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Lewis-base system and the role of the Lewis base in initiation
Polymerization of vinyl chloride by the AI Et3/benzoyl-peroxide/Lewisbase system and the role of the Lewis base in initiation E. B. Milovskaya, E. L. Kopp, O. S. Mikhailicheva, V. M. Denisov and A. I. Koltsov Institute of High Molecular Compounds, Bolshoi 31, Leningrad, USSR (Received 1 January 1971) Kinetics of polymerization of vinyl chloride under the influence of the AIEts/ benzoyl-peroxide initiation system in the presence of the complex[ng agents allyl acetate, dibutyl ether and pyridine have been studied. It follows from the kinetic dependences established (which were confirmed by infra-red and nuclear magnetic resonance spectroscopies) that the role of polar agents, irrespective of their complexing ability, is to reduce the initial concentration of aluminium alkyl. Freeradical formation proper occurs by the interaction of the uncomplexed form of AIEt3 with peroxide. The process as a whole has SN1 kinetics. When the polar monomer vinyl acetate is used as a complexing agent, the complexed form of AIEts participates in the initiation. A suggestion is put forward concerning the possibility of the occurrence of a quite different mechanism of interaction between the components of the initiation system in the presence of polar monomers.
INTRODUCTION In recent years systems based on aluminium alkyls and acyl peroxides have been used to initiate the polymerization of the vinyl monomers--vinyl acetate (VA)~, 2, methyl methacrylate (MMA) 3, acrylonitrile (AN) 4 and vinyl chloride (VC) 5, 6. The process involves free-radical polymerization and can be carried out at much lower temperatures than the thermal decomposition of peroxide. The efficiency of these systems lies in the separation of the initiating system components by the polar agent or the monomer since direct reaction between A1Ra and peroxide is explosive 7, s. The electron-donating ability of such monomers as VA, AN and MMA proves to be sufficient for the development of polymerization. For all the systems investigated the same expression has been derived for the dependence of the overall rate of the process on the concentrations of the monomer and of the initiator components: V= k[M]l'o-z'2[A1Rz]O'5[P]O'5
(1)
where M is AN, MMA and VA, A1R3 is A1Et3 and Al(iBu)3, P is benzoyl peroxide (BP) and dicyclohexyl peroxidicarbonate (PC).
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POLYMER, 1972, Vol 13, June
On the basis of the kinetic results and the fact that the monomer should participate in the initiation, the formation of free radicals has been represented as a series of reactions 5: fast
AIRs+M
BP
> A1Rs"M
• (AIR3M.BP) kl
> k2
A1R2COOPh + R" + PhCOO" + (M) (2) Under the condition that kl
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al. overall rate of the process in the presence of catalytic amounts of EA on the concentrations of the monomer, EA, and the initiator components has the form: {[A1Et3]'~ °'5 V=k~[VC] 1"21 [E~]-/! [BP] °'25
(3)
A similar dependence on the concentration o f monomer and initiator components was shown for the case of VC polymerization without CA. The mechanism for the initiation reactions is: Keq A1Etz + E A . • AIEta. EA (4) KI KII 2AIEt3 + BP ~ " [(A1Et3)eBP] -, " k5 2(AIEtzCOOPh)" ~ 2A1Et2COOPh+2Et" (5) If the equilibria with K~ and Ku shifts to the left, the equilibrium of reaction (4) shifts to the right and the overall rate of interaction (5) is determined by ks, the expression for the initiation reaction rate in the system with EA can be written: Via = kin [A1Et3]rBP10.5 ~X-]- ~ ~
(3a)
From the results obtained, it follows that the role of EA is to reduce the initial concentration of aluminium alkyl by complex formation (equation 4); free-radical formation occurs by reaction of uncomplexed aluminium alkyl with peroxide (i.e., it occurs in the same manner as under model conditions7-9). Comparison of the kinetic dependence for the polymerization of polar' monomers (equation 1) with that found for VC polymerization in the presence of EA (equation 3), and comparison of reactions responsible for the initiation (reactions 2 and 4-5) both showed that EA did not simulate the polar monomer function in the initiation. Further investigations were carried out to determine the true role of the polar monomer in initiation. It is clear that this problem cannot be solved under the conditions of the polymerization of the polar monomer; therefore, an artificial device was used, namely VC polymerization was studied in the presence of catalytic amounts of a polar monomer (VA). On the other hand. it was interesting to establish to what extent the reactions for the EA system are valid for different classes of CA. Our results allowed us to obtain general ideas about the mechanism of free-radical formation in the A1Rz/acyl peroxide systems in the presence of electron-donors, including polar monomers.
EXPERIMENTAL Experiments were conducted in the absence of moisture and air. The solvents benzene and octane were purified in the usual way and then dried with calcium hydride and concentrated butyl lithium. The complexing agents were purified according to the methods 1° previously described and then dried with calcium hydride and distilled under vacuum.
VA was pre-polymerized with the AI(iBu)a-BP system. At low concentrations these compounds were used in octane solution. Commercial A1Eta was used without additional purification in octane solution. Its concentration was determined according to the method of Razuvaev and Graevsky 11. The concentration of the solution of the complexing compound was determined from the AIEt3 solution of a known concentration. Al(iBu)3 was distilled under vacuum and the fraction boiling at 41 °C (1 mmHg) was used. Benzoyl peroxide was purified from its chloroform solution, the active oxygen content being 98.5-99 ~ . The BP-benzene solution was added. VC was carefully degassed, dried at - 3 0 ° C over the concentrated butyl lithium, the polymerization being allowed to go to 3-5 ~o conversion. VC was then pre-polymerized using the A1Et~-Bu20-BP system. The purity of the monomer was checked chromatographically. Polymerization was carried out in a single-cell dilatometer (with a magnetic stirrer) having a capacity of about 15 ml and with a capillary calibrated to 0.01 ml. The dilatometer, previously heated under vacuum, was filled with argon and the AIEtz and CA solutions and octane were introduced from Schlenk vessels supplied with a needle. The monomer was added in weighed amounts from the balloon through the needle. The order in which the components were added was always the same: CA, A1Eta, solvent, monomer. The temperature of the reaction mixture was thermostatically controlled to within + 0" 1°C, and the BP solution was injected with a syringe. In the kinetic experiments the polymerization rate was calculated for a 3 - 5 ~ conversion, this being estimated from contraction. The contraction coefficient was found as a mean value from several experiments on the basis of gravimetric measurements of the polymer yield. Infra-red spectra were taken using a UR-10 spectrometer. Sealed cells of potassium bromide were used to protect the organo-aluminium compounds from moisture and air. The cell was carefully dried under vacuum and filled with the previously prepared solutions of the analysed components in octane. The equilibrium c o n s t a n t ( K o q ) of reaction (4) was calculated from : Keq = [AlEtz.__EA] [EA][AIEta]
(6)
The following expression was used to calculate the concentration of free EA [EX] from the 1752cm -1 band intensity: _ Ex[EA]0 [E-A] E0
(7)
where [EA] 0 is the initial EA concentration; E0 is the molar extinction coefficient of the absorption band of the EA carbonyl oxygen, equal to (562_+ 14) 1 mol -x cm-l; Ex is the extinction coefficient of the 1752cm t band in the spectra of the complex. The concentration of the A1Et3.EA complex was determined as the difference between [EA]0 and [EA], and the concentration of [AIEt3] from {[AIEt3]0[AIEta. EA]}. The preparation of samples for proton magnetic resonance (p.m.r.) and ZTAl nuclear magnetic resonance
POLYMER, 1972, Vol 13, June
289
Polymerization of VC by AIEta/benzoyl-peroxide/Lewis-base system : E, B. Milovskaya et aL Table 1 VC polymerization with the AIEt3-BP system in the absence and presence of CA. [VC]=8mol/I; solvents: octane, benzene; temperature= 0°C
0.7 x 0-6 o 0.5
Concentration x 102 (mol/I)
Conversion (%)
~- 0"4
Exp. No.
CA*
AIEt3
CA
BP
lh
2h
4h
1 2 3
VA EA --
1 "0 5"0 5.0
16'0 25'0 --
1-0 5"0 5.0
11 "8 30"0 2.2
15-6 45-5 2"9
18'1 56'6 3'3
0"3' O-2
RESULTS AND DISCUSSION
AIEt3/vinyl-acetate/benzoyl-peroxide system In the presence of catalytic amounts of VA the AIEt3BP system can be successfully used for VC polymerization. Table 1 shows the data obtained and gives the results for the system without CA and in the presence of EA. The reaction kinetics were studied at 0°C. The rate was estimated gravimetrically from the polymer yield. When the ratio [VA]/[A1Et3] is equal to or greater than one, the rate practically coincides with the value calculated from the contraction. Figure 1 shows the rate dependence of the VA concentration, from which one can conclude that under the experimental conditions the composition of the complex resulting from the reaction between AIEt3 and VA is 1 : 1 (8)
With the same concentration ratio of VA and AIEt3 the rate is fixed by the initial concentration of A1Et3, from which it follows that the free-radical formation under given conditions is a result of the interaction between benzoyl peroxide and the A1Eta.VA complex. When the concentrations of VA are lower than those of AIEt3 two processes occur simultaneously (with comparable rates): they result from the reaction of BP with free aluminium alkyl and with aluminium alkyl complexed with VA. The dependence in Figure 1 for the region of [VA]/[A1Et3] < 1 can be explained by the fact that the rate was estimated from the polymer yield and the interaction
POLYMER,
1972, V o l 13, J u n e
i
O'5
I
i
i
0"7
I
O.9
I
I
I.I
, s I
I
1.3
I
I
1.5
I
:: I
1.7
I
I
1.9
Figure I Dependence of the polymerization rate on the VA
(n.m.r.) study was carried out as follows: 10~o benzene solutions of Al(iBu)3 with a certain amount of added CA were placed into the sample tubes--5 mm and 18 mm outside diameter for p.m.r, and 27Al-n.m.r., respectively --which had been dried at high temperature in vacuum. All operations including sealing the tubes were performed under vacuum. The samples were stored in liquid nitrogen and n.m.r, spectra were recorded (4-5 times) just after the samples had reached room temperature. High resolution p.m.r, spectra were recorded with a YEOL YNM-3 spectrometer (40 MHz). No changes were observed in the spectra for one hour. Weak 27Al-n.m.r. signals were observed with a RYa-2301 spectrometer (8 MHz) using phase detection techniques. The peak-topeak line widths were measured allowing for overmodulation 12. Modulation amplitude and frequency were equal to 1.5 gauss and 35 Hz, respectively. The speed of field sweep was equal to 50mg/sec and response time was 20 sec.
