Mayr, Herbert

Schade, Christian

English

Crossref

Open Access LMU ( Ludwig-Maximilians-Univ. München)

Makromol. Chem., Rapid Commun. 9,477-482 (1988) 477 Tritylium ions as initiators and co-initiators in cationic polymerizations Christian Schade, Herbert Mayr * Institut fur Chemie der Medizinischen Universitat zu Lubeck, Ratzeburger Allee 160, D-2400 Lubeck, Federal Republic of Germany (Date of receipt: February 19, 1988) Introduction The unsubstituted tritylium cation (1 a) does not initiate the cationic polymeri- zation of isobutene (2)’). This observation was attributed to the difference of the carbenium ion stabilization of l a  and 3a, which cannot be compensated by the free energy, gained from the conversion of a n- into a a-bond [Eq. (l)]’). On the other hand, the less stabilized tris(4chlorophenyl)methyl cation (1 b) was reported to initiate the polymerization of isobutene3). In accordance, p-methoxy and p-methyl substituted tritylium ions do not initiate the polymerization of a-methyl- styrene, in contrast to the less stable 1 a and 1 b4). Only for few reactive monomers, however, the initiation was proven to proceed via addition of trityl cations 1 to the monomer5), and in most of these cases the site of attack at the trityl cation has not been determined”. As some nucleophiles were shown to attack the para-position of trityl cations’), the reaction might occur either at the central carbon or at one of the aromatic rings. Due to their sluggish addition towards alkenes, tritylium ions have been introduced as Lewis acidic catalysts for the addition of acetals and aldehydes to (silyl) enol ethers and allyltrimethylsilanes by Mukaiyama et al.*). In this work, we have studied electrophilic alkylations of isobutene (2) and allyltri- methylsilane, in which tritylium ions alternatively act as electrophilic alkylating agents or as Lewis acidic catalysts. As in previous investigations, conditions were used under which the reactions terminate after the inclusion of one alkene unit”. Initiation by tritylium ions The reaction of tritylium perchlorate with isobutene (2) in acetonitrile at 30 “C was reported to yield 28% of 111-dimethyl-3,3-diphenylindane (5) along with polymeric material’”. The corresponding ZnC1, 9 Et,O catalyzed reaction of trityl chloride (4) 0173-2803/88/$01 .oO 478 C. Schade, H. Mayr with 2 in CH,Cl, at 0°C gave 5 and 2-methyl-4,4,4-triphenyl-l-butene (6) in small yieid9b). We found now that tritylium tetrachloroborate (7a) does not react with isobutene in CH,CI, at - 10°C. Under the same conditions, the less stabilized chloro derivative 7b gave 79% of the regular addition product 8b, when treated with 70 equivalents of isobutene. 7 8 CH3 7 n . h  7b,8b Tt-T-7- Allyltrimethylsilane (9) is more reactive than 2 and gives the expected 4,4,4-triaryl- 1-butenes 10a, b with both trityl tetrachloroborates 7a") and 7b. The structures of compounds 5 ,6 ,8b  and 10 indicate that both n-nucleophiles 2 and 9 attack tritylium ions at the central carbon atom, not in p-position of the aromatic rings. C H z C I z  7 + H2C=CH-CH2-Si(CH3), - @-R-CgHq)3C-CH2-CH=CH2 (4) 9 10 109: R = H b: R = C1 In contrast to the common viewpoint'), the sluggishness of tritylium ion additions towards z-systems cannot be explained by the inherent stability of these carbenium ions. Whereas the unsubstituted trityl cation (pKR+ = -6,63)") does not react with 2 below O"C, the even better stabilized his@-anisy1)methyl cation (pKR+ = - $71)") reacts with 2 even at - 7 0 ° C " ~ ~ ~ ) .  