'In cases where the value of /1~n was difficult to find from GPC, values of h,Tp were taken as the polymer molecular weight. A pooling of the Ctr values for all systems from Jl,~n and ~'p gave the empirical relation Ct,~. = (3.0 _+0.3)Ctr#, bFor the dihydrochloride. *ST = styrene. **EA = ethyl acrylate.
(1)
[1
Anomalous chain transfer behaviour of fl-aminomercaptans
813
8
•
e,i
"8
I 20
10
I 7,0
Fig. 1. Determination of chain transfer constant for styrene: +, BAET; A, BAET HCI. _=
rqe-i
,-4,,6
"8 E
~6 II
2" a~ <~
'.6',/
aee~ ,.4,.4
--~
eqeq
o=
z
. J
[s] /[M] lO3
r~
o
/ ÷
2 O0 "--
E
_0
<_
4
"E
agents as other thiols [5] (¢fTable 1). It can be further seen that the electron-withdrawing effect of the groups in the fl-position does not modify the chain transfer property of the mereaptan group from the high C,, value for BAET-HCI towards the acrylate (where one would otherwise have expected a decrease--see later diseusion) and the slight reduction in the chain transfer property of the aminomercaptans towards styrene upon transformation to their salts. In the latter case, a reverse trend would be expected since the transition state for transfer is envisaged as one in which an electron is transferred from the electron-rich polystyryl radical to the hydrogen atom. In that case, the electron-withdrawing groups should facilitate the transfer• It is worth noting that for styrene the aminomercaptans are very good transfer agents. Hence the chain transferretardation is a special feature of the acrylates. Retardation in transfer was found to different extents for the 2-aminomercaptans studied. To examine the influence of the NH group on the transfer, in one case the secondary amine was transformed to its trimethyl silyl derivative as follows (BSAET) but did not bring about any change. This results showed that the NH group is not involved in transfer and that the N-silyl group is not capable of modifying the property of the aminothiol,
[-
B u - - N H - - C H 2 - - C H 2 - - S H + CI--Si(CH3)3
o
z
b-
(Et3)N
, Bu--N--CH2--CH2--SH
(BSAET)
(
Si(CH3)3 ~
rC
10.0
~0,~ 7 . 5 2 . , 5 5 . 0
.............................
E
0.00L .
tI
2I 3I [S]/[M] 103
4I
5I
Fig. 2. Determination of chain transfer constant for MMA: +, DAET-HCI; O, 4-mercaptopyridine.
814
C.P. REGHU/qADrlANNAIRet
Literature values of Ctrs for N-phosphorylated 2aminoethane thiols show that they are higher than those of even the corresponding hydrochlorides [6, 7]. This fact indicates that the retardation is caused by the intervention of the non-bonding basic electrons on nitrogen. Blocking them by acids or masking them by acylation (or phosphorylation) confers good transfer property on the thiol. Parallel studies carried out for isomeric mercaptopyridines and their hydrochlorides indicate that the presence of a basic amino group in a mercaptan cannot interfere with its transfer process with MMA because, in these cases, the thiols and their salts do not have the same behaviour. Interference of any amine present in the system with the transfer process can also be ruled out from the result obtained for an equimolar mixture of 1-butane thiol and dibutylamine studied in the case of MMA, where the thiol retained its transfer property. In one case, dibutyl amine and its hydrochloride were studied for their transfer behaviour towards MMA and were found to have identical Ctr. This result confirms that the increase in transfer constant of the aminomercaptan hydrochloride is caused by the activation of the hydrogen atoms on the carbon ~ to the ammonium group. All these results indicate one point: the retardation in transfer behaviour is a peculiar feature of the fl-aminomercaptans with the acrylates. In classical free radical reactions, the transition state for chain transfer has been described as one in which electron-transfer takes place between the attacking radical and the mercaptan group in either of the following ways [8]. 6+
6
~-
R-"H...
S--R'
R...H...