290
i
'' 3+log { VA ]
* CA = complexing agent
Keq AIEta + V A . • AIEta. VA
111,1:/.:
concentration with the system AIEta-VA-BP in the solvents octane and benzene at 0°C. [VC]=4mol/I; [AIEta]=lxl0-2mol/I; [ B P ] = 5 x 10-3 mol/I
~r
o x
[AIEt3] °'s [ B p ] ° ' S x iO 3
Figure 2 Dependence of the polymerization rate on the concentration of the initiator components in the system AIEt3-VA-BP in the solvents octane and benzene at 0°C. [VC]=4mol/I; [ V A ] =
5x 10-2 mol/I
of large amounts of free AIEta with BP was followed by the formation of considerable amounts of oligomers 6. In order to avoid the superposition of two effects, a further study of the kinetics was made in the concentration range 10> [VA]/[AIEta] > 1. (As noted, the region of low concentration ratios is characterized by appreciable oligomerization, whereas at large values one should take into account the copolymerization processes.) Figure 2 demonstrates the rate dependence of the process on the concentration of the components of the initiation system. From these data, under conditions of bimolecular termination of growing chains, the expression for the initiation reaction rate may be written as follows: Vin = ka[AlEta] [BP]
(9)
Figure 3 gives the dependence of the overall rate constant of the process on the inverse temperature from which E o is estimated to be 9.7 + 1.2 kcal/mol, Ein being 16"5kcal/ tool. In the calculation of Ein we used the value (Ep - l e t ) = 1.5 kcal/mol for VC 13, 14 The kinetic results obtained enable the initiation in the AIEtz-VA-BP system to be represented as a sequence of reactions expressed by reaction (2). The similarity of the kinetic results (equations la and 9) and the Ein value for the A1Eta-VA-BP system with the corresponding parameters, as established in the polymerization proper of VA 1, is evidence that in both cases the free radicals are formed by the same mechanism. Thus, this work suggests that in the presence of a polar monomer (VA) the free radicals are formed as a result of reaction between the complexed form of aluminium alkyl with peroxide,
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al. 1.6
free-radical formation in the systems with the participation of these CA is the result of the same reactions (4 and 5). As mentioned above, a similar mechanism of the interaction of the initiator components is also realized without CA. This permits one to calculate, from the kinetic data, the thermodynamic parameters for the complexing reaction of AA with A1Et3 (reaction of type 4), the equilibrium constant (Keq) and the enthalpy (AH).
1"8
2"0 2-2 ~3n o
2"4
I
-->.
2'6 2"8 3.5
3.6
3.7
3.8
3.9
4.O
/% = (2f)1/2 kl k~, t ~ . klii'~2 in the absence of
and
/,p
tO3/T perature for the system AIEt3-VA-BP
¥
L'I
I.,FI
0"9 0"7
I
0"8
I
I.O
i
l
l.i. ~ /t
\ [ ,~) -
ki'=(2f)l"~ k] '~ ' \K,,~
Figure 3 Dependence of the rate constant on the inverse tem-
i
1.2
I
1.4
i
i
t
CA
in the presence of CA
Using the k 0 values without CA equal to 1.41 x 10 ..2 (I tool 1)0.75sec-1 (corrected as compared with the value given in reference 6), and k o with AA as given above, the value of Keq is obtained as 1.4 × 1041 mol I-1 at 0°C. (The assumption about the equality of f efficiencies of the initiation in the absence and presence of CA makes this calculation somewhat tentative.) The value for the heat of complexing reaction is derived from:
_ _
Ein (without CA) = Ein (with AA) + AH
1.6
(11)
Taking Ein (without CA) = 1"8 kcal/mol (ref. 6) the value of AH is - 9.8 kcal/mol.
3 + l o g [AA]
Figure 4 Dependence of the polymerization rate on the A A concentration with the system AIEt3-AA-BP in the solvents octane and benzene at 0°C. [VC]=4 mol/I; [AIEt3]= [BP]= 5 x 10-3 mol/I
3"0
2"0
whereas for EA this process is described by the reactions of the SNI type.
•
f
r""
x 2x
I.O
AIEtz/allyl-acetate/benzoyl-peroxide system The reason for the sharp difference in behaviour of compounds of similar structure was investigated. The complexing agent used was allyl acetate (AA), a compound combining all the structural features of EA and VA. AA itself is not polymerized under the influence of the AIEt3-BP system, but its presence in catalytic amounts ensures the effective polymerization of VC. Thus, under conditions similar to those in Table 1 (experiment 2) the polymer yield is 25.3, 38.3 and 46.4 ~o. The kinetics of VC polymerization with the A1Et 3-AA-BP system was studied at 0°C. Figure 4 shows the dependence of the rate of the process on the AA concentration; Figure 5 shows the same dependence on the concentration of the initiator components. On the basis of the above data the expression for the initiation reaction rate in this system can be written as:
Vin~-~-/~.10[AIEt3]rnp]0.5 [/&x/~k]L ~ - - ,
I
I
I.I
1.3
I
1"5
I
1.7
1.9
I
2"1
0.5
[ AIEt3 ] [EA]
[ B P ] ° 2 S x IO
Figure 5 Dependence of the polymerization rate on the concentration of the initiator components in the system AIEt3-AA-BP in the solvents octane and benzene at 0°C. [VC]-- 4 mol/I; molar ratio AA/AIEt3= constant= 4 3.4 3.6 3-8
(10)
4.0
The overall constant of the process is 1.18x 10-4 (1 mol 1)0.2s sec 1 at 0°C. The overall activation energy for the VC polymerization is estimated to be 7.3 _+ 1.0 kcal/mol (Figure 6), hence Ein is 11"6kcal/mol. From the equality of the expressions obtained for EA (equation 3a) and for AA (equation 10) it follows that the
4.2
I
--
3.5
I
3.6
___~
3-7
,L
3-8
103/T Figure 6 Dependence of the rate constant on the inverse temperature for the system AIEt3-AA-BP
POLYMER,
1972, V o l 13, J u n e
291
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et aL The values of the heat effect and the equilibrium constant of the complexing reaction of A1Et3 with AA practically coincide with the corresponding parameters established for EA ( A H = -9.2kcal/mol; Keq= 1.5 × 104 lmo1-1 *), which suggests a rather similar electrondonating ability for these compounds. Infra-red spectra of CA complexes with AIEtz were studied to check the conclusions, obtained from kinetic data, about reversibility and the stoichiometric relations of the complexing reaction of EA and AA with AIEta and also to find which oxygen atom of the ester group takes part in this interaction. Figures 7 and 8 compare the spectra of A1Eta/EA and A1Et3/AA complexes with those of the corresponding free CA. From the data obtained it follows that in the presence of AIEta the carbonyl-oxygen absorption band is split into two bands, one of which corresponds to the carbonyl oxygen of the free CA (1752 cm-1), the other one corresponding to the carbonyl oxygen of the CA in the complex with A1Et3 (1678 cm-Z). The absorption band of the ether oxygen (1230cm -1) is shifted towards higher frequencies (1320cm-1), suggesting the absence of complexing between this group and A1Et3, since such interaction would cause the reverse effect. The results obtained lead to the conclusion that the complexing between AIEta and the ester investigated occurs only with the carbonyl oxygen, and the shift in the absorption band of the ether oxygen is caused by this interaction. These facts agree with the information obtained from the study of the i.r. spectra of benzoic and caprylic esters complexes with AIEt3 15 A quantitative study of the complexing reaction with i.r. spectroscopy was conducted with EA. The presence of the absorption band in the 1752cm -1 region with [EA]/[A1Eta] ~<1 (Figures 9A and B)indicates the existence of non-complexed EA, i.e. the reversibility of the complexing reaction. The band intensity is low, which suggests a large value of Keq. At [EA]/[AIEta] > 1 a sharp increase in the intensity of this band occurs (Figure 9C). On this basis we can state that stoichiometry of the formation of the AIEt3/EA complex corresponds to I : 1. We have estimated Keq from the intensity of the free EA band using equation (6). The value of the constant obtained is 7 x 1031mol -1 at 25°C. To compare the Keq value obtained under model conditions (at 25°C) and in the kinetic experiments (at 0°C), it was calculated for 25°C. From the usual equations - RTlnKeq = A H - TAS (using A H = - 9.2 kcal/mol and AS= -- 14.8 e.u.) Keq(25oc) was estimated to be 2 x l0 a lmol -z. Sufficiently good agreement of the equilibrium constant obtained for the reaction of EA with AIEta by two independent methods confirms both the validity of the proposed kinetic scheme and shows that the kinetic data obtained can be used for the calculation of this value. As may be expected, one of the reasons for different behaviour of EA and VA in the initiation is connected with a much larger stability of the A1Eta complex with the polar monomer• This, in turn, could be due to chelate formation (with the participation of double bond). It is clear that such a possibility is also valid for the case of
* For the calculation of Keq the value ko = 1.41 × 10-2 (I mol-Z) °'75 sec-z was used, i.e. the same value as in the calculation of Keq for AA. This led to a change in Kea for EA compared to that given by K o p p and Milovskayas
292
POLYMER, 1972, Vol 13, June
Ii ii
I t
m r
j
r' ,
rt
|
t
J
t~ t I
I |
Figure 7 Infra-red spectra of EA complexed with AIEta ( of free EA ( . . . . . . .