This conclusion is corroborated by kinetic experiments. When the consumption of 7b by 2 was followed photometrically by monitoring the absorbance at 490 nm, the rate of the reaction was found to decrease considerably after a small degree of conver- sion. In contrast, the decrease of [7b] in presence of a large excess (770 equivalents) of 9 followed a pseudo first order rate law up to 80% conversion of 7 b. The resulting second order rate constant k, = 0,44 1 .mol-' . s - '  (at - 10°C in CH,CI,) is 5000 times smaller than k, of the corresponding reaction of a-(p-methoxypheny1)benzyl tetrachloroborate (BMMT) with 9 (k, (at -10°C)  = 2040)''), although both Tritylium ions as initiators and co-initiators in cationic polymerizations 419 carbenium ions are of similar stability (pKR+ of 7 b  = -7,7412) and pKR+ of a-@- methoxypheny1)benzyl tetrachloroborate = - 7,9”)). Steric strain, therefore, accounts for the relatively low reactivity of the trityl cation 7b. Force field calculations (MMPMI)”) have been used to estimate the steric strain generated in such reactionsI6). Eqs. ( 5 )  and (6) show that the addition reactions of trityl chloride to 2 and styrene are less exothermic than the corresponding diarylmethyl chloride additions. The difference is much more pronounced in the additions to 2 ( A M  = 24 kJ * mol-I) than in the additions to styrene ( A M  = 14 kJ-mol-I) ,  indicating that steric hindrance is more important in additions to isobutene. Therefore, styrene polymeri- zation in contrast to isobutene polymerization can be initiated by the unsubstituted trityl cation’, ”) , although diarylcarbenium ions were shown to react faster with isobutene than with styrene’*). Co-initiation by tritylium ions The discussion in the previous section has shown that trityl cations, though not having an extraordinarily high stability (PKR + ), exhibit a very low reactivity towards olefinic x-systems. They can, therefore, act as Lewis acids and accept halide anions from other alkyl halides. Since the carbenium ions, thus generated, may be capable of attacking olefins, trityl salts can act as co-initiators for the carbocationic polymeri- zation of alkenes (Scheme I). Scheme I: Ar3C’ + RX R+ + Ar,CX 1: + c=c : *# I‘ ‘ Experimental evidence for these conclusions comes from the formation of 94% of 12 from 11 and 2 in presence of tritylium tetrachloroborate (7a). Whereas 11 and 2 do 480 C. Schade, H. Mayr not react in dichloromethane in absence of a Lewis acid, the reaction proceeds at considerably lower temperatures ( - 78 "C) when 7a is replaced by the stronger Lewis acid BCl, 14). CH3 l a /  - 2SQC I I 11 12 CH3 (7) @-H3C-C6H4)2HC-C1 + 2 ' @-H~C-C~H~)~HC-CHZ-C-CI Since the rates of electrophilic alkylations of alkenes (Scheme I )  depend on the equilibrium concentration of R+  , the co-initiating efficiency of trityl salts can be controlled by appropriate substituents in the tritylium ions. Co-initiation by tritylium tetrachloroborates may either proceed via direct C1- abstraction of tritylium ions from R-Cl or via the small equilibrium concentration of BCl, (7a * 4 + BCl,). Assuming fast C1- transfer processes, both mechanisms are in agreement with our conclusions. Experimental part CH,Cl, was dried with CaCI, , washed with cone. H,SO, , stirred over K,CO, , distilled over P,O, and finally CaH, . 