~i+
(1)
S-R'
(2)
1 is the preferred state for electron-rich radicals like styryl, vinyl acetate etc. and 2 for electron-deficient radicals like the acrylates. Neither has been unambigously proved and, in the non-bonded canonical forms for the transition state, each of the above structures has a definite weighting factor. The phenomenon of the fl-aminomercaptans can be rationalized since the transition state 2 is the more reasonable for the electron-deficient acrylate radical, in which an electron is transferred from the mercaptan to the radical. In that case, it is possible that the electron on the fl-nitrogen can interact with the electron-seeking attacking radical through a six-membered cyclic transition state thus: ,,'°
CH2
"°°, N ~
bond formation between the carbon radical and the hydrogen. In amine salts, the electrons are not free and, in acylated amines, they are already conjugated with the carbonyl (or phosphonyl or phosphazyl) groups. The mechanism is to some extent analogous to the neighbouring group participation in organic nucleophilic substitutions. Similar shielding of the radical by the non-bonding electrons on certain heteroatom (like CI) has been used to explain the reduced chain transfer activity of the radical at the oligomerization stage in PMMA telomerization using BrCCI3 as the telogen [9]. For electron-rich radicals like styrene or vinyl acetate, the transition state 1 is the more probable. The radical is already conjugated with the electron-rich aromatic ring or the non-bonding orbital on the adjacent hetero atom. Since the radical is already electron-rich, the interaction with the nitrogen can only facilitate the transfer process. This may perhaps explain the enhanced Ctr of BAET, with respect to its hydrochloride, towards styrene. In other words, the observation in the case of styrene indirectly favours the postulated mechanism. In oxygen analogues, a similar mechanism may be expected to operate. But the electrons on oxygen are so firmly held (e.g. they are not easily protonated unlike amines) that they cannot participate in this kind of perturbation. Table 1 shows the chain transfer constants for typical primary, secondary and tertiary aminomercaptans and their salts. Another aminothiol in which the amine forms part of a six-membered ring (MEP) has been included. The perturbation is not proportional to the basicity of the amine. On the other hand, it depended upon the steric crowding on the amine. It can be seen that the greater the hindrance on the nitrogen, the lesser the perturbation with the result that in MEP, by virtue of the six-membered ring, no perturbation is observed in the transfer. The amine like its salt has excellent transfer property because the bulkier substituents on N hinder sterically the geometrical requirement for the formation of the above transition state. The highest chain transfer constant in the case of mercaptoethyl piperazine is hence not surprising. It is well known that reactions (or interactions) involving polymer radicals, much as transfer, are sensitive to steric hindrance on the substrate [5, 10]. In the studied cases, steric hindrance to the above mentioned transition state decreases in the order. H
/---xN ~ HN
H .... S
R"
al.
/ CH2
I
R°
It can be envisaged that the non-bonding sp 3 orbital on nitrogen overlaps with the electron-seeking unpaired p-orbital of the attacking carbon which is in the course of overlapping with the bonded s-orbital of the hydrogen atom in the transition state. Although this may be expected to lower the activation energy of the transition state, it can slow down
\
/
>
I
BuN~
> Et2N
~ H2N~
Figure 3 shows the molecular models (computer graphics) for the carbon skeleton of MEP and BAET. It can be seen that, in the most stable conformation (calculated by computer) for MEP, the fl-nitrogen is well shielded by the neighbouring carbon (and also by the hydrogen, although not shown) with the result that the nitrogen is not easily accessible to the radical whereas for BAET, as can be seen, it is well exposed. It is possible therefore to explain the difference in transfer properties.