i
;I i I i
) and
) in octane at 25°C
,,',
P
i
II
Figure8
Infra-red spectra of A A complexed with AIEta ( ~ ) of free A A ( . . . . . . . ) in octane at 25°C
and
A
Figure 9 Infra-red spectra of the complex AIEta-EA (100mol/I) in octane at 25°C. (A) Molar ratio [EA]/[AIEta]=0.9; EA=7.4mol/I ( ). (B) Molar ratio [EA]/[AIEta]=I '0; EA=8-0mol/I ( . . . . . . )• (C) Molar ratio [EAI/EAIEt3]=1.1; EA=8-8mol/I (. . . . . . . . )
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al.
0.6 o~ _OO
+
--:
0.4
0"2
O.O O'8
I.O
1.2
1.4
1.6
1.8
2.0
2 + log [ B u 2 0 ]
Figure 10 Dependence of the polymerization rate on the Bu~O concentration with the system AIEt3-Bu20-BP in the solvents octane and benzene at 0°C. [VC] = 8 mol/I ; [AIEta]= [BP] = 1 x 10-3
mol/I
1.0
centrations of the initiator components and of the monomer and at 20°C. (At 0°C, polymerization also occurs but, because of extremely low rates, the process cannot be studied over a sufficiently large range of pyridine concentrations.) It follows from the dependence of polymerization rates on the BuzO and Py concentrations (Figures 10 and 11) that the order for CA is -0"5, i.e. it corresponds to the same dependence as in the case of EA and AA. Hence we can affirm that for all the cases investigated the function of non-polymerized CA is uniform. Hence free-radical formation, i.e. the reaction proper between A1Et3 and BP, can be described by equation (5). On this basis and using equation (3), a calculation can be made of the overall rate constants (k~) of the processes with Bu20 and Py. These are 5'03 x 10-6 (1 mol-1) °'~5 sec -1 and 2.54 x 10- 4 (1 mol-a) °~5 sec -1 at 0°C for Py and BuzO, respectively. The data necessary for the calculation of the overall activation energy of the VC polymerization with the A1Et3-Py-BP and AIEt3-Bu20-BP systems are given in Figures 12a and b. The calculated values were 8-9 + 0.8 kcal/mol (Bu20) and 12-1 + 1 kcal/mol (Py).
0.8
a 3"0
0'6
3.2 3.4
0"4 0"2
0"4
0"6
0"8
3.6
I+log [Py]
3"8 Figure 11 Dependence of the polymerization rate on the Py
,
~
,
,
t,'N,~"
3"5
3"6
3.7
3"8
3-9
concentration with the system AIEt3-Py-BP in the solvents octane
and benzene at 20°C. [VC]=8mol/I; [AIEt3]=[BP]=5× 10-~mol/I
AA, but the kinetic studies showed that the initiation process is similar for EA and AA, which suggests that the chelate reaction cannot be responsible for the observed effect.
3.4 ~3n o
I
b 4,4
AlEtz/pyridine/benzoyl-peroxide peroxide systems
and AlEta/ether/benzoyl-
In a further study, the complexing agents used were selected on the basis of the stability of the complexes formed. Pyridine (py)16, whose complexes with A1R3 a r e among the most stable* and dibutyl ether (Bu20), whose complexes are much less stable, were chosen. A study of kinetics using these complexing agents was restricted to evaluating the dependence on the CA concentration. The process with Bu20 participation was investigated at 0°C. The appreciable complexing ability of Py, as compared with other complexing agents, was shown by the fact that polymerization had a noticeable rate only at higher con* Py unlike RaN does not form a low temperature initiating system with BP 17
4.6
4.8 5"0 5"2
3"4
3.5
3.6
3.7
to31r Figure 12 Dependence of the rate constant on the inverse temperature. (a) AIEt3-Bu~O-BP; (b) AIEta-Py-BP
POLYMER, 1972, Vol 13, J u n e
293
Polymerization of VC by AIEtz/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al, Table 2 Characteristics of the complexing reaction of EA, A A Bu20 and Py with AIEt8
CA EA AA Bu20 Py
Keqx 10-a at 0°C
(mol-z)
--AH (kcal/mol)
15 14 3 5000
9"2 9"8 13-0 19"4
-AS (e.u.) 14'8 17'0 32'7 40' 6
Table3 The line width (3H) in the AI(iBu)a. CA complexes 8H of Al(iBu)a=4.8 gauss CA
3H (gauss)
Bu20
6'5+0'4
EA AA
4.8+0'2
VA Bu20+EA Bu20+AA EA+AA
Bu20+VA EA+VA AA+VA
5.2+0.2 4.1_+0.3 4'8_+0"4 4.8_+0"3 5.3_+0.2 4.9_+0.2 4.6_+0.3 5.2_+0.2
The Keq and AH values of the complexing reaction for these complexing agents were calculated from kinetic parameters (k 0 and Ein) in the same manner as for the AA system. The thermodynamic values obtained are given in Table 2 which also presents, for comparison, the information about EA and AA. Values for AH of the reaction with Bu20 and Py as estimated from kinetic data are in a good agreement with those obtained from calorimetric measurements under model conditions 16, is. Thus the AH values of the complexing reaction of A1Etz with EA and AA calculated in the same manner can be considered as valid; published data for these CA are unavailable. The Keq values in Table 2 enable the electron donors studied to be arranged in the following series, according to the stability of their complexes: Py~EA~AA>Bu20
(12)
To confirm the correctness of these suggestions we have studied the n.m.r, spectra on 27A1 nuclei of A1Rz complexes with different Lewis bases. Al(iBu)z was used as the organo-aluminium compoundt. In n.m.r, spectroscopy on 27A1 the complex formation leads to a change in the 27A1 nuclear quadrupolar relaxation time and to the corresponding change in the line width 19, 2o. In Table 3 peak-to-peak 27A1 n.m.r, line widths (3H) are presented for free Al(iBu)a and for Al(iBu)a with two moles of ligand added. (The line width depends only slightly on CA concentrations in the range 1-4mol/mol Al(iBu)a 19.) In the same cases 3H values are quite different and it is possible to compare the abilities of the two ligands, CA1 and CA2, to compete for a place in the complex. In this case one can measure 3H for the t It is possible to extend the data obtained with Al(iBu)3 to the reactions of CA with A1Eta, since it has been shown that aluminium alkyls behave in the same manner both in the polymerization processa, 5 and under model conditions7, a
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P O L Y M E R , 1972, Vol 13, J u n e
AI(iBu)z+CAI+CAz system. If a fast exchange takes place, the preference of the AI(iBu)z.CA1 complex will be shown by the closeness of the 8H values for the Al(iBu)z+ CA1 + CA2 and AI(iBu)z.CA1 systems (Table 3). The complexing agents studied can be arranged according to their complexing ability: EA~AA>Bu20
(13)
The established regularity agrees with that obtained in the polymerization process. The kinetic investigation (the results of which are independently confirmed) showed that the mechanism of initiation by the AIEtz-CA-BP systems, where CA is a non-polymerized polar compound, is always the same and does not depend on the complexing ability of the electron donor used. The function of the electron donor is to reduce the concentration of free aluminium alkyl: this prevents the polymerization from being instantaneous since the free-radical formation is a result of the interaction of the uncomplexed form of A1Rz with peroxide.