3-Chloro-3-methyl-I, I I-tris(4-chlorophenyl)butune (8 b): Isobutene (2) (28,l g; 500 mmol) was condensed into a flask which contained 2,64 g (6,91 mmol) of the chloride of 1 b. After dilution with 100 ml of CH,CI,, 50 ml(2,O mmol) of gaseous BCl, was added. The reaction mixture was stored for 70 h at - 10°C, washed with 6~ HC1, dried over CaCI, , and the solvent was evaporated. The residue (3,02 g; 6,89 mmol; 99Vo) was purified by recrystallization from petroleum ether; colorless crystals, which decompose at 159 - 160 "C with elimination of HCI. Yield: 2,4 g (79%). ' H  NMR (90 MHz, CDCI,): 6 = 1,18 (s; 6H, CH,), 3,33 (s; 2H, CH,), 7,27 (mc; 12H, Ar- HI. I3C NMR (CDCI,): 6 = 3435 (q; 2CH3), 52,34 (t; C-2), 55,58 (s; C-l), 70,lO (s; C-3), 128,10, 130,56 (2d; ortho-C, meta-C), 132,28 (s; para-C), 144,51 (s; ipso-C). MS (70 eV): m/z = 402,400 (3%, 370, M +  - 36), 349 (31), 348 (20), 347 (94), 346 (21), 345 (loo), 277 (6), 275 (17), 239 (20), 235 (lo), 233 (15). 4,4,4-Tris(4-chlorophenyl)-l-butene (lob): Gaseous BCI, (100 ml; 4,O mmol) and 1 ,O ml(6,3 mmol) of 9 were successively added to a solution of 1,11 g ( 2 9  mmol) of the chloride of 1 b in 50 ml of CH,CI, at -20°C. After 24 h, another 1 ml of 9 was added, and the solution was kept at -20°C for 24 h. The mixture was washed with 25% aq. NH,CI, dried over CaC1, , and the solvent was evaporated. Purification was done by MPLC (Merck Lichroprep Si 60 RP-18, MeOH, 12,5 ml/min, 12 bar, tR = 16,3 min). Yield: 0,94 g (84%) of a colorless oil. ' H  NMR (200 MHz, CDCl,): 6 = 3,33 (br. d; J = 6,6 Hz, 2H, 3-H), 4,94-5,08 (m; 2H, 1-H), 5,47-5,67 (m; 1 H, 2-H), 7,01, 7,27 (AA'BB'system; JAB = 8,8 Hz, 12H, Ar-H). I3C NMR (CDCI,): 6 = 45,33 (t; C-3), 55,26 (s; C-4), 118,45 (t; C-1), 128,15, 130,51 (2d; ortho-C, meta-C), 132,33 (s; para-C), 134,58 (d; C-2), 144,85 (s; ipso-C). MS (70 eV): m/z = 349, 347, 345 ( 5 % ,  13%, 14%, M +  - C,H,), 88 (12). 86 (77), 84 (loo), 75 (15). 73 (45). IR (neat): 3065, 3025, 2960, 2919, 1660, 1588, 1570, 1490, 1444, 1400, 1094, 1011, 975, 916, 811, 748, 732 cm-I. 4,4,4-Triphenyl-I-butene (108): It was prepared as described by R. Schneider") in an analogous manner; m.p. 68-68,5"C. Tritylium ions as initiators and co-initiators in cationic polymerizations 48 1 ‘ H  NMR (200 MHz, CDCI,): 6 = 3,43 (br. dt; J = 6,6 Hz, J = 1,5 Hz, 2H, 3-H), 4,89-5,07 (m; 2H, 1-H), 5,55-5.75 (m; 1 H, 2-H), 7,21 (mc; 15H, Ar-H). Kinetic e~per iment ’~) :  Compound 9 (3,16 mmol) was added to a solution of 7b (0,0041 mmol) in 100 ml of CH,Cl, , and the decreasing absorbance of 7 b was subsequently monitored at 490 nm. The reaction followed a pseudo first order rate law, and the second order rate constants k ,  were derived from the pseudo first order rate constants k , ,  , using the equation k,  = k /[Alkene]. Two independent experiments gave k,  = 0,436 and 0,453 1. mol- I . s-  I. Tritylium tetrachloroborate catalyzed additions of bis(p-toly1)methvl chloride (11) to wards isobutene (2): Compound 11 (258 mg; I ,  12 mmol) was added to a solution of 7a prepared from 4 (52,2 mg; 0,187 mmol) and BC1, (4 ml; 0,16 mrnol) in 100 ml of CH,CI, . After the addition of isobutene (50 rnl; 2,3 mmol), the mixture was kept at -25°C for 24 h and worked up with 6 M HC1. After removal of the solvent, 144 mg (1,33 mmol) of anisole was added as internal standard. I H NMR analysis showed the formation of 94% of 