Anomalous chain transfer behaviour of fl-aminomercaptans BAET
815
for all amines, then the effective transfer constant for the salt Ct+~ is given as,
MEP
Ct+~= ctCtr + (1
Fig. 3. Molecular models (computer graphic) for the carbon skeleton of BAET and MEP. Since the free amine is presumed to perturb the transfer reaction, all the fl-aminoethane thiol hydrochlorides should be expected to possess the same chain transfer properties for a given monomer. Examination of the results shows that they are dependent on the nature of the amine. In fact in alcoholic solutions (employed for the polymerization) the salts can dissociate according to the scheme. R:N +--CH2CHE--SH~-R2N--CH2CHs--SH + H +
I
H
1-~
~
:t
(2) The effective Ctr of the salt is the sum of Ctr of the free amines and that of the salt. Hence it depends upon the degree of ionization of the salt which in turn depends upon the pKa of the amine salt. The higher the basicity of the amine, the lower the degree of dissociation of the complementary acid (amine salt) and higher the effective Cir. The present observation is consistent with this concept. The basicities of the fl-amines are theoretically in the order. H H2N~
=HN
N~
k___/
~
L
BuN~
The observed Ctr of the salts are in the increasing order of basicity of the amines. H H2N ~ <
/---xN ~
HN
\
/
< EHN--
I
< BuN~
If ct is the degree of dissociation, then from equation (2) the equilibrium constant K is Cot g = --l-ct
(3)
where C is the concentration of the ammonium salt. If C~ is the chain transfer constant for the amines and Ct~÷ the theoretical value for the salt (supposing complete blocking of the electrons) which is constant
- - a ) -"rth+ -tr •
(4)
From equation (3), ~t is dependent on K, the equilibrium constant, which is sensitive to changes in the polarity, dielectric constant and the basicity of the polymerizing medium. ~ is also inversely related to the concentration. Hence the effective Ctr can be expected to be affected by the above mentioned variables during polymerization. The ratio of the chain transfer constants of the amines and the salts is given as: R
~-- C t r+ /Ctr.
(5)
From equation (4) it is clear that R = :t + (1 - ~)C~ +/C~r.
(6)
For the methacrylates, R is generally very high so that equation (6) can be approximated as R - (1
-
@)Ctth+/Ctr
(7)
from which it becomes evident that
C,+r = (1
- - ~ )"C""+ --~..
(8)
Equation (8) gives the relation between the observed chain transfer constant and its degree of dissociation in the polymerization medium for a fl-aminoethane thiol hydrochioride. On the other hand, the above equation permits calculation of ~ if a reasonable constant value can be given to C~ +. CONCLUSION The reduced chain transfer property of fl-aminomercaptans towards the alkyl acrylate can be attributed to perturbations in the transition state for transfer caused by the non-bonding electrons on the N-atom. Blocking them, through salt formation or imposing sterically hindering substituents on the N-atom, appears to increase the chain transfer constants and hence the amine functionality of the resulting polymers. REFERENCES
l. R. D. Athey Jr and W. A. Mosher. J. Polym. Sci.; Polym. Chem. Edn 15, 1423 (1977). 2. K. K. Roy, D. Pramanick and S. R. Palit. Makromolek. Chem. 153, 71 (1972). 3. Y. Ikada, H. Iwata and S. Nagaoka. Macromolecules 10(6), 1364 (1977). 4. A. G. De Boos. Polymer 14(11), 587 (1973). 5. J. L. O'Brien and F. Gornick. J. Am. chem. Soc. 77, 4757 (1955). 6. G. Clouet and M. Knipper. Makromolek. Chem. 188, 2597 (1987). 7. M. Knipper. PhD thesis, University Louis Pasteur, Strasbourg 0985). 8. C. Walling. J. Am. chem. Soc. 70, 2561 (1948). 9. C. A. Barson and R. Ensor. Fur. Polym. J. 13, 53 0977). 10. R. A. Gregg, D. M. Alderman and F. R. Mayo. J. Am. chem. Soc. 70, 3740 0948).