Role of polar monomers in the initiation The reason for the different behaviour of nonpolymerizable electron donors and the polar monomer (VA) remained unsolved. The difference in the kinetic schemes in the A1Et3-BP and A1Etz-VA-BP systems prevents estimation of Keq of reaction (8) from kinetic data. The independence of the rate of VC polymerization initiated by the A1Et3-VA-BP system (at [VA]> [AIEt3]) on the VA concentration (Figure 1) suggests a relatively high value for Keq of reaction (8). An attempt to estimate this value by the i.r. method did not yield the desired data since under experimental conditions, T=25°C, [VA]= [A1Etz], a comparatively rapid irreversible consumption of [VA] is observed. To find where VA comes in the established series of basicity of CA we have estimated the stability of its complex with Al(iBu)z by 27Al-n.m.r. and by p.m.r, methods. The data from 27Al-n.m.r. (Table 3) showed that, by its complexing ability, VA is situated in the middle of the series (eqn. 13). Additional data on the Al(iBu)z behaviour in the presence of polar agents were obtained from p.m.r. High resolution p.m.r, spectroscopy shows complex formation from the shielding of Al(iBu)z methylene protons, caused by electron donation to the aluminium atom from the ligand hetero-atom. Figure 13 shows the dependence of the internal chemical shift of Al(iBu)3 (A = ~-erI2 - ~'eHa) on the VA concentration. Almost identical plots were obtained for all the ligands studied: CA (EA, Bu20, AA) and the polar monomers (VA, MMA, AN). The data presented suggest the formation of a 1 : 1 type labile complex with a short life time ( < 0.1 sec). This is proved by the coalescence of the free and coupled Al(iBu)a p.m.r, signals into one common signal with the averaged A value. The same pattern was maintained for different ether-AIRa complexesZl-2a. Another pattern is observed for Py. Here the p.m.r. signal coalescence does not take place owing to a more rigid complex formation with life time exceeding one second. This information is consistent with published data on the behaviour of the complex of AIEt3-Py 21. The data suggest that the reason for the unusual behaviour of VA
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al. 1,0
/
0.9 I
/
0.8
0.7 o
X
×
i
i
I
l
2
3
[VA]/
[ A I ( i B u ) 31
Figure 13
Dependence of the internal chemical shift forAl(iBu)8 (A=rcH ~ -rCHs) on the molar ratio of the VA and Al(iBu)3 concentrations. The concentration of Al(iBu)3 is 10% by wt. in benzene
,,^~ i
suppose that in the formation of the complex between A1Ra and the polar monomer a certain polarization of its double bond occurs followed by the electron cloud shift towards the electron acceptor, aluminium alkyl. This process is obviously feasible for the monomers having a direct conjugation between the double bond and the hetero-atom, i.e. for A N and MMA. In the VA molecule the movement of the double bond electron cloud can, obviously, occur through the uncoupled electron pair of the other oxygen (as an inductive effect). The decrease in the electron density on the monomer double bond in the complex is shown by the example of MMA.A1Et3 using i.r. spectroscopy. Figure 14 gives a comparison of the i.r. spectral regions of M M A and the complex A1Et~. MMA. It is established that the vibration band of the C = C double bond normally at 1632cm -1 is shifted to lower frequencies (1624 cm -1) in the complex. The distortion of the double-bond symmetry leads to a sharp increase in the intensity of the corresponding band. (With i.r. spectroscopy we failed to find a similar shift in the VA molecule: this might be connected with the difference in the structure of these monomers.) Hence the complex A1Et3. M is a kind of bifunctional compound in which the electron-deficient centres are represented by aluminium alkyl and the double bond of the complexing monomer. We may assume that in the subsequent reaction with BP (a bi-functional electron donor) this complex acts as a single bi-accepting compound. The transition state of the interaction can be schematically shown as follows:
i r f R3
Figure 14
Infra-red spectra of MMA complexed with AIEt3 and of free MMA in octane at 25°C. Molar ratio [MMA]/[AIEt3]=I ( - - ) ; M M A = 8 x 10-2mol/I; free MMA ( . . . . . . . )
,,c !
where X= - C = iq O+ C H 2
in the initiation process is not connected with the stability of the complex. The experimental data available indicate another possibility--a fundamentally different mechanism of interaction between AIEtz and BP in the presence of a polar monomerL Both the information obtained under simulated conditions 7, 8 and the kinetic results (obtained in an earlier investigation and in the present research) show that stoichiometry of the BP reaction corresponds to 1 : 2, i.e. the interaction with both carbonyl oxygens is necessary for the rupture of the peroxide bond. As already stated, the same expression was obtained for the initiation for all the polar monomers (equation la). This enables us to pass from a particular case of VA to the general analysis of the phenomenon*. It is natural to * It is not possible to use other polar monomers (AN and MMA) in catalytic amounts in the VC polymerization with these systems because of a rather unfavourable ratio between the copolymerization constants of VC-AN and VC-MMA. Using the catalytic system with MMA a simultaneous occurrence of the initiation and inhibition was established
/ .-
""
_O_C
z, ~ ~
o o')
o
LJ~c
~
~0
~'CH 3
- - c ~°
I
\ OCH3
C6Hs
Stoichiometry of this interaction corresponds to the experimentally established ratio A1Eta : BP equal to 1 : 1 (see equations la and 9). In accordance with the proposed interaction scheme not only the alkyl radicals (the only radicals obtained under the experimental conditions 8) but also benzoate radicals are formed. Occurrence of the latter was shown with P M M A a for which the chain transfer reaction to BP is known to be absent 24, therefore the presence of terminal benzoate groups in the polymer must be entirely due to the initiation reaction. The fact that the rate constant of the initiation reaction is strongly dependent on the nature of the medium provides a certain confirmation of the conception of the free-
POLYMER,
1972, V o l 13, J u n e
295
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system: E. B. Milovskaya et al. radical formation resulting from the interaction between the highly polarized particles. Thus with a complete agreement of the kinetic orders for the initiator components (equations la and 9) and for the Ein values, the kin values with the AIEta-BP system in the VC medium in the presence of catalytic amount of VA, and in the VA medium are 7 . 2 5 x 1 0 - a l m o l - l s e c -1 and 7.2x10-51 mo1-1 sec -1 (ref. 1), respectively. Since the systems studied are efficient as initiators for the low temperature polymerization, analogous systems based on other organometallic compounds might also be effective. As was shown recently in similar experiments, the replacement of M - C bonds (where M is B, AI etc.) by M - O bonds is energetically favourable ~5,~6. The energetic gain must be still greater in the interaction between metal alkyls and acyl peroxide. In particular, in the interaction between aluminium alkyls and BP the driving force of the free radical's formation is the transformation of the covalent A I - C bond into the partly ionic bond A I - O C O -5. It is quite evident that the advantages of this process will be principally determined by the metallicity of the central atom. Thus it has been established that a trialkyl boron with BP does not yield a system of high activity 27. No data exist about the use of organic compounds of Zn and Cd in this combination but we may assume that they will be less active than organoaluminium compounds for the same reason. Conversely, organic compounds of alkaline and alkaline earth metals can hardly be used with acyl peroxide as initiating systems. In this case the ionic contribution to the M - C bond is too great. For instance, this causes the interaction with BP to develop as a heterolytic process rather than a homolytic one 2s. Besides, we have found that the reaction of MgR2 with BP, even in the presence of a considerable excess of an electron donating compound (dioxan), occurs practically instantaneously. Thus this interaction cannot be a source of continuous formation of free radicals.
296
POLYMER, 1972, Vol 13, June
REFERENCES 1 Milovskaya, E. B. and Zhuravleva, T. G. Vysokomol. Soedin. 1964, 6, 1035 2 Milovskaya, E. B. and Zamoiskaja, L. W. Vysokomol. Soedin. 1965, 7, 670 3 Milovskaya, E. B., Zamoiskaja, L. W. and Winogradowa, S. I. Fur. Polym. J. 1970, 6, 1589 4 Milovskaya, E. B. and Zhuravleva, T. G. Vysokomol. Soedin. (A) 1967, 9, 1128 5 Milovskaya, E. B., Zhuravleva, T. G. and Zamoiskaja, L. W. J. Polym. Sci. (C) 1967, 16, 899 6 Kopp, E. L. and Milovskaya, E. B. Vysokomol. Soedin. (A) 1969, 11,750 7 Milovskaya, E. B., Pokrovsky, E. I. and Fedorova, E. F. lzv. Akad. Nauk S.S.S.R., Ser. Khim. 1967, p 1093 8 Zamoiskaja, L. W., Milovskaya, E. B. and Orestova, W. A. Izv. Akad. Nauk S.S.S.R., Ser. Khim. 1970, p 2053 9 Kopp, E. L. and Milovskaya, E. B. Izv. Akad. Nauk S.S.S.R., Ser. Khim. in press I0 Weisberger, A. 'Organic Solvents', Interscience, New York, 1955, Vol VII 11 Razuvaev, G. A. and Graevsky, A. I. DokL Akad. Nauk. S.S.S.R. 1959, 128, 309 12 Myers, O. and Pitzer, E. J. AppL Phys. 1959, 30, 1987 13 Danusso, F. and Sianesi, D. Chim. Ind. 1955, 37, 695 14 Burnett, G. and Wright, W. Proc. R. Soc. 1954, 231, 28 15 Pasynkiewiez, S. and Starowieyski, K. Roczn. Chem. 1967, 41, 1139 16 Bonitz, E. Chem. Ber. 1955, 88, 742 17 Margaritova, M. F. and Rysanova, K. A. Vysokomol. Soedin. (A) 1969, 11, 2741 18 Emerson, W. and Ramirez, E. Anal. Chem. 1965, 37, 806 19 Poole, C. P., Swift, H. E. and Itzel, Y. E. J. Chem. Phys. 1965, 42, 2576 20 Swift, H. E., Poole, C. P. and Itzel, Y. E. J. Phys. Chem. 1964, 68, 2509 21 Takashi, Y. Bull. Chem. Soc. Japan, 1967, 40, 612 22 Takeshita, T. and Frankle, W. Tetrahedron Lett. 1968, 5913 23 Hatade, K. and Yuki, H. Tetrahedron Lett. 1968, p 213 24 Baysal, B. and Tobolsky, H. J. Polym. Sci. 1952, 8, 529 25 Davies, A. and Roberts, B. J. Organomet. Chem. 1969, 19, 17 26 Krusic, P. and Kochi, J. J. Am. Chem. Soc. 1969, 91, 3942 27 Contreras, J., Grotewold, J., Lissi, E. and Rozas, R. J. Polym. Sci. (A-l) 1969, 7, 2341 28 Lawesson, S. and Wang, W. J. Am. Chem. Soc. 1959, 81, 4230
INTRODUCTION In recent years systems based on aluminium alkyls and acyl peroxides have been used to initiate the polymerization of the vinyl monomers--vinyl acetate (VA)~, 2, methyl methacrylate (MMA) 3, acrylonitrile (AN) 4 and vinyl chloride (VC) 5, 6. The process involves free-radical polymerization and can be carried out at much lower temperatures than the thermal decomposition of peroxide. The efficiency of these systems lies in the separation of the initiating system components by the polar agent or the monomer since direct reaction between A1Ra and peroxide is explosive 7, s. The electron-donating ability of such monomers as VA, AN and MMA proves to be sufficient for the development of polymerization. For all the systems investigated the same expression has been derived for the dependence of the overall rate of the process on the concentrations of the monomer and of the initiator components: V= k[M]l'o-z'2[A1Rz]O'5[P]O'5
(1)
where M is AN, MMA and VA, A1R3 is A1Et3 and Al(iBu)3, P is benzoyl peroxide (BP) and dicyclohexyl peroxidicarbonate (PC).