12. ‘ W  Conclusion Tritylium ions can attack at the double bond of isobutene and thus are potential initiators of the polymerization of isobutene. Their additions to alkenes are consider- ably retarded, however, by the steric strain, generated during the addition reactions. Diarylcarbenium ions attack alkenes with rate constants several orders of magnitude higher than trityl cations of similar stability (derived from pKR+ ). Because of their Lewis acidity and low reactivity towards CC x-bonds, tritylium ions may also act as co-initiators of alkene polymerizations. They may ionize other types of alkyl halides to give less bulky carbenium ions, which add to CC x-bonds and thus initiate polymerization. The main advantage of such initiating systems is the well-defined character of the individual steps, for which thermodynamic and kinetic data are frequently availablei9). C. S. wishes to thank the Stiftung Volkswagenwerk/Fonds der Chemischen Industrie for a Kekulk grant. I )  a) J .  P. Kennedy, E. Markhal, “Carbocationic Polymerization”, Wiley, New York 1982, p. 94; b) A. Gandini, H. Cheradarne, “Cationic Polymerization”, Adv. Polym. Sci. 34/35, 1 (1980) Ref.”), p. 255 ’) a) S. D. Pask, P. H. Plesch, Eur. Pofym. J. 18, 839 (1982); b) ref.Ia), p. 58 and 94 4, G. Heublein, P. Hallpap, P. Adler, Acta Polym. 30, 582 (1979) 5, See, e.g. a) G. Sauvet, J. P. Vairon, P. Sigwalt, J. Polym. Sci., Part A-I,  7 ,  983 (1969); b) R. Cotrel, G. Sauvet, J. P. Vairon, P. Sigwalt, Macromolecules 9, 931 (1976); c) N. G. Gaylord, M. Svestka, J. Macromol. Sci., Chem. A3, 897 (1969); d) A. F. Johnson, D. A. Pearce, J. Polym. Sci., Polym. Symp. 56, 57 (1976); e) T. Kunitake, K. Takarabe, S. Tsugawa, Polym. J. 8, 363 (1976); f) P. M. Bowyer, A. Ledwith, D. C. Sherrington, Polymer 12, 505 (1971); g) M. Villesange, G. Sauvet, J. P. Vairon, P. Sigwalt, Polym. Bull. (Berlin) 2,  131 (1980) 6, a) A. Ledwith, Adu. Chem. Ser. 91, 317 (1969); b) J .  A. Kamprneier, R. P. Geer, A. J.  Meskin, R. M. D’Silva, J. Am. Chem. SOC. 88, 1257 (1966) 482 C. Schade, H. Mayr ’) a) G. A. Olah, J. J .  Svoboda, J. Am. Chem. SOC. 95, 3794 (1973); b) R. H. Heck, P. S. ’) Review: T. Mukaiyama, M. Murakami, Synthesis 1987, 1043 9, a) H. May,  R. Schneider, R. Pock, Makromol. Chem., Macromol. Symp. 3, 19 (1986); b) H. Mayr, W. Striepe, J. Org. Chem. 48, 1159 (1983) lo) H. G. Richey Jr., R. K. Lustgarten, J .  M. Richey, J. Org. Chem. 33, 4543 (1%8) R. Schneider, Dissertation, Erlangen 1987 N. C. Deno, J. J. Jaruzelski, A. Schriesheim, J.  Am. Chem. Soc. 77, 3044 (1955) Magee, S .  Winstein, Tetrahedron Lett. 1964, 2033 1 3 )  N. C. Deno, A. Schriesheim, J. Am. Chem. Soc. 77, 3051 (1955) 14) R. Schneider, U. Grabis, H. Mayr, Angew. Chem. 98, 94 (1986); Angew. Chem., Znt. Ed. Engl. 25, 89 (1986) Is) a) U. Burkert, N. L. Allinger “Molecular Mechanics”; ACS Monograph 177, ACS Washington, DC, 1982; b) J.  J. Gajewski, K. E. Gilbert, “Molecular Mechanics Package (MS-DOS Computers)”, Serena Software, Bloomington, Indiana For experimental values of related reactions see: C. Schade, H. Mayr, E. M. Arnett, J. Am. Chem. Soc. 110, 567 (1988) 17) T. Higashimura, T. Fukushima, S. Okamura, J. Macromol. Sci., Chem. Al ,  683 (1967) Is) a) H. Mayr, R. Pock, Chem. Ber. 119,2473 (1986); b) R. Pock, H. Mayr, Chem. Ber. 119, 19) Review: H. Mayr, R. Schneider, C. Schade, Makromol. Chern., Macromol. Symp. 13/14, 2497 (1986) 43 (1988) 

