J. Phys. Chem. 1991, 95,4932-4935
4932
Cationic Polymerization within Gas-Phase Clusters of Isoprene M. Samy El-Shall* and C. Marks Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006 (Received: March 4, 1991; In Final Form: May 9, 1991) Consecutive bimolecular ion-molecule addition and elimination reactions have been observed within van der Waals clusters of isoprene following electron impact ionization at pressures of Torr. A magic number has been observed for C14H21+ ion and attributed to a stable cyclic structure. The results suggest that cationic polymerization of isoprene proceeds via the formation of the stable dimer CloHI6+which, most likely, has a structure similar to limonene and can undergo further addition reactions to generate larger ions.
Introduction Gas-phase cationic polymerization has been known for several years and is a subject of growing interest.’ Findings in this field will contribute to fundamental understanding of a wide range of problems, including the mechanism of polymerization, the production of large molecules in interstellar media, nucleation on polymer molecules in the vapor phase, soot formation in hydrocarbon flames, and photoemission from polymeric ions generated from highly exothermic processes. In addition to early work using conventional gas-phase techniques, some recent papers addressed the feasibility of studying the kinetics and mechanism of gas-phase polymerization by using nucleation of liquid monomer drops to observe slow thermally or photochemically initiated reactions.* These studies involved free-radical polymerization within a monomer vapor held in a supersaturatedstate. When the growing polymer attains a certain critical size, determined by the degree of supersaturation, it participates in the formation of a condensation nucleus resulting in the formation of a macroscopic drop of liquid monomer. The rate of production of such observable drops can be related to the rate of formation of polymers of a particular size, and therefore the rate of propagation can be measured. This technique has been valuable in measuring the rate of an ultraslow process such as that involved in the thermal self-initiated polymerization of styrene vapore3 However, it is not clear that this method can be used to study fast ionic polymerization reactions. Furthermore, the nucleation studies, at the present time, are limited by the unknown identity of the nucleating species, the difficulty to access information on the mechanism of the reaction, and the dependence on nucleation theories or statistical models to predict the size of polymer molecules involved in the proce~s.~ One interesting possibility that has not been explored yet is the study of ionic polymerization within van der Waals (vdW) clusters of the monomer molecules. The reaction can be initiated following the ionization of a neutral cluster beam formed in a supersonic expansion. Addition and elimination reactions can take place within the ionized clusters, resulting in a product ion distribution that reflects both the stability of the polymeric ions and the kinetics of the reaction. The dependence of the rate constants of the addition and elimination reactions on the degree of polymerization ( I ) See for example: (a) Grossoleil, J.; Herman, J. A. Can. J. Chem. 1971, 49,363. (b) Kebarle, P.; Haynes, R. M. J . Chem. fhys. 1967,47, 1676. (c) Sieck, L. W.; Gorden, R., Jr.; Lias, S.G.; Ausloos, P. fnr. J . Mass Specrrom. fon fhys. 1974,15, 181. (d) Tiernan, T. 0.; Futrell, J. H. J . fhys. Chem. 1968,72, 3080. (e) Abramson, F. P.; Futrell, J. H. J. Am. Chem. Soc. 1968, 72. 1994. (0Henis. J. M. S.J . Chem. fhys. 1970,52, 282. (g) Mwt-Ner (Mautner), M.; Hunter, E. P.; Field, F. H. J . Am. Chem. Soc. 1977, 99, 5576. (h) Ono. Y.; Ng, C. Y. J. Am. Chem. Soc. 1982,104,4752. (i) Buckley, T. J.; Sieck, L. W.; Metz, R.; Lias, S.G.; Liebman, J. F. fnr. J . Mass Specrrom.
...
Ian Processes 1985. . .. . ....~ . -..
--.