0014-3057/90 $3.00 + 0.00 Pergamon Press plc
ANOMALOUS CHAIN TRANSFER BEHAVIOUR OF fl-AMINOMERCAPTANS IN THE RADICAL POLYMERIZATIONS OF ACRYLATES C. P. REGHUNADHANNAIR, M. C. RICHOU,P. CHAUMONTand G. CLOUET* Institut Charles Sadron (CRM-EAHP), 6, rue Boussingault, 67083 Strasbourg C6dex, France
(Received 5 September 1989) Abstract--In contrast to thiols in general, fl-aminomercaptans were found to have low chain transfer constants in the free radical polymerizations of acrylates. Transformation of the amines to their salts, or other modifications masking the basicity of the amines, produced dramatic improvements in their chain transfer capability. The chain transfer constants of typical primary, secondary and tertiary fl-aminomercaptans and their hydrochlorides were determined at 60 ° for MMA and in some cases also for styrene and ethyl acrylate. In the case of styrene, normal chain transfer behaviour was observed for the amines and their salts, suggesting that the retardation accompanying transfer is a special feature of the electron-deficient acrylate radicals. The observed trends in relative reactivities of the amines and their salts and complementary studies with other aminomercaptans led to the postulate that the decreased reactivity of the aminomercaptans towards the acrylate radicals may be due to perturbations from specific interaction between the non-bonding electrons on the amine and the electron-deficient attacking radical, through a six membered transition state which is sensitive to the steric crowding on the N-atom.
INTRODUCTION
EXPERIMENTAL PROCEDURES
Use of functional chain transfer agents in radical polymerization is an effective means for chain-endfunctionalization of vinyl polymers. The functionality of the resultant polymers are dependent on the chain transfer constants [1]. Generally, a very high chain transfer constant is desirable but the preferred value is unity, in which case the polymer chains have uniform length at comparatively high conversions. Mercaptans generally have excellent chain transfer properties. Amino-terminated vinly polymers are desirable for a variety of polymer modifications through reaction and modification of the chain-end groups. Their syntheses warrant utilization of aminomercaptans as chain transfer agents. All commercially available aliphatic aminomercaptans are fl-aminomercaptans probably because of ease of synthesis. A few published data cite utilization of 2-mecapto-ethylamine hydrochloride as a good chain transfer agent [2-4]. The salt was used because the amine was available quite pure in that form. We had special interest in amine-terminated vinyl polymers for use in the synthesis of block copolymers via the thermal "iniferter" technique. During our investigation, it was observed that the fl-aminomercaptans had very poor chain transfer capability and that they required to be transformed to their salts to have any meaningful transfer property in the case of alkyl acrylates. This finding prompted a detailed study of the chain transfer behaviour of various aminomercaptans with a view to elucidate the mechanism of chain transfer and retardation for these chain transfer agents.
Materials 2-Aminoethane thiol (AET) and its hydrochloride, 2-(butylamino) ethane thiol (BAET), 2-(diethylamino) ethane thiol (DAET) hydrochloride, piperazine, ethylene sulphide and 2- and 4-mercaptopyridines (Aldrich) were used as received. Methyl methacrylate (MMA), styrene and ethyl acrylate were purified by distillation under reduced pressure from finely powdered Call 2. AIBN was recrystallized from methanol. BAET hydrochloride was prepared from the amine by treating an ethereal solution with excess conc. HCI. The aqueous layer was treated with chloroform, dried thoroughly with Na2SO4 and the solvent evaporated to give a white fine powder which was dried in vacuum. DAET was liberated from its hydrochloride, by treating its aqueous solution with a slight excess of NaOH. The liberated amine was extracted with ether and dried; the ether was evaporated and the residue vacuum distilled. Purity was determined by titration of SH groups. Instruments ~H-NMR spectra were recorded on a Brucker 60 MHz spectrometer. Molecular weights were found by GPC using THF as eluent, with a refractive index detector. Synthesis of 2-[N-(l-butyl) N-trimethyl silyll aminoethane thiol (BSAET) To 0.05 mol of BAET in 50ml dry ether containing 0.06tool triethylamine was added dropwise 0.05mol trimethyl chlorosilane under argon at ca 10°. After the addition, the system was reacted for 2 hr. The amine hydrochloride was filtered. After recovering the ether from the residue, it was vacuum distilled. The fraction boiling at 73°/5mm was collected and characterized by ~H-NMR; purity determined from the SH content.