Tritylium ions as initiators and co-initiators in cationic polymerizations

Universität München: Elektronischen Publikationen

Makromol. Chem., Rapid Commun. 9,477-482 (1988) 477 
Tritylium ions as initiators and co-initiators in cationic 
polymerizations 
Christian Schade, Herbert Mayr * 
Institut fur Chemie der Medizinischen Universitat zu Lubeck, 
Ratzeburger Allee 160, D-2400 Lubeck, Federal Republic of Germany 
(Date of receipt: February 19, 1988) 
Introduction 
The unsubstituted tritylium cation (1 a) does not initiate the cationic polymeri- 
zation of isobutene (2)’). This observation was attributed to the difference of the 
carbenium ion stabilization of l a  and 3a, which cannot be compensated by the free 
energy, gained from the conversion of a n- into a a-bond [Eq. (l)]’). 
On the other hand, the less stabilized tris(4chlorophenyl)methyl cation (1 b) was 
reported to initiate the polymerization of isobutene3). In accordance, p-methoxy and 
p-methyl substituted tritylium ions do not initiate the polymerization of a-methyl- 
styrene, in contrast to the less stable 1 a and 1 b4). Only for few reactive monomers, 
however, the initiation was proven to proceed via addition of trityl cations 1 to the 
monomer5), and in most of these cases the site of attack at the trityl cation has not 
been determined”. As some nucleophiles were shown to attack the para-position of 
trityl cations’), the reaction might occur either at the central carbon or at one of the 
aromatic rings. Due to their sluggish addition towards alkenes, tritylium ions have 
been introduced as Lewis acidic catalysts for the addition of acetals and aldehydes to 
(silyl) enol ethers and allyltrimethylsilanes by Mukaiyama et al.*). 
In this work, we have studied electrophilic alkylations of isobutene (2) and allyltri- 
methylsilane, in which tritylium ions alternatively act as electrophilic alkylating 
agents or as Lewis acidic catalysts. As in previous investigations, conditions were used 
under which the reactions terminate after the inclusion of one alkene unit”. 
Initiation by tritylium ions 
The reaction of tritylium perchlorate with isobutene (2) in acetonitrile at 30 “C was 
reported to yield 28% of 111-dimethyl-3,3-diphenylindane (5) along with polymeric 
material’”. The corresponding ZnC1, 9 Et,O catalyzed reaction of trityl chloride (4) 
0173-2803/88/$01 .oO 
478 C. Schade, H. Mayr 
with 2 in CH,Cl, at 0°C gave 5 and 2-methyl-4,4,4-triphenyl-l-butene (6) in small 
yieid9b). 
We found now that tritylium tetrachloroborate (7a) does not react with isobutene 
in CH,CI, at - 10°C. Under the same conditions, the less stabilized chloro derivative 
7b gave 79% of the regular addition product 8b, when treated with 70 equivalents of 
isobutene. 
7 8 CH3 
7 n . h  7b,8b 
Tt-T-7- 
Allyltrimethylsilane (9) is more reactive than 2 and gives the expected 4,4,4-triaryl- 
1-butenes 10a, b with both trityl tetrachloroborates 7a") and 7b. The structures of 
compounds 5 ,6 ,8b  and 10 indicate that both n-nucleophiles 2 and 9 attack tritylium 
ions at the central carbon atom, not in p-position of the aromatic rings. 
C H z C I z  
7 + H2C=CH-CH2-Si(CH3), - @-R-CgHq)3C-CH2-CH=CH2 (4) 
9 10 
109: R = H 
b: R = C1 
In contrast to the common viewpoint'), the sluggishness of tritylium ion additions 
towards z-systems cannot be explained by the inherent stability of these carbenium 
ions. Whereas the unsubstituted trityl cation (pKR+ = -6,63)") does not react with 
2 below O"C, the even better stabilized his@-anisy1)methyl cation (pKR+ = - $71)") 