288
POLYMER, 1972, Vol 13, June
On the basis of the kinetic results and the fact that the monomer should participate in the initiation, the formation of free radicals has been represented as a series of reactions 5: fast
AIRs+M
BP
> A1Rs"M
• (AIR3M.BP) kl
> k2
A1R2COOPh + R" + PhCOO" + (M) (2) Under the condition that kl
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al. overall rate of the process in the presence of catalytic amounts of EA on the concentrations of the monomer, EA, and the initiator components has the form: {[A1Et3]'~ °'5 V=k~[VC] 1"21 [E~]-/! [BP] °'25
(3)
A similar dependence on the concentration o f monomer and initiator components was shown for the case of VC polymerization without CA. The mechanism for the initiation reactions is: Keq A1Etz + E A . • AIEta. EA (4) KI KII 2AIEt3 + BP ~ " [(A1Et3)eBP] -, " k5 2(AIEtzCOOPh)" ~ 2A1Et2COOPh+2Et" (5) If the equilibria with K~ and Ku shifts to the left, the equilibrium of reaction (4) shifts to the right and the overall rate of interaction (5) is determined by ks, the expression for the initiation reaction rate in the system with EA can be written: Via = kin [A1Et3]rBP10.5 ~X-]- ~ ~
(3a)
From the results obtained, it follows that the role of EA is to reduce the initial concentration of aluminium alkyl by complex formation (equation 4); free-radical formation occurs by reaction of uncomplexed aluminium alkyl with peroxide (i.e., it occurs in the same manner as under model conditions7-9). Comparison of the kinetic dependence for the polymerization of polar' monomers (equation 1) with that found for VC polymerization in the presence of EA (equation 3), and comparison of reactions responsible for the initiation (reactions 2 and 4-5) both showed that EA did not simulate the polar monomer function in the initiation. Further investigations were carried out to determine the true role of the polar monomer in initiation. It is clear that this problem cannot be solved under the conditions of the polymerization of the polar monomer; therefore, an artificial device was used, namely VC polymerization was studied in the presence of catalytic amounts of a polar monomer (VA). On the other hand. it was interesting to establish to what extent the reactions for the EA system are valid for different classes of CA. Our results allowed us to obtain general ideas about the mechanism of free-radical formation in the A1Rz/acyl peroxide systems in the presence of electron-donors, including polar monomers.
EXPERIMENTAL Experiments were conducted in the absence of moisture and air. The solvents benzene and octane were purified in the usual way and then dried with calcium hydride and concentrated butyl lithium. The complexing agents were purified according to the methods 1° previously described and then dried with calcium hydride and distilled under vacuum.
VA was pre-polymerized with the AI(iBu)a-BP system. At low concentrations these compounds were used in octane solution. Commercial A1Eta was used without additional purification in octane solution. Its concentration was determined according to the method of Razuvaev and Graevsky 11. The concentration of the solution of the complexing compound was determined from the AIEt3 solution of a known concentration. Al(iBu)3 was distilled under vacuum and the fraction boiling at 41 °C (1 mmHg) was used. Benzoyl peroxide was purified from its chloroform solution, the active oxygen content being 98.5-99 ~ . The BP-benzene solution was added. VC was carefully degassed, dried at - 3 0 ° C over the concentrated butyl lithium, the polymerization being allowed to go to 3-5 ~o conversion. VC was then pre-polymerized using the A1Et~-Bu20-BP system. The purity of the monomer was checked chromatographically. Polymerization was carried out in a single-cell dilatometer (with a magnetic stirrer) having a capacity of about 15 ml and with a capillary calibrated to 0.01 ml. The dilatometer, previously heated under vacuum, was filled with argon and the AIEtz and CA solutions and octane were introduced from Schlenk vessels supplied with a needle. The monomer was added in weighed amounts from the balloon through the needle. The order in which the components were added was always the same: CA, A1Eta, solvent, monomer. The temperature of the reaction mixture was thermostatically controlled to within + 0" 1°C, and the BP solution was injected with a syringe. In the kinetic experiments the polymerization rate was calculated for a 3 - 5 ~ conversion, this being estimated from contraction. The contraction coefficient was found as a mean value from several experiments on the basis of gravimetric measurements of the polymer yield. Infra-red spectra were taken using a UR-10 spectrometer. Sealed cells of potassium bromide were used to protect the organo-aluminium compounds from moisture and air. The cell was carefully dried under vacuum and filled with the previously prepared solutions of the analysed components in octane. The equilibrium c o n s t a n t ( K o q ) of reaction (4) was calculated from : Keq = [AlEtz.__EA] [EA][AIEta]
(6)
The following expression was used to calculate the concentration of free EA [EX] from the 1752cm -1 band intensity: _ Ex[EA]0 [E-A] E0
(7)
where [EA] 0 is the initial EA concentration; E0 is the molar extinction coefficient of the absorption band of the EA carbonyl oxygen, equal to (562_+ 14) 1 mol -x cm-l; Ex is the extinction coefficient of the 1752cm t band in the spectra of the complex. The concentration of the A1Et3.EA complex was determined as the difference between [EA]0 and [EA], and the concentration of [AIEt3] from {[AIEt3]0[AIEta. EA]}. The preparation of samples for proton magnetic resonance (p.m.r.) and ZTAl nuclear magnetic resonance
POLYMER, 1972, Vol 13, June
289
Polymerization of VC by AIEta/benzoyl-peroxide/Lewis-base system : E, B. Milovskaya et aL Table 1 VC polymerization with the AIEt3-BP system in the absence and presence of CA. [VC]=8mol/I; solvents: octane, benzene; temperature= 0°C
0.7 x 0-6 o 0.5
Concentration x 102 (mol/I)
Conversion (%)
~- 0"4
Exp. No.
CA*
AIEt3
CA
BP
lh
2h
4h
1 2 3
VA EA --
1 "0 5"0 5.0
16'0 25'0 --
1-0 5"0 5.0
11 "8 30"0 2.2
15-6 45-5 2"9
18'1 56'6 3'3
0"3' O-2
RESULTS AND DISCUSSION
AIEt3/vinyl-acetate/benzoyl-peroxide system In the presence of catalytic amounts of VA the AIEt3BP system can be successfully used for VC polymerization. Table 1 shows the data obtained and gives the results for the system without CA and in the presence of EA. The reaction kinetics were studied at 0°C. The rate was estimated gravimetrically from the polymer yield. When the ratio [VA]/[A1Et3] is equal to or greater than one, the rate practically coincides with the value calculated from the contraction. Figure 1 shows the rate dependence of the VA concentration, from which one can conclude that under the experimental conditions the composition of the complex resulting from the reaction between AIEt3 and VA is 1 : 1 (8)
With the same concentration ratio of VA and AIEt3 the rate is fixed by the initial concentration of A1Et3, from which it follows that the free-radical formation under given conditions is a result of the interaction between benzoyl peroxide and the A1Eta.VA complex. When the concentrations of VA are lower than those of AIEt3 two processes occur simultaneously (with comparable rates): they result from the reaction of BP with free aluminium alkyl and with aluminium alkyl complexed with VA. The dependence in Figure 1 for the region of [VA]/[A1Et3] < 1 can be explained by the fact that the rate was estimated from the polymer yield and the interaction
POLYMER,
1972, V o l 13, J u n e
i
O'5
I
i
i
0"7
I
O.9
I
I
I.I
, s I
I
1.3
I
I
1.5
I
:: I
1.7
I
I
1.9
Figure I Dependence of the polymerization rate on the VA
(n.m.r.) study was carried out as follows: 10~o benzene solutions of Al(iBu)3 with a certain amount of added CA were placed into the sample tubes--5 mm and 18 mm outside diameter for p.m.r, and 27Al-n.m.r., respectively --which had been dried at high temperature in vacuum. All operations including sealing the tubes were performed under vacuum. The samples were stored in liquid nitrogen and n.m.r, spectra were recorded (4-5 times) just after the samples had reached room temperature. High resolution p.m.r, spectra were recorded with a YEOL YNM-3 spectrometer (40 MHz). No changes were observed in the spectra for one hour. Weak 27Al-n.m.r. signals were observed with a RYa-2301 spectrometer (8 MHz) using phase detection techniques. The peak-topeak line widths were measured allowing for overmodulation 12. Modulation amplitude and frequency were equal to 1.5 gauss and 35 Hz, respectively. The speed of field sweep was equal to 50mg/sec and response time was 20 sec.