65. 18 - - -I .-
(2) (a) Reiss, H. Science 1987,238, 1368. (b) El-Shall, M. S.;Rabeony, M. H.; Reiss, H. J. Chem. fhys. 1989, 91, 7925. (c) El-Shall, M. S.; Reiss, H. J. fhys. Chem. 1988.92,1021. (d) Reiss, H.; Chowdhury, M. A. J . fhys. Chem. 1984,88, 6667. (3) El-Shall, M. S.;Bahta, A.; Rabeony, H.; Reiss, H. J . Chem. fhys. 1987, 87, 1329. (4) (a) Reiss, H.; Rabeony, H.; El-Shall. M. S.; Bahta. A. J . Chem. fhys. 1987,87, 1315. (b) Rabeony, H.; Reiss, H. Macromolecules 1988,21,912.
will manifest itself in the appearanceof ‘magic numbers”, i.e. ions that exhibit abundance maxima at sizes corresponding to some stable structures. These are probably cyclic structures which could provide special stability to the polymeric ions. The survival of a condensation ion formed in a highly exothermic process within the cluster proceeds via evaporative loss where the cluster boils off its vdW bonded molecules, leaving a bare polymeric ion. If sufficient energy is available, the condensation ions generated from the cluster beam may undergo unimolecular decomposition resulting in fragment ions with branching ratios similar to those observed from other stable polymer ions. In this Letter we report the observation of addition-elimination reactions within van der Waals clusters of isoprene following their electron impact (EI) ionization. Our study will provide new information on the mechanism involved in the first stages of cationic polymerization of isoprene. In addition to its atmospheric importan~e,~ isoprene constitutes the building block of natural rubber, terpenes, camphors, and important biological compounds such as vitamin A, vitamin K, and chlorophyll. From a mechanistic point of view, it is well-known that isoprene, similar to other 2-substituted 1,3-butadienes,undergoes very rapid Diels-Alder reactions with its molecular ion? This makes isoprene an ideal example for demonstrating the feasibility of studying cationic polymerization within vdW clusters. The preliminary results discussed here deal only with observations of ion distributions and magic numbers. Subsequent publications will focus on kinetic aspects of the polymerization reactions, as measured by pulsed high-pressure mass spectrometry. This data will be compared with cluster beam results. Experimental Section Isoprene clusters were generated by pulsed adiabatic expansion in a supersonic cluster beam apparatus. The essential elements of the apparatus are jet and beam chambers coupled to a coaxial quadrupole mass spectrometer. During operation, saturated isoprene vapor is formed by flowing ultrahigh-purityHe (Spectra Gases, 99.999%) at a pressure of 2-4 atm through a reservoir filled with liquid isoprene (99.99%) at 250 K. The saturated gas is then expanded through a conical nozzle ( 0 5 “ diameter, General Valve Series 9) in pulses of 150-200-ps duration at repetition rates of 10-20 Hz. The average pressure during operation in this chamber is typically (4-8) X 10” Torr. The jet is skimmed by a 3-mm aluminum conical skimmer and passed into a high-vacuum chamber. At the end of the beam chamber is the ion source region of a quadrupole mass spectrometer (-70 cm from the nozzle). The beam chamber is maintained at 7 X lo-* to 1 X lo-’ Torr during operation. The cluster beam enters the ion source and quadrupole mass filter in an axial configuration. The mass spectrometer is an (5) (a) Kleindienst, T. E.; Harris, G . W.; Pitts, J. N., Jr. Enuiron. Sci. Technol. 1982,16,844. (b) Atkinson, R.; Damell, K. R.; Lloyd, A. C.; Winer, A. M.; Pitts, J. N., Jr. Ado. fhorochem. 1979, 11, 375. (c) Killus, J. P.; Whitten, G. Z. Enuiron. Sci. Techtwl. 1984,18, 142. (d) Ants. R. R.; Meeks, S.A. Armos. Enuiron. 1981, 15, 1643. (6) Groenewold. G . S.;Gross, M. L. J . Am. Chem. Soc. 1984,106.6569.
0022-365419112095-4932$02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 4933
Letters
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I
I
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200 250
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I
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500
550
80
1
I
I
90
100
110
I
I
I
I
120
130
I
140 ' 150
160
"2 "2
Figure 1. Raw 50-eV E1 mass spectrum of isoprene clusters.
----e In (30eV)
-t
In (70 ev)
--b Bln.1 ( 3 0 ~ )
-t
Bln-1 (70 ev)
1-
150
I
\60
I
I70
I
180
I
190 200
I
210
I
220
1
230
m/z
Figure 3. High-resolutionsegments of the mass spectrum (70-eV EI) of isoprene products ions.