reacts with 2 even at - 7 0 ° C " ~ ~ ~ ) .  
This conclusion is corroborated by kinetic experiments. When the consumption of 
7b by 2 was followed photometrically by monitoring the absorbance at 490 nm, the 
rate of the reaction was found to decrease considerably after a small degree of conver- 
sion. In contrast, the decrease of [7b] in presence of a large excess (770 equivalents) of 
9 followed a pseudo first order rate law up to 80% conversion of 7 b. The resulting 
second order rate constant k, = 0,44 1 .mol-' . s - '  (at - 10°C in CH,CI,) is 5000 
times smaller than k, of the corresponding reaction of a-(p-methoxypheny1)benzyl 
tetrachloroborate (BMMT) with 9 (k, (at -10°C)  = 2040)''), although both 
Tritylium ions as initiators and co-initiators in cationic polymerizations 419 
carbenium ions are of similar stability (pKR+ of 7 b  = -7,7412) and pKR+ of a-@- 
methoxypheny1)benzyl tetrachloroborate = - 7,9”)). Steric strain, therefore, 
accounts for the relatively low reactivity of the trityl cation 7b. 
Force field calculations (MMPMI)”) have been used to estimate the steric strain 
generated in such reactionsI6). 
Eqs. ( 5 )  and (6) show that the addition reactions of trityl chloride to 2 and styrene 
are less exothermic than the corresponding diarylmethyl chloride additions. The 
difference is much more pronounced in the additions to 2 ( A M  = 24 kJ * mol-I) 
than in the additions to styrene ( A M  = 14 kJ-mol-I) ,  indicating that steric 
hindrance is more important in additions to isobutene. Therefore, styrene polymeri- 
zation in contrast to isobutene polymerization can be initiated by the unsubstituted 
trityl cation’, ”) , although diarylcarbenium ions were shown to react faster with 
isobutene than with styrene’*). 
Co-initiation by tritylium ions 
The discussion in the previous section has shown that trityl cations, though not 
having an extraordinarily high stability (PKR + ), exhibit a very low reactivity towards 
olefinic x-systems. They can, therefore, act as Lewis acids and accept halide anions 
from other alkyl halides. Since the carbenium ions, thus generated, may be capable of 
attacking olefins, trityl salts can act as co-initiators for the carbocationic polymeri- 
zation of alkenes (Scheme I). 
Scheme I: 
Ar3C’ + RX R+ + Ar,CX 1: + c=c : *# I‘ ‘ 
Experimental evidence for these conclusions comes from the formation of 94% of 
12 from 11 and 2 in presence of tritylium tetrachloroborate (7a). Whereas 11 and 2 do 
480 C. Schade, H. Mayr 
not react in dichloromethane in absence of a Lewis acid, the reaction proceeds at 
considerably lower temperatures ( - 78 "C) when 7a is replaced by the stronger Lewis 
acid BCl, 14). 
CH3 
l a /  - 2SQC I 
I 
11 12 CH3 
(7) @-H3C-C6H4)2HC-C1 + 2 ' @-H~C-C~H~)~HC-CHZ-C-CI 
Since the rates of electrophilic alkylations of alkenes (Scheme I )  depend on the 
equilibrium concentration of R+  , the co-initiating efficiency of trityl salts can be 
controlled by appropriate substituents in the tritylium ions. Co-initiation by tritylium 
tetrachloroborates may either proceed via direct C1- abstraction of tritylium ions 
from R-Cl or via the small equilibrium concentration of BCl, (7a * 4 + BCl,). 
Assuming fast C1- transfer processes, both mechanisms are in agreement with our 
conclusions. 
Experimental part 
CH,Cl, was dried with CaCI, , washed with cone. H,SO, , stirred over K,CO, , distilled 
over P,O, and finally CaH, . 
3-Chloro-3-methyl-I, I I-tris(4-chlorophenyl)butune (8 b): Isobutene (2) (28,l g; 500 mmol) 