290
i
'' 3+log { VA ]
* CA = complexing agent
Keq AIEta + V A . • AIEta. VA
111,1:/.:
concentration with the system AIEta-VA-BP in the solvents octane and benzene at 0°C. [VC]=4mol/I; [AIEta]=lxl0-2mol/I; [ B P ] = 5 x 10-3 mol/I
~r
o x
[AIEt3] °'s [ B p ] ° ' S x iO 3
Figure 2 Dependence of the polymerization rate on the concentration of the initiator components in the system AIEt3-VA-BP in the solvents octane and benzene at 0°C. [VC]=4mol/I; [ V A ] =
5x 10-2 mol/I
of large amounts of free AIEta with BP was followed by the formation of considerable amounts of oligomers 6. In order to avoid the superposition of two effects, a further study of the kinetics was made in the concentration range 10> [VA]/[AIEta] > 1. (As noted, the region of low concentration ratios is characterized by appreciable oligomerization, whereas at large values one should take into account the copolymerization processes.) Figure 2 demonstrates the rate dependence of the process on the concentration of the components of the initiation system. From these data, under conditions of bimolecular termination of growing chains, the expression for the initiation reaction rate may be written as follows: Vin = ka[AlEta] [BP]
(9)
Figure 3 gives the dependence of the overall rate constant of the process on the inverse temperature from which E o is estimated to be 9.7 + 1.2 kcal/mol, Ein being 16"5kcal/ tool. In the calculation of Ein we used the value (Ep - l e t ) = 1.5 kcal/mol for VC 13, 14 The kinetic results obtained enable the initiation in the AIEtz-VA-BP system to be represented as a sequence of reactions expressed by reaction (2). The similarity of the kinetic results (equations la and 9) and the Ein value for the A1Eta-VA-BP system with the corresponding parameters, as established in the polymerization proper of VA 1, is evidence that in both cases the free radicals are formed by the same mechanism. Thus, this work suggests that in the presence of a polar monomer (VA) the free radicals are formed as a result of reaction between the complexed form of aluminium alkyl with peroxide,
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al. 1.6
free-radical formation in the systems with the participation of these CA is the result of the same reactions (4 and 5). As mentioned above, a similar mechanism of the interaction of the initiator components is also realized without CA. This permits one to calculate, from the kinetic data, the thermodynamic parameters for the complexing reaction of AA with A1Et3 (reaction of type 4), the equilibrium constant (Keq) and the enthalpy (AH).
1"8
2"0 2-2 ~3n o
2"4
I
-->.
2'6 2"8 3.5
3.6
3.7
3.8
3.9
4.O
/% = (2f)1/2 kl k~, t ~ . klii'~2 in the absence of
and
/,p
tO3/T perature for the system AIEt3-VA-BP
¥
L'I
I.,FI
0"9 0"7
I
0"8
I
I.O
i
l
l.i. ~ /t
\ [ ,~) -
ki'=(2f)l"~ k] '~ ' \K,,~
Figure 3 Dependence of the rate constant on the inverse tem-
i
1.2
I
1.4
i
i
t
CA
in the presence of CA
Using the k 0 values without CA equal to 1.41 x 10 ..2 (I tool 1)0.75sec-1 (corrected as compared with the value given in reference 6), and k o with AA as given above, the value of Keq is obtained as 1.4 × 1041 mol I-1 at 0°C. (The assumption about the equality of f efficiencies of the initiation in the absence and presence of CA makes this calculation somewhat tentative.) The value for the heat of complexing reaction is derived from:
_ _
Ein (without CA) = Ein (with AA) + AH
1.6
(11)
Taking Ein (without CA) = 1"8 kcal/mol (ref. 6) the value of AH is - 9.8 kcal/mol.
3 + l o g [AA]
Figure 4 Dependence of the polymerization rate on the A A concentration with the system AIEt3-AA-BP in the solvents octane and benzene at 0°C. [VC]=4 mol/I; [AIEt3]= [BP]= 5 x 10-3 mol/I
3"0
2"0
whereas for EA this process is described by the reactions of the SNI type.
•
f
r""
x 2x
I.O
AIEtz/allyl-acetate/benzoyl-peroxide system The reason for the sharp difference in behaviour of compounds of similar structure was investigated. The complexing agent used was allyl acetate (AA), a compound combining all the structural features of EA and VA. AA itself is not polymerized under the influence of the AIEt3-BP system, but its presence in catalytic amounts ensures the effective polymerization of VC. Thus, under conditions similar to those in Table 1 (experiment 2) the polymer yield is 25.3, 38.3 and 46.4 ~o. The kinetics of VC polymerization with the A1Et 3-AA-BP system was studied at 0°C. Figure 4 shows the dependence of the rate of the process on the AA concentration; Figure 5 shows the same dependence on the concentration of the initiator components. On the basis of the above data the expression for the initiation reaction rate in this system can be written as:
Vin~-~-/~.10[AIEt3]rnp]0.5 [/&x/~k]L ~ - - ,
I
I
I.I
1.3
I
1"5
I
1.7
1.9
I
2"1
0.5
[ AIEt3 ] [EA]
[ B P ] ° 2 S x IO
Figure 5 Dependence of the polymerization rate on the concentration of the initiator components in the system AIEt3-AA-BP in the solvents octane and benzene at 0°C. [VC]-- 4 mol/I; molar ratio AA/AIEt3= constant= 4 3.4 3.6 3-8
(10)
4.0
The overall constant of the process is 1.18x 10-4 (1 mol 1)0.2s sec 1 at 0°C. The overall activation energy for the VC polymerization is estimated to be 7.3 _+ 1.0 kcal/mol (Figure 6), hence Ein is 11"6kcal/mol. From the equality of the expressions obtained for EA (equation 3a) and for AA (equation 10) it follows that the
4.2
I
--
3.5
I
3.6
___~
3-7
,L
3-8
103/T Figure 6 Dependence of the rate constant on the inverse temperature for the system AIEt3-AA-BP
POLYMER,
1972, V o l 13, J u n e
291
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et aL The values of the heat effect and the equilibrium constant of the complexing reaction of A1Et3 with AA practically coincide with the corresponding parameters established for EA ( A H = -9.2kcal/mol; Keq= 1.5 × 104 lmo1-1 *), which suggests a rather similar electrondonating ability for these compounds. Infra-red spectra of CA complexes with AIEtz were studied to check the conclusions, obtained from kinetic data, about reversibility and the stoichiometric relations of the complexing reaction of EA and AA with AIEta and also to find which oxygen atom of the ester group takes part in this interaction. Figures 7 and 8 compare the spectra of A1Eta/EA and A1Et3/AA complexes with those of the corresponding free CA. From the data obtained it follows that in the presence of AIEta the carbonyl-oxygen absorption band is split into two bands, one of which corresponds to the carbonyl oxygen of the free CA (1752 cm-1), the other one corresponding to the carbonyl oxygen of the CA in the complex with A1Et3 (1678 cm-Z). The absorption band of the ether oxygen (1230cm -1) is shifted towards higher frequencies (1320cm-1), suggesting the absence of complexing between this group and A1Et3, since such interaction would cause the reverse effect. The results obtained lead to the conclusion that the complexing between AIEta and the ester investigated occurs only with the carbonyl oxygen, and the shift in the absorption band of the ether oxygen is caused by this interaction. These facts agree with the information obtained from the study of the i.r. spectra of benzoic and caprylic esters complexes with AIEt3 15 A quantitative study of the complexing reaction with i.r. spectroscopy was conducted with EA. The presence of the absorption band in the 1752cm -1 region with [EA]/[A1Eta] ~<1 (Figures 9A and B)indicates the existence of non-complexed EA, i.e. the reversibility of the complexing reaction. The band intensity is low, which suggests a large value of Keq. At [EA]/[AIEta] > 1 a sharp increase in the intensity of this band occurs (Figure 9C). On this basis we can state that stoichiometry of the formation of the AIEt3/EA complex corresponds to I : 1. We have estimated Keq from the intensity of the free EA band using equation (6). The value of the constant obtained is 7 x 1031mol -1 at 25°C. To compare the Keq value obtained under model conditions (at 25°C) and in the kinetic experiments (at 0°C), it was calculated for 25°C. From the usual equations - RTlnKeq = A H - TAS (using A H = - 9.2 kcal/mol and AS= -- 14.8 e.u.) Keq(25oc) was estimated to be 2 x l0 a lmol -z. Sufficiently good agreement of the equilibrium constant obtained for the reaction of EA with AIEta by two independent methods confirms both the validity of the proposed kinetic scheme and shows that the kinetic data obtained can be used for the calculation of this value. As may be expected, one of the reasons for different behaviour of EA and VA in the initiation is connected with a much larger stability of the A1Eta complex with the polar monomer• This, in turn, could be due to chelate formation (with the participation of double bond). It is clear that such a possibility is also valid for the case of
* For the calculation of Keq the value ko = 1.41 × 10-2 (I mol-Z) °'75 sec-z was used, i.e. the same value as in the calculation of Keq for AA. This led to a change in Kea for EA compared to that given by K o p p and Milovskayas
292
POLYMER, 1972, Vol 13, June
Ii ii
I t
m r
j
r' ,
rt
|
t
J
t~ t I
I |
Figure 7 Infra-red spectra of EA complexed with AIEta ( of free EA ( . . . . . . .
i
;I i I i
) and
) in octane at 25°C
,,',
P
i
II
Figure8
Infra-red spectra of A A complexed with AIEta ( ~ ) of free A A ( . . . . . . . ) in octane at 25°C
and
A
Figure 9 Infra-red spectra of the complex AIEta-EA (100mol/I) in octane at 25°C. (A) Molar ratio [EA]/[AIEta]=0.9; EA=7.4mol/I ( ). (B) Molar ratio [EA]/[AIEta]=I '0; EA=8-0mol/I ( . . . . . . )• (C) Molar ratio [EAI/EAIEt3]=1.1; EA=8-8mol/I (. . . . . . . . )
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al.