For example, the monomer fragment ions produced upon ionization of the cluster beam do not include ions such as C2H2+,G H + , C2+, CH+, and C+. The monomer spectrum consists mainly of the following ions, with relative intensities as indicated: CsHs+ (77), C5H7+(loo), C4H5+(52), C3H3+(44). and C2H3+(23). The (C5He),,+and the C4H5+(CSHs),, sequences show maxima at n n = 6 and 3, respectively. These features are illustrated in Figure Figure 2. Normalized ion intensity as a function of n for I, = (CsH,),' 2 where the normalized intensities are plotted vs n for different and Bl,, = C4H,+(C5H,),I sequencesobtained at different El energies. electron impact energies. It is clear that the relative enhancements of the (C5H8)6+and C4H5+(C5Hs)2 ions are only weakly dependent Extrel-500 equipped with a 324-9 mass filter (pole diameter 1.9 on the electron impact energies and are observed at all energies. cm)which provides a high transmission efficiency. The mass filter However, at lower electron energy (30 eV) there is an increase is typically operated at better than 0.3 amu mass resolution up in the normalized intensity of the (C5H8)6+ion while the intensity to 550 amu. Emission current in the source is maintained at 1 of the C4H5+(C5H8),ion increases at higher electron energy (70 mA by current regulation at a fixed electron energy ranging from eV). This trend argues against the generation of (C5H&+ ion 15 to 100 eV. The amplified signal from the particle multiplier from subsequent fragmentation of larger clusters since this process is processed by using a boxcar integrator (EGBrG Model 166) set to sample at arrival times appropriate for the detected ions. should be enhanced at higher electron energy. As the electron energy increases, more (CSH8)6+ions are produced with sufficient At least 50 shots per amu are averaged during each run. The mass excess energy to decompose into (C5H&+ and (CsH&+. This spectrometer is routinely calibrated against the E1 pattern of is clearly seen in the 70-eV distribution shown in Figure 2. The perfluorotributylamine. same argument suggests that a significant amount of the Results C4H5+(C5H8),, ions are produced by fragmentation of larger ions. Figure 3A is a segment of the cluster spectrum taken at higher Figure 1 displays a typical 50-eV E1 mass spectrum observed resolution and includes ions smaller than the dimer, C&l6+ ( m / z for a cluster beam of isoprene. In addition to the major sequence, = 136). The ions observed are consistent with their formation (C5H8),,+. the spectrum contains other series such as C ~ H ~ + ( C ~ H ~ ) ,C- I H , ~ + ( C ~ H I ) ~ -CI ~, H I + ( C ~ H S ) ~ - Iby ~ eliminative ion-molecule reactions between isoprene radical cation and neutral isoprene within the clusters. These ions are C ~ H ~ + ( C ~ HG~ H ) , +I ( C J % L i r G'(C5Hs)ri. CH'(C5Hs)n-i similar to those observed by Kascheres and Cooks from their c + ( C ~ H g ) p iC. ~ H ~ + ( C S H and ~ ) PC~~~H ~ + ( C ~ H B These ) + seion-molecule reactions of isoprene molecular ion with neutral quences cannot be explained by simple fragment ion solvation.
4934 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
Letters ADDITION AND ELIMINATION REACTIONS IN ISOPRENE CLUSTERS
TABLE I: Relative Abvlldums of tk Fragment tom (Smrlki Tban Q/Z
/----
= 136) Observed in Cluster Beam, Ion-Molecule Reactions of
Isoopmn, a d E1 Ioniution of Umoneoe
cluster beam" 81 68 91 37 77 100 48
m l r chem formula 121 108 107 95 94 93 92 91 81 80 79
lC5HSf + Cs He) lCsH81n.2
ion-molecule reactions (300 ms)b 87 60 100 27 53 93
6 24 21 13
33 20
I C, :H
+ C 5Hgl I C5 Hg)n.2
I
E1
ELIMINATION i n ;21
D I E L S - A L D E R ADDITION
limonene' 34 8 31 14 42 100 32 23 22 20 43
"This work, electron energy = 7 0 eV. bReference 7. 'Reference 9, electron energy = 70 eV.
isoprene after 300-ms reaction time in an ion trap.' Table I compares the relative peak intensity of the product ions arising from isoprene ion-molecule reactions with those obtained from our cluster beam experiment. It is interesting that the relative intensities of the ions generated within the clusters (reaction time is on the order of a few microseconds) are in very good agreement with those obtained from ion-molecule reactions after 300-ms reaction time. It is also significant to note that these ions are the characteristic fragment ions observed in the daughter spectrum of limonene, which is formed by a Diels-Alder reaction of isoprene radical cation with its neutral molecule* (reaction 1).