was condensed into a flask which contained 2,64 g (6,91 mmol) of the chloride of 1 b. After 
dilution with 100 ml of CH,CI,, 50 ml(2,O mmol) of gaseous BCl, was added. The reaction 
mixture was stored for 70 h at - 10°C, washed with 6~ HC1, dried over CaCI, , and the solvent 
was evaporated. The residue (3,02 g; 6,89 mmol; 99Vo) was purified by recrystallization from 
petroleum ether; colorless crystals, which decompose at 159 - 160 "C with elimination of HCI. 
Yield: 2,4 g (79%). 
' H  NMR (90 MHz, CDCI,): 6 = 1,18 (s; 6H, CH,), 3,33 (s; 2H, CH,), 7,27 (mc; 12H, Ar- 
HI. 
I3C NMR (CDCI,): 6 = 3435 (q; 2CH3), 52,34 (t; C-2), 55,58 (s; C-l), 70,lO (s; C-3), 
128,10, 130,56 (2d; ortho-C, meta-C), 132,28 (s; para-C), 144,51 (s; ipso-C). 
MS (70 eV): m/z = 402,400 (3%, 370, M +  - 36), 349 (31), 348 (20), 347 (94), 346 (21), 345 
(loo), 277 (6), 275 (17), 239 (20), 235 (lo), 233 (15). 
4,4,4-Tris(4-chlorophenyl)-l-butene (lob): Gaseous BCI, (100 ml; 4,O mmol) and 1 ,O ml(6,3 
mmol) of 9 were successively added to a solution of 1,11 g ( 2 9  mmol) of the chloride of 1 b in 
50 ml of CH,CI, at -20°C. After 24 h, another 1 ml of 9 was added, and the solution was 
kept at -20°C for 24 h. The mixture was washed with 25% aq. NH,CI, dried over CaC1, , and 
the solvent was evaporated. Purification was done by MPLC (Merck Lichroprep Si 60 RP-18, 
MeOH, 12,5 ml/min, 12 bar, tR = 16,3 min). Yield: 0,94 g (84%) of a colorless oil. 
' H  NMR (200 MHz, CDCl,): 6 = 3,33 (br. d; J = 6,6 Hz, 2H, 3-H), 4,94-5,08 (m; 2H, 
1-H), 5,47-5,67 (m; 1 H, 2-H), 7,01, 7,27 (AA'BB'system; JAB = 8,8 Hz, 12H, Ar-H). 
I3C NMR (CDCI,): 6 = 45,33 (t; C-3), 55,26 (s; C-4), 118,45 (t; C-1), 128,15, 130,51 (2d; 
ortho-C, meta-C), 132,33 (s; para-C), 134,58 (d; C-2), 144,85 (s; ipso-C). 
MS (70 eV): m/z = 349, 347, 345 ( 5 % ,  13%, 14%, M +  - C,H,), 88 (12). 86 (77), 84 (loo), 
75 (15). 73 (45). 
IR (neat): 3065, 3025, 2960, 2919, 1660, 1588, 1570, 1490, 1444, 1400, 1094, 1011, 975, 
916, 811, 748, 732 cm-I. 
4,4,4-Triphenyl-I-butene (108): It was prepared as described by R. Schneider") in an 
analogous manner; m.p. 68-68,5"C. 
Tritylium ions as initiators and co-initiators in cationic polymerizations 48 1 
‘ H  NMR (200 MHz, CDCI,): 6 = 3,43 (br. dt; J = 6,6 Hz, J = 1,5 Hz, 2H, 3-H), 
4,89-5,07 (m; 2H, 1-H), 5,55-5.75 (m; 1 H, 2-H), 7,21 (mc; 15H, Ar-H). 
Kinetic e~per iment ’~) :  Compound 9 (3,16 mmol) was added to a solution of 7b (0,0041 
mmol) in 100 ml of CH,Cl, , and the decreasing absorbance of 7 b was subsequently monitored 
at 490 nm. The reaction followed a pseudo first order rate law, and the second order rate 
constants k ,  were derived from the pseudo first order rate constants k , ,  , using the equation 
k,  = k /[Alkene]. Two independent experiments gave k,  = 0,436 and 0,453 1. mol- I . s-  I. 
Tritylium tetrachloroborate catalyzed additions of bis(p-toly1)methvl chloride (11) to wards 
isobutene (2): Compound 11 (258 mg; I ,  12 mmol) was added to a solution of 7a prepared from 
4 (52,2 mg; 0,187 mmol) and BC1, (4 ml; 0,16 mrnol) in 100 ml of CH,CI, . After the addition 
of isobutene (50 rnl; 2,3 mmol), the mixture was kept at -25°C for 24 h and worked up with 
6 M HC1. After removal of the solvent, 144 mg (1,33 mmol) of anisole was added as internal 
standard. I H NMR analysis showed the formation of 94% of 12. 
‘ W  
Conclusion 
Tritylium ions can attack at the double bond of isobutene and thus are potential 
initiators of the polymerization of isobutene. Their additions to alkenes are consider- 