0.6 o~ _OO
+
--:
0.4
0"2
O.O O'8
I.O
1.2
1.4
1.6
1.8
2.0
2 + log [ B u 2 0 ]
Figure 10 Dependence of the polymerization rate on the Bu~O concentration with the system AIEt3-Bu20-BP in the solvents octane and benzene at 0°C. [VC] = 8 mol/I ; [AIEta]= [BP] = 1 x 10-3
mol/I
1.0
centrations of the initiator components and of the monomer and at 20°C. (At 0°C, polymerization also occurs but, because of extremely low rates, the process cannot be studied over a sufficiently large range of pyridine concentrations.) It follows from the dependence of polymerization rates on the BuzO and Py concentrations (Figures 10 and 11) that the order for CA is -0"5, i.e. it corresponds to the same dependence as in the case of EA and AA. Hence we can affirm that for all the cases investigated the function of non-polymerized CA is uniform. Hence free-radical formation, i.e. the reaction proper between A1Et3 and BP, can be described by equation (5). On this basis and using equation (3), a calculation can be made of the overall rate constants (k~) of the processes with Bu20 and Py. These are 5'03 x 10-6 (1 mol-1) °'~5 sec -1 and 2.54 x 10- 4 (1 mol-a) °~5 sec -1 at 0°C for Py and BuzO, respectively. The data necessary for the calculation of the overall activation energy of the VC polymerization with the A1Et3-Py-BP and AIEt3-Bu20-BP systems are given in Figures 12a and b. The calculated values were 8-9 + 0.8 kcal/mol (Bu20) and 12-1 + 1 kcal/mol (Py).
0.8
a 3"0
0'6
3.2 3.4
0"4 0"2
0"4
0"6
0"8
3.6
I+log [Py]
3"8 Figure 11 Dependence of the polymerization rate on the Py
,
~
,
,
t,'N,~"
3"5
3"6
3.7
3"8
3-9
concentration with the system AIEt3-Py-BP in the solvents octane
and benzene at 20°C. [VC]=8mol/I; [AIEt3]=[BP]=5× 10-~mol/I
AA, but the kinetic studies showed that the initiation process is similar for EA and AA, which suggests that the chelate reaction cannot be responsible for the observed effect.
3.4 ~3n o
I
b 4,4
AlEtz/pyridine/benzoyl-peroxide peroxide systems
and AlEta/ether/benzoyl-
In a further study, the complexing agents used were selected on the basis of the stability of the complexes formed. Pyridine (py)16, whose complexes with A1R3 a r e among the most stable* and dibutyl ether (Bu20), whose complexes are much less stable, were chosen. A study of kinetics using these complexing agents was restricted to evaluating the dependence on the CA concentration. The process with Bu20 participation was investigated at 0°C. The appreciable complexing ability of Py, as compared with other complexing agents, was shown by the fact that polymerization had a noticeable rate only at higher con* Py unlike RaN does not form a low temperature initiating system with BP 17
4.6
4.8 5"0 5"2
3"4
3.5
3.6
3.7
to31r Figure 12 Dependence of the rate constant on the inverse temperature. (a) AIEt3-Bu~O-BP; (b) AIEta-Py-BP
POLYMER, 1972, Vol 13, J u n e
293
Polymerization of VC by AIEtz/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al, Table 2 Characteristics of the complexing reaction of EA, A A Bu20 and Py with AIEt8
CA EA AA Bu20 Py
Keqx 10-a at 0°C
(mol-z)
--AH (kcal/mol)
15 14 3 5000
9"2 9"8 13-0 19"4
-AS (e.u.) 14'8 17'0 32'7 40' 6
Table3 The line width (3H) in the AI(iBu)a. CA complexes 8H of Al(iBu)a=4.8 gauss CA
3H (gauss)
Bu20
6'5+0'4
EA AA
4.8+0'2
VA Bu20+EA Bu20+AA EA+AA
Bu20+VA EA+VA AA+VA
5.2+0.2 4.1_+0.3 4'8_+0"4 4.8_+0"3 5.3_+0.2 4.9_+0.2 4.6_+0.3 5.2_+0.2
The Keq and AH values of the complexing reaction for these complexing agents were calculated from kinetic parameters (k 0 and Ein) in the same manner as for the AA system. The thermodynamic values obtained are given in Table 2 which also presents, for comparison, the information about EA and AA. Values for AH of the reaction with Bu20 and Py as estimated from kinetic data are in a good agreement with those obtained from calorimetric measurements under model conditions 16, is. Thus the AH values of the complexing reaction of A1Etz with EA and AA calculated in the same manner can be considered as valid; published data for these CA are unavailable. The Keq values in Table 2 enable the electron donors studied to be arranged in the following series, according to the stability of their complexes: Py~EA~AA>Bu20
(12)
To confirm the correctness of these suggestions we have studied the n.m.r, spectra on 27A1 nuclei of A1Rz complexes with different Lewis bases. Al(iBu)z was used as the organo-aluminium compoundt. In n.m.r, spectroscopy on 27A1 the complex formation leads to a change in the 27A1 nuclear quadrupolar relaxation time and to the corresponding change in the line width 19, 2o. In Table 3 peak-to-peak 27A1 n.m.r, line widths (3H) are presented for free Al(iBu)a and for Al(iBu)a with two moles of ligand added. (The line width depends only slightly on CA concentrations in the range 1-4mol/mol Al(iBu)a 19.) In the same cases 3H values are quite different and it is possible to compare the abilities of the two ligands, CA1 and CA2, to compete for a place in the complex. In this case one can measure 3H for the t It is possible to extend the data obtained with Al(iBu)3 to the reactions of CA with A1Eta, since it has been shown that aluminium alkyls behave in the same manner both in the polymerization processa, 5 and under model conditions7, a
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P O L Y M E R , 1972, Vol 13, J u n e
AI(iBu)z+CAI+CAz system. If a fast exchange takes place, the preference of the AI(iBu)z.CA1 complex will be shown by the closeness of the 8H values for the Al(iBu)z+ CA1 + CA2 and AI(iBu)z.CA1 systems (Table 3). The complexing agents studied can be arranged according to their complexing ability: EA~AA>Bu20
(13)
The established regularity agrees with that obtained in the polymerization process. The kinetic investigation (the results of which are independently confirmed) showed that the mechanism of initiation by the AIEtz-CA-BP systems, where CA is a non-polymerized polar compound, is always the same and does not depend on the complexing ability of the electron donor used. The function of the electron donor is to reduce the concentration of free aluminium alkyl: this prevents the polymerization from being instantaneous since the free-radical formation is a result of the interaction of the uncomplexed form of A1Rz with peroxide.