,f
+
CHZ
-
\ O y E H z
(1)
The relative abundances of the daughter ions in the limonene E1 spectra9 are also included in Table I. The fragment ion at m / z = 93, resulting from loss of a propyl radical, is the base peak in the product ions generated from the cluster beam of isoprene as well as the E1 fragmentations of limonene.
Discussion The daughter spectra listed in Table I share many similarities that strongly support the idea that the ions formed in the cluster beam (Figure 3a) originate directly via fragmentation sequences of limonene ( m / z = 136). In fact, the ions produced within these clusters, as shown in Table I, are very similar to the E1 fragmentations from limonene. However, the relative abundances of the limonene fragments are different from those observed within the clusters. This may be rationalized in terms of isomerization. It is well-known that several fragmentations leading to ions at m / z 121,107,94,and 92 are preceded by isomerization of the molecular limonene ion.I0 Isomerization via 1,3-hydrogen migration resulting from rearrangements of the double bond within the cyclohexene ring is believed to involve a low-energy barrier and therefore is most likely to occur.*Jo Some of the possible isomers are shown below.
1
(7 Kwheres, C.; Cooks,R. G . Anal. Chim. Acra 1988, 215, 223. (8 Vincenti, M.; Homing, S.R.; Cooks,R.G. Org. Mass Specrrom. 1988, 23, 585. (9) Mass Spectral Data Base, National Institute of Standards and Technology, Gaithersburg, MD. (10) (a) Mabud, M. A.; Ast, T.; Verma, S.;Jiang, Y. X.; Cooks, R.G.J . Am. Chem. Soc. 1987, 109, 7597. (b) Harris, D.; McKinnon, S.;Boyd, R. K. Org. Mass. Specrrom. 1976, 14, 265. (c) Brittain, E. F. H.;Wells, C. H. J.; Paisley, M. H. J. J . Chem. Soc. E. 1968, 304. (d) Bank, S.;Rowe, C. A., Jr.; Schriesheim, A.; Naslund, L. A. J . Org. Chem. 1968, 33, 221.
i Figure 4. Overall reaction mechanism accounting for all the observed mass peaks in isoprene clusters. Figure 3B shows another segment of the cluster spectrum. As indicated above, the ions observed can be accounted for by a series of ion-molecule consecutive addition-elimination reactions within the clusters. The proposed mechanism is given in Figure 4. This mechanism accounts for the reactions of both C5H8+and C4HS+ ions (the major ions produced from the E1 ionization of the monomer) with neutral isoprene within its vdW clusters. In addition to the elimination reactions between CSH8+and neutral isoprene, Diels-Alder addition and conventional 1,2- and 1,4addition polymerization can take place. A Diels-Alder reaction will generate a limonene type molecular ion which can also be formed by cyclization of the dimer cation resulting from the 1,2or 1,4-addition. Similar reactions are possible between C4HS+ and CSHB which can account for the observed C4HS+(CSH8)rl series. As shown in Figure 2, this sequence shows a maximum a t n = 3 corresponding to the ion C14H21+. We believe that the stability of this ion is associated with its closed-shell electronic structure (even number of electrons), and we propose the cyclic structure shown in Figure 4. It must be noted that several cyclic ion are possible, for example isomers for the C14HZI+
a
b
C
It is worth noting that cyclization of 1,4-poly(isoprene) in bulk cationic polymerization has been confirmed by IR and NMR studies." In fact, depending on the polymerization conditions, monocyclic, bicyclic, and tricyclic structures have been reported.8J' Interestingly, the bicyclic structure (b) is similar to that already found in the bulk cationic polymerization of 2-alkyl-l,3-butadienes.l* We now turn to the observation of a maximum intensity distribution for the (C,H&+ sequence which appears at n = 6. This feature is more pronounced under conditions that facilitate the formation of larger neutral isoprene clusters. This conclusion is based on a series of repetitive experiments under different stagnation pressures and seeding ratios. For example, a t stagnation pressure less than 1.5 atm, the relative intensities of the n = 4, ( I 1 ) (a) Agnihotri, R. K.; Falcon, D.; Fdericks, E. C. J . Polym. Scf., Parr A 1972, IO, 1839. (b) Stolka, M.; Vodehnal, J.; Kossler, 1. J. Polym. Sei. Part A 1964, 2, 3987. (12) Hasegawa, K.; Asami, R.; Higashimura, T. Macromolecules 1977,
IO, 592.