*To whom all correspondence should be addressed. 811
C.P. REGHUNADHANNAIR et
812
~H-NMR (CC14): 6 ppm--0.22 (s, 3H, Si---CH3), 0.77 (t, 3H, ---CH3), 1.26 (m, 4H, N---C---CH2--CH2--), 1.62 (s, IH, --SH), 4.3-4.7 (m, broad, 4H, ---CH2---CH2--S) SH%: Calc. 16.1; Found 14.5.
al.
thiols). The transfer constants (Ct~) were determined by using Mayo's equation (1) for low thiol concentration where retardation by the thiol can be neglected.
Synthesis of N-(2-mercapto ethyl) piperazine (MEP) Piperazine (0.05 mol) was reacted overnight with 0.01 mol of ethylene sulphide in dry T H F under reflux. The T H F was evaporated and the residue vacuum distilled, collecting the fraction boiling at 73°/5 mm Hg. Yield: 45%.
C6HI4N2S Calc. Found SH% Calc. Amine*°/0 Calc.
C 49.30 48.50 22.60 19.20
H 9.60 9.60 Found Found
N 19.20 19.20 22.52 18.00
S 21.92 20.82
Its dihydrochloride was prepared by treating an alcoholic solution with HC1 and then precipitating in ether.
Polymerization The polymerizations were carried out in sealed evacuated glass tubes containing the monomer, initiator, solvent and chain transfer agent in a bath at 60 °. Ethanol was used as the solvent. After the polymerization, the contents were precipitated in a large excess of methanol or heptane. The precipitates were filtered, washed and dried. Conversions were limited to < 7 % . Molecular weights were determined from GPC, calibrated with PMMA and PS as standards, using T H F as eluent. RESULTS AND DISCUSSION T h e c h a i n t r a n s f e r c o n s t a n t s (Ctr) o f v a r i o u s m e r c a p t a n s were d e t e r m i n e d f o r M M A at 60 °. I n the cases o f B A E T a n d A E T , the studies were e x t e n d e d to styrene. B A E T w a s studied also w i t h ethyl acrylate. T h e c h a i n t r a n s f e r c o n s t a n t s f o r v a r i o u s s y s t e m s are cited in T a b l e 1 a n d typical e x p e r i m e n t a l c o n d i t i o n s a n d p o l y m e r c h a r a c t e r i s t i c s are given in T a b l e 2 (similar c o n d i t i o n s h a v e b e e n e m p l o y e d f o r the o t h e r
1
1
Ctr [~-] +
P,, and P,,o are the degrees of polymerization of polymer with and without the chain transfer agent respectively. The values of Ctr for various systems are tabulated in Table 1. Representative Mayo plots are shown in Figs 1 and 2. An examination of the Ctr values for the flaminomercaptans reveals that they are considerably lower than expected for the acrylate monomers. Transformation of the amines to the salts brings about drastic improvements in their chain transfer values, although they do not attain the normal values (e.g. that of butane thiol). Sometimes a 50-fold increase is observed. Ikada et al. [3], observed that AET does not give rise to amine functional polymers unlike its hydrochloride; no explanation was given for this observation. Palit et al. [2], attributed the lower Ctr values of 2-aminoethane thiol hydrochloride itself to the strong electron-withdrawing ammonium group which opposes the electron transfer from the hydrogen to the radical. They did not however study the free amine. This explanation does not appear valid since we have observed a reverse trend for the amines and their hydrochloride. Further, the electron-withdrawing theory cannot explain the lower Ctr of fl-aminomercaptans since, in the case of 2-mercaptoethanol, a still lower Ctr would be expected. Published values show that 2-mercaptoethanol and mercaptoacetic acid and their esters, possessing strong, electron-withdrawing groups adjacent to the mercapto groups, are as good transfer
Table 1. Chain transfer constants for the various thiols Chain transfer constant
Chain transfer agent AET AET BAET BAET BAET DAET MEP BSAET 2-Mercaptopyridine 4-Mercaptopyridine (EtO)2P(O)NH(CH2)2 SH [6] C15N3P3NH(CH2), SH [7] Dibutylamine I-Butane thiol t + Di-butylamine
l-Butanethiol
2-Mercaptoethanol Ethyl mercaptoacetate
Monomer MMA ST* MMA ST EA** MMA MMA MMA MMA MMA MMA MMA MMA
Amine C~ 3.1 x 10-5 11.09 1.6 x 10 2 22 2.6 x 10-2 7.3 × 10-5 0.3 9 x 10 5 1.56 x 10-2 0.17 0.28 0.20 7.4 x 10-4
Amine hydrochloride C~ 0.11 [3] 10.8 0.48a 8.82 0.23a 0.39 0.28b __ 1.59 x 10-2 0.28 --8 x 10 4
R = Ct+' C~ 35 1 30 0.4 9 54 1 __ 1 1.7 --1.1
MMA
0.6
--
--
MMA MMA MMA
0.655 0.605 0.625
----
----
'In cases where the value of /1~n was difficult to find from GPC, values of h,Tp were taken as the polymer molecular weight. A pooling of the Ctr values for all systems from Jl,~n and ~'p gave the empirical relation Ct,~. = (3.0 _+0.3)Ctr#, bFor the dihydrochloride. *ST = styrene. **EA = ethyl acrylate.