ably retarded, however, by the steric strain, generated during the addition reactions. 
Diarylcarbenium ions attack alkenes with rate constants several orders of magnitude 
higher than trityl cations of similar stability (derived from pKR+ ). 
Because of their Lewis acidity and low reactivity towards CC x-bonds, tritylium 
ions may also act as co-initiators of alkene polymerizations. They may ionize other 
types of alkyl halides to give less bulky carbenium ions, which add to CC x-bonds and 
thus initiate polymerization. The main advantage of such initiating systems is the 
well-defined character of the individual steps, for which thermodynamic and kinetic 
data are frequently availablei9). 
C. S. wishes to thank the Stiftung Volkswagenwerk/Fonds der Chemischen Industrie for a 
Kekulk grant. 
I )  a) J .  P. Kennedy, E. Markhal, “Carbocationic Polymerization”, Wiley, New York 1982, 
p. 94; b) A. Gandini, H. Cheradarne, “Cationic Polymerization”, Adv. Polym. Sci. 34/35, 
1 (1980) 
Ref.”), p. 255 
’) a) S. D. Pask, P. H. Plesch, Eur. Pofym. J. 18, 839 (1982); b) ref.Ia), p. 58 and 94 
4, G. Heublein, P. Hallpap, P. Adler, Acta Polym. 30, 582 (1979) 
5, See, e.g. a) G. Sauvet, J. P. Vairon, P. Sigwalt, J. Polym. Sci., Part A-I,  7 ,  983 (1969); b) 
R. Cotrel, G. Sauvet, J. P. Vairon, P. Sigwalt, Macromolecules 9, 931 (1976); c) N. G. 
Gaylord, M. Svestka, J. Macromol. Sci., Chem. A3, 897 (1969); d) A. F. Johnson, D. A. 
Pearce, J. Polym. Sci., Polym. Symp. 56, 57 (1976); e) T. Kunitake, K. Takarabe, S. 
Tsugawa, Polym. J. 8, 363 (1976); f) P. M. Bowyer, A. Ledwith, D. C. Sherrington, 
Polymer 12, 505 (1971); g) M. Villesange, G. Sauvet, J. P. Vairon, P. Sigwalt, Polym. Bull. 
(Berlin) 2,  131 (1980) 
6, a) A. Ledwith, Adu. Chem. Ser. 91, 317 (1969); b) J .  A. Kamprneier, R. P. Geer, A. J.  
Meskin, R. M. D’Silva, J. Am. Chem. SOC. 88, 1257 (1966) 
482 C. Schade, H. Mayr 
’) a) G. A. Olah, J. J .  Svoboda, J. Am. Chem. SOC. 95, 3794 (1973); b) R. H. Heck, P. S. 
’) Review: T. Mukaiyama, M. Murakami, Synthesis 1987, 1043 
9, a) H. May,  R. Schneider, R. Pock, Makromol. Chem., Macromol. Symp. 3, 19 (1986); b) 
H. Mayr, W. Striepe, J. Org. Chem. 48, 1159 (1983) 
lo) H. G. Richey Jr., R. K. Lustgarten, J .  M. Richey, J. Org. Chem. 33, 4543 (1%8) 
R. Schneider, Dissertation, Erlangen 1987 
N. C. Deno, J. J. Jaruzelski, A. Schriesheim, J.  Am. Chem. Soc. 77, 3044 (1955) 
Magee, S .  Winstein, Tetrahedron Lett. 1964, 2033 
1 3 )  N. C. Deno, A. Schriesheim, J. Am. Chem. Soc. 77, 3051 (1955) 
14) R. Schneider, U. Grabis, H. Mayr, Angew. Chem. 98, 94 (1986); Angew. Chem., Znt. Ed. 
Engl. 25, 89 (1986) 
Is) a) U. Burkert, N. L. Allinger “Molecular Mechanics”; ACS Monograph 177, ACS 
Washington, DC, 1982; b) J.  J. Gajewski, K. E. Gilbert, “Molecular Mechanics Package 
(MS-DOS Computers)”, Serena Software, Bloomington, Indiana 
For experimental values of related reactions see: C. Schade, H. Mayr, E. M. Arnett, J. Am. 
Chem. Soc. 110, 567 (1988) 
17) T. Higashimura, T. Fukushima, S. Okamura, J. Macromol. Sci., Chem. Al ,  683 (1967) 
Is) a) H. Mayr, R. Pock, Chem. Ber. 119,2473 (1986); b) R. Pock, H. Mayr, Chem. Ber. 119, 
19) Review: H. Mayr, R. Schneider, C. Schade, Makromol. Chern., Macromol. Symp. 13/14, 
2497 (1986) 
43 (1988) 


https://epub.ub.uni-muenchen.de/3932/1/077.pdf

Tritylium ions as initiators and co-initiators in cationic polymerizations

Abstract

Similar works

Full text

Available Versions

Crossref

Open Access LMU ( Ludwig-Maximilians-Univ. München)

Universität München: Elektronischen Publikationen