Role of polar monomers in the initiation The reason for the different behaviour of nonpolymerizable electron donors and the polar monomer (VA) remained unsolved. The difference in the kinetic schemes in the A1Et3-BP and A1Etz-VA-BP systems prevents estimation of Keq of reaction (8) from kinetic data. The independence of the rate of VC polymerization initiated by the A1Et3-VA-BP system (at [VA]> [AIEt3]) on the VA concentration (Figure 1) suggests a relatively high value for Keq of reaction (8). An attempt to estimate this value by the i.r. method did not yield the desired data since under experimental conditions, T=25°C, [VA]= [A1Etz], a comparatively rapid irreversible consumption of [VA] is observed. To find where VA comes in the established series of basicity of CA we have estimated the stability of its complex with Al(iBu)z by 27Al-n.m.r. and by p.m.r, methods. The data from 27Al-n.m.r. (Table 3) showed that, by its complexing ability, VA is situated in the middle of the series (eqn. 13). Additional data on the Al(iBu)z behaviour in the presence of polar agents were obtained from p.m.r. High resolution p.m.r, spectroscopy shows complex formation from the shielding of Al(iBu)z methylene protons, caused by electron donation to the aluminium atom from the ligand hetero-atom. Figure 13 shows the dependence of the internal chemical shift of Al(iBu)3 (A = ~-erI2 - ~'eHa) on the VA concentration. Almost identical plots were obtained for all the ligands studied: CA (EA, Bu20, AA) and the polar monomers (VA, MMA, AN). The data presented suggest the formation of a 1 : 1 type labile complex with a short life time ( < 0.1 sec). This is proved by the coalescence of the free and coupled Al(iBu)a p.m.r, signals into one common signal with the averaged A value. The same pattern was maintained for different ether-AIRa complexesZl-2a. Another pattern is observed for Py. Here the p.m.r. signal coalescence does not take place owing to a more rigid complex formation with life time exceeding one second. This information is consistent with published data on the behaviour of the complex of AIEt3-Py 21. The data suggest that the reason for the unusual behaviour of VA
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system : E. B. Milovskaya et al. 1,0
/
0.9 I
/
0.8
0.7 o
X
×
i
i
I
l
2
3
[VA]/
[ A I ( i B u ) 31
Figure 13
Dependence of the internal chemical shift forAl(iBu)8 (A=rcH ~ -rCHs) on the molar ratio of the VA and Al(iBu)3 concentrations. The concentration of Al(iBu)3 is 10% by wt. in benzene
,,^~ i
suppose that in the formation of the complex between A1Ra and the polar monomer a certain polarization of its double bond occurs followed by the electron cloud shift towards the electron acceptor, aluminium alkyl. This process is obviously feasible for the monomers having a direct conjugation between the double bond and the hetero-atom, i.e. for A N and MMA. In the VA molecule the movement of the double bond electron cloud can, obviously, occur through the uncoupled electron pair of the other oxygen (as an inductive effect). The decrease in the electron density on the monomer double bond in the complex is shown by the example of MMA.A1Et3 using i.r. spectroscopy. Figure 14 gives a comparison of the i.r. spectral regions of M M A and the complex A1Et~. MMA. It is established that the vibration band of the C = C double bond normally at 1632cm -1 is shifted to lower frequencies (1624 cm -1) in the complex. The distortion of the double-bond symmetry leads to a sharp increase in the intensity of the corresponding band. (With i.r. spectroscopy we failed to find a similar shift in the VA molecule: this might be connected with the difference in the structure of these monomers.) Hence the complex A1Et3. M is a kind of bifunctional compound in which the electron-deficient centres are represented by aluminium alkyl and the double bond of the complexing monomer. We may assume that in the subsequent reaction with BP (a bi-functional electron donor) this complex acts as a single bi-accepting compound. The transition state of the interaction can be schematically shown as follows:
i r f R3
Figure 14
Infra-red spectra of MMA complexed with AIEt3 and of free MMA in octane at 25°C. Molar ratio [MMA]/[AIEt3]=I ( - - ) ; M M A = 8 x 10-2mol/I; free MMA ( . . . . . . . )
,,c !
where X= - C = iq O+ C H 2
in the initiation process is not connected with the stability of the complex. The experimental data available indicate another possibility--a fundamentally different mechanism of interaction between AIEtz and BP in the presence of a polar monomerL Both the information obtained under simulated conditions 7, 8 and the kinetic results (obtained in an earlier investigation and in the present research) show that stoichiometry of the BP reaction corresponds to 1 : 2, i.e. the interaction with both carbonyl oxygens is necessary for the rupture of the peroxide bond. As already stated, the same expression was obtained for the initiation for all the polar monomers (equation la). This enables us to pass from a particular case of VA to the general analysis of the phenomenon*. It is natural to * It is not possible to use other polar monomers (AN and MMA) in catalytic amounts in the VC polymerization with these systems because of a rather unfavourable ratio between the copolymerization constants of VC-AN and VC-MMA. Using the catalytic system with MMA a simultaneous occurrence of the initiation and inhibition was established
/ .-
""
_O_C
z, ~ ~
o o')
o
LJ~c
~
~0
~'CH 3
- - c ~°
I
\ OCH3
C6Hs
Stoichiometry of this interaction corresponds to the experimentally established ratio A1Eta : BP equal to 1 : 1 (see equations la and 9). In accordance with the proposed interaction scheme not only the alkyl radicals (the only radicals obtained under the experimental conditions 8) but also benzoate radicals are formed. Occurrence of the latter was shown with P M M A a for which the chain transfer reaction to BP is known to be absent 24, therefore the presence of terminal benzoate groups in the polymer must be entirely due to the initiation reaction. The fact that the rate constant of the initiation reaction is strongly dependent on the nature of the medium provides a certain confirmation of the conception of the free-
POLYMER,
1972, V o l 13, J u n e
295
Polymerization of VC by AIEt3/benzoyl-peroxide/Lewis-base system: E. B. Milovskaya et al. radical formation resulting from the interaction between the highly polarized particles. Thus with a complete agreement of the kinetic orders for the initiator components (equations la and 9) and for the Ein values, the kin values with the AIEta-BP system in the VC medium in the presence of catalytic amount of VA, and in the VA medium are 7 . 2 5 x 1 0 - a l m o l - l s e c -1 and 7.2x10-51 mo1-1 sec -1 (ref. 1), respectively. Since the systems studied are efficient as initiators for the low temperature polymerization, analogous systems based on other organometallic compounds might also be effective. As was shown recently in similar experiments, the replacement of M - C bonds (where M is B, AI etc.) by M - O bonds is energetically favourable ~5,~6. The energetic gain must be still greater in the interaction between metal alkyls and acyl peroxide. In particular, in the interaction between aluminium alkyls and BP the driving force of the free radical's formation is the transformation of the covalent A I - C bond into the partly ionic bond A I - O C O -5. It is quite evident that the advantages of this process will be principally determined by the metallicity of the central atom. Thus it has been established that a trialkyl boron with BP does not yield a system of high activity 27. No data exist about the use of organic compounds of Zn and Cd in this combination but we may assume that they will be less active than organoaluminium compounds for the same reason. Conversely, organic compounds of alkaline and alkaline earth metals can hardly be used with acyl peroxide as initiating systems. In this case the ionic contribution to the M - C bond is too great. For instance, this causes the interaction with BP to develop as a heterolytic process rather than a homolytic one 2s. Besides, we have found that the reaction of MgR2 with BP, even in the presence of a considerable excess of an electron donating compound (dioxan), occurs practically instantaneously. Thus this interaction cannot be a source of continuous formation of free radicals.
296
POLYMER, 1972, Vol 13, June
REFERENCES 1 Milovskaya, E. B. and Zhuravleva, T. G. Vysokomol. Soedin. 1964, 6, 1035 2 Milovskaya, E. B. and Zamoiskaja, L. W. Vysokomol. Soedin. 1965, 7, 670 3 Milovskaya, E. B., Zamoiskaja, L. W. and Winogradowa, S. I. Fur. Polym. J. 1970, 6, 1589 4 Milovskaya, E. B. and Zhuravleva, T. G. Vysokomol. Soedin. (A) 1967, 9, 1128 5 Milovskaya, E. B., Zhuravleva, T. G. and Zamoiskaja, L. W. J. Polym. Sci. (C) 1967, 16, 899 6 Kopp, E. L. and Milovskaya, E. B. Vysokomol. Soedin. (A) 1969, 11,750 7 Milovskaya, E. B., Pokrovsky, E. I. and Fedorova, E. F. lzv. Akad. Nauk S.S.S.R., Ser. Khim. 1967, p 1093 8 Zamoiskaja, L. W., Milovskaya, E. B. and Orestova, W. A. Izv. Akad. Nauk S.S.S.R., Ser. Khim. 1970, p 2053 9 Kopp, E. L. and Milovskaya, E. B. Izv. Akad. Nauk S.S.S.R., Ser. Khim. in press I0 Weisberger, A. 'Organic Solvents', Interscience, New York, 1955, Vol VII 11 Razuvaev, G. A. and Graevsky, A. I. DokL Akad. Nauk. S.S.S.R. 1959, 128, 309 12 Myers, O. and Pitzer, E. J. AppL Phys. 1959, 30, 1987 13 Danusso, F. and Sianesi, D. Chim. Ind. 1955, 37, 695 14 Burnett, G. and Wright, W. Proc. R. Soc. 1954, 231, 28 15 Pasynkiewiez, S. and Starowieyski, K. Roczn. Chem. 1967, 41, 1139 16 Bonitz, E. Chem. Ber. 1955, 88, 742 17 Margaritova, M. F. and Rysanova, K. A. Vysokomol. Soedin. (A) 1969, 11, 2741 18 Emerson, W. and Ramirez, E. Anal. Chem. 1965, 37, 806 19 Poole, C. P., Swift, H. E. and Itzel, Y. E. J. Chem. Phys. 1965, 42, 2576 20 Swift, H. E., Poole, C. P. and Itzel, Y. E. J. Phys. Chem. 1964, 68, 2509 21 Takashi, Y. Bull. Chem. Soc. Japan, 1967, 40, 612 22 Takeshita, T. and Frankle, W. Tetrahedron Lett. 1968, 5913 23 Hatade, K. and Yuki, H. Tetrahedron Lett. 1968, p 213 24 Baysal, B. and Tobolsky, H. J. Polym. Sci. 1952, 8, 529 25 Davies, A. and Roberts, B. J. Organomet. Chem. 1969, 19, 17 26 Krusic, P. and Kochi, J. J. Am. Chem. Soc. 1969, 91, 3942 27 Contreras, J., Grotewold, J., Lissi, E. and Rozas, R. J. Polym. Sci. (A-l) 1969, 7, 2341 28 Lawesson, S. and Wang, W. J. Am. Chem. Soc. 1959, 81, 4230