(1)
[1
Anomalous chain transfer behaviour of fl-aminomercaptans
813
8
•
e,i
"8
I 20
10
I 7,0
Fig. 1. Determination of chain transfer constant for styrene: +, BAET; A, BAET HCI. _=
rqe-i
,-4,,6
"8 E
~6 II
2" a~ <~
'.6',/
aee~ ,.4,.4
--~
eqeq
o=
z
. J
[s] /[M] lO3
r~
o
/ ÷
2 O0 "--
E
_0
<_
4
"E
agents as other thiols [5] (¢fTable 1). It can be further seen that the electron-withdrawing effect of the groups in the fl-position does not modify the chain transfer property of the mereaptan group from the high C,, value for BAET-HCI towards the acrylate (where one would otherwise have expected a decrease--see later diseusion) and the slight reduction in the chain transfer property of the aminomercaptans towards styrene upon transformation to their salts. In the latter case, a reverse trend would be expected since the transition state for transfer is envisaged as one in which an electron is transferred from the electron-rich polystyryl radical to the hydrogen atom. In that case, the electron-withdrawing groups should facilitate the transfer• It is worth noting that for styrene the aminomercaptans are very good transfer agents. Hence the chain transferretardation is a special feature of the acrylates. Retardation in transfer was found to different extents for the 2-aminomercaptans studied. To examine the influence of the NH group on the transfer, in one case the secondary amine was transformed to its trimethyl silyl derivative as follows (BSAET) but did not bring about any change. This results showed that the NH group is not involved in transfer and that the N-silyl group is not capable of modifying the property of the aminothiol,
[-
B u - - N H - - C H 2 - - C H 2 - - S H + CI--Si(CH3)3
o
z
b-
(Et3)N
, Bu--N--CH2--CH2--SH
(BSAET)
(
Si(CH3)3 ~
rC
10.0
~0,~ 7 . 5 2 . , 5 5 . 0
.............................
E
0.00L .
tI
2I 3I [S]/[M] 103
4I
5I
Fig. 2. Determination of chain transfer constant for MMA: +, DAET-HCI; O, 4-mercaptopyridine.
814
C.P. REGHU/qADrlANNAIRet
Literature values of Ctrs for N-phosphorylated 2aminoethane thiols show that they are higher than those of even the corresponding hydrochlorides [6, 7]. This fact indicates that the retardation is caused by the intervention of the non-bonding basic electrons on nitrogen. Blocking them by acids or masking them by acylation (or phosphorylation) confers good transfer property on the thiol. Parallel studies carried out for isomeric mercaptopyridines and their hydrochlorides indicate that the presence of a basic amino group in a mercaptan cannot interfere with its transfer process with MMA because, in these cases, the thiols and their salts do not have the same behaviour. Interference of any amine present in the system with the transfer process can also be ruled out from the result obtained for an equimolar mixture of 1-butane thiol and dibutylamine studied in the case of MMA, where the thiol retained its transfer property. In one case, dibutyl amine and its hydrochloride were studied for their transfer behaviour towards MMA and were found to have identical Ctr. This result confirms that the increase in transfer constant of the aminomercaptan hydrochloride is caused by the activation of the hydrogen atoms on the carbon ~ to the ammonium group. All these results indicate one point: the retardation in transfer behaviour is a peculiar feature of the fl-aminomercaptans with the acrylates. In classical free radical reactions, the transition state for chain transfer has been described as one in which electron-transfer takes place between the attacking radical and the mercaptan group in either of the following ways [8]. 6+
6
~-
R-"H...