J. Phys. Chem. 1991,95,4935-4939 5, and 6 cluster ions are almost identical. However, at stagnation pressure above 2 atm, the intensity of the n = 6 ion is significantly enhanced. We believe that the (C&J6+ ion represents a maximum in the polymeric ion distribution generated, under conditions of extensive clustering, via the addition reactions described in Figure 4. Recently, a similar observation of a magic number for the (C2H4)4+ion in the ethene cluster ion distribution has been reported and attributed to a single molecular ion.I3 The exothermicity of the condensation reactionsIbfj resulting in the formation of a polymeric ion within the cluster can be efficiently dissipated through evaporative loss of vdW bonded monomers in the cluster. For smaller clusters this mechanism may not be very efficient, and considerable amounts of energy may be deposited within the polymeric ions which can cause their characteristic fragmentations. The abundance of the polymeric ions is therefore expected to depend on the exothermicity of the addition reactions, the rate constants of the individual reactions, and the size of the cluster together with the time scale for observation and the critical energies and frequency factors of competing fragmentations. The factors that control the product ion distributions in cationic polymerization within vdW clusters are of continued interest in our laboratory, and other experimental techniques are currently being employed in order to fully understand these processes.I4 In summary, the experimental observations that support the conclusion that the (C,H,),,+ and C4H5+(C5H8),I ions are condensation polymeric ions formed via intracluster ion-molecule reactions are as follows: (1) The fragment ions observed from (C5H8)2+ are identical with those from E1 ionization of limonene, CIOHI6,which is the major product of Diels-Alder reaction of isoprene radical cation with its neutral molecule. (2) The fragment ions are identical with the products of the ion-molecule reactions observed in an ion trap after 300-ms reaction time. This is consistent with their formation of fragmentation of an internally excited limonene ion. (3) The relative intensities of the fragments C9HI3+,CsHI2+,C8HII+,C7Hll+,C8Hlo+,C7H9+,C6H9+, and C7H8+observed in the cluster beam and those reported from the ion-molecule reactions were found to be in excellent agreement. (4) The magic number observed for CI4H21+ ion is consistent with (13) (a) Cmlbaugh, M. T.; Peifcr, W. R.; Garvey, J. F. Chem. Phys. Lelr. 1990, 168, 337. (b) Garvey, J. F.; Peifer, W. R.; Coolbaugh, M. T. Acc. Chem. Res. 1991, 21, 48. (14) El-Shall, M. S.;Meot-Ner (Mautner), M. To be. published.
4935
a stable cyclic structure which can be derived from a series of ion-molecule consecutive addition-eliiination reactions. ( 5 ) The magic number observed for (CSH8)6+under experimental conditions where larger clusters are formed is consistent with a polymeric ion distribution characterized by a maximum which appears at n = 6 under our experimental conditions. (6) The observed ions generated within the clusters cannot be explained by simple monomer fragment ion solvation. Addition and elimination reactions within the clusters according to the suggested scheme in Figure 4 seem to account for all the observed ions. Conclusions a d Outlook
The main conclusions that can be drawn from this study are as follows: 1. Isoprene clusters provide a good example for demonstrating the feasibility of studying cationic polymerization within vdW clusters. The system seems to be sensitive to both the kinetic and internal energies of the reacting ions. The greater reactivity of the molecular ion leads to its consumption in a rapid Diels-Alder reaction. 2. The cationic polymerization of isoprene proceeds via the formation of the stable dimer C1oH16+ which, most likely, has a structure similar to limonene and can undergo further addition reactions to generate larger ions. 3. The formation of stable ionic species (cyclic structure) interrupts the general pattern of successive addition reactions. A clear example in isoprene study is the observation of a magic number corresponding to CI4H2'+ion. It is interesting that similar cyclic structures have been found in polyisoprene formed by bulk cationic polymerization of isoprene. 4. Clusters provide a feasible and valuable approach to understand both the mechanism of ionic polymerization and how the size of polymer chains is controlled in such a process. Work is under way in our laboratory to study ionic polymerization initiated by photoionization within the clusters.
Acknowledgment. The authors thank Professor R. M. Ottenbrite for helpful discussions regarding Diels-