S--R'
R...H...
~i+
(1)
S-R'
(2)
1 is the preferred state for electron-rich radicals like styryl, vinyl acetate etc. and 2 for electron-deficient radicals like the acrylates. Neither has been unambigously proved and, in the non-bonded canonical forms for the transition state, each of the above structures has a definite weighting factor. The phenomenon of the fl-aminomercaptans can be rationalized since the transition state 2 is the more reasonable for the electron-deficient acrylate radical, in which an electron is transferred from the mercaptan to the radical. In that case, it is possible that the electron on the fl-nitrogen can interact with the electron-seeking attacking radical through a six-membered cyclic transition state thus: ,,'°
CH2
"°°, N ~
bond formation between the carbon radical and the hydrogen. In amine salts, the electrons are not free and, in acylated amines, they are already conjugated with the carbonyl (or phosphonyl or phosphazyl) groups. The mechanism is to some extent analogous to the neighbouring group participation in organic nucleophilic substitutions. Similar shielding of the radical by the non-bonding electrons on certain heteroatom (like CI) has been used to explain the reduced chain transfer activity of the radical at the oligomerization stage in PMMA telomerization using BrCCI3 as the telogen [9]. For electron-rich radicals like styrene or vinyl acetate, the transition state 1 is the more probable. The radical is already conjugated with the electron-rich aromatic ring or the non-bonding orbital on the adjacent hetero atom. Since the radical is already electron-rich, the interaction with the nitrogen can only facilitate the transfer process. This may perhaps explain the enhanced Ctr of BAET, with respect to its hydrochloride, towards styrene. In other words, the observation in the case of styrene indirectly favours the postulated mechanism. In oxygen analogues, a similar mechanism may be expected to operate. But the electrons on oxygen are so firmly held (e.g. they are not easily protonated unlike amines) that they cannot participate in this kind of perturbation. Table 1 shows the chain transfer constants for typical primary, secondary and tertiary aminomercaptans and their salts. Another aminothiol in which the amine forms part of a six-membered ring (MEP) has been included. The perturbation is not proportional to the basicity of the amine. On the other hand, it depended upon the steric crowding on the amine. It can be seen that the greater the hindrance on the nitrogen, the lesser the perturbation with the result that in MEP, by virtue of the six-membered ring, no perturbation is observed in the transfer. The amine like its salt has excellent transfer property because the bulkier substituents on N hinder sterically the geometrical requirement for the formation of the above transition state. The highest chain transfer constant in the case of mercaptoethyl piperazine is hence not surprising. It is well known that reactions (or interactions) involving polymer radicals, much as transfer, are sensitive to steric hindrance on the substrate [5, 10]. In the studied cases, steric hindrance to the above mentioned transition state decreases in the order. H
/---xN ~ HN
H .... S
R"
al.
/ CH2
I
R°
It can be envisaged that the non-bonding sp 3 orbital on nitrogen overlaps with the electron-seeking unpaired p-orbital of the attacking carbon which is in the course of overlapping with the bonded s-orbital of the hydrogen atom in the transition state. Although this may be expected to lower the activation energy of the transition state, it can slow down
\
/
>
I
BuN~
> Et2N
~ H2N~
Figure 3 shows the molecular models (computer graphics) for the carbon skeleton of MEP and BAET. It can be seen that, in the most stable conformation (calculated by computer) for MEP, the fl-nitrogen is well shielded by the neighbouring carbon (and also by the hydrogen, although not shown) with the result that the nitrogen is not easily accessible to the radical whereas for BAET, as can be seen, it is well exposed. It is possible therefore to explain the difference in transfer properties.
Anomalous chain transfer behaviour of fl-aminomercaptans BAET
815
for all amines, then the effective transfer constant for the salt Ct+~ is given as,
MEP
Ct+~= ctCtr + (1
Fig. 3. Molecular models (computer graphic) for the carbon skeleton of BAET and MEP. Since the free amine is presumed to perturb the transfer reaction, all the fl-aminoethane thiol hydrochlorides should be expected to possess the same chain transfer properties for a given monomer. Examination of the results shows that they are dependent on the nature of the amine. In fact in alcoholic solutions (employed for the polymerization) the salts can dissociate according to the scheme. R:N +--CH2CHE--SH~-R2N--CH2CHs--SH + H +
I
H
1-~
~
:t
(2) The effective Ctr of the salt is the sum of Ctr of the free amines and that of the salt. Hence it depends upon the degree of ionization of the salt which in turn depends upon the pKa of the amine salt. The higher the basicity of the amine, the lower the degree of dissociation of the complementary acid (amine salt) and higher the effective Cir. The present observation is consistent with this concept. The basicities of the fl-amines are theoretically in the order. H H2N~
=HN
N~
k___/
~
L
BuN~
The observed Ctr of the salts are in the increasing order of basicity of the amines. H H2N ~ <
/---xN ~
HN
\
/
< EHN--
I
< BuN~
If ct is the degree of dissociation, then from equation (2) the equilibrium constant K is Cot g = --l-ct
(3)
where C is the concentration of the ammonium salt. If C~ is the chain transfer constant for the amines and Ct~÷ the theoretical value for the salt (supposing complete blocking of the electrons) which is constant
- - a ) -"rth+ -tr •
(4)
From equation (3), ~t is dependent on K, the equilibrium constant, which is sensitive to changes in the polarity, dielectric constant and the basicity of the polymerizing medium. ~ is also inversely related to the concentration. Hence the effective Ctr can be expected to be affected by the above mentioned variables during polymerization. The ratio of the chain transfer constants of the amines and the salts is given as: R
~-- C t r+ /Ctr.
(5)
From equation (4) it is clear that R = :t + (1 - ~)C~ +/C~r.
(6)
For the methacrylates, R is generally very high so that equation (6) can be approximated as R - (1
-
@)Ctth+/Ctr
(7)
from which it becomes evident that
C,+r = (1
- - ~ )"C""+ --~..
(8)
Equation (8) gives the relation between the observed chain transfer constant and its degree of dissociation in the polymerization medium for a fl-aminoethane thiol hydrochioride. On the other hand, the above equation permits calculation of ~ if a reasonable constant value can be given to C~ +. CONCLUSION The reduced chain transfer property of fl-aminomercaptans towards the alkyl acrylate can be attributed to perturbations in the transition state for transfer caused by the non-bonding electrons on the N-atom. Blocking them, through salt formation or imposing sterically hindering substituents on the N-atom, appears to increase the chain transfer constants and hence the amine functionality of the resulting polymers. REFERENCES
l. R. D. Athey Jr and W. A. Mosher. J. Polym. Sci.; Polym. Chem. Edn 15, 1423 (1977). 2. K. K. Roy, D. Pramanick and S. R. Palit. Makromolek. Chem. 153, 71 (1972). 3. Y. Ikada, H. Iwata and S. Nagaoka. Macromolecules 10(6), 1364 (1977). 4. A. G. De Boos. Polymer 14(11), 587 (1973). 5. J. L. O'Brien and F. Gornick. J. Am. chem. Soc. 77, 4757 (1955). 6. G. Clouet and M. Knipper. Makromolek. Chem. 188, 2597 (1987). 7. M. Knipper. PhD thesis, University Louis Pasteur, Strasbourg 0985). 8. C. Walling. J. Am. chem. Soc. 70, 2561 (1948). 9. C. A. Barson and R. Ensor. Fur. Polym. J. 13, 53 0977). 10. R. A. Gregg, D. M. Alderman and F. R. Mayo. J. Am. chem. Soc. 70, 3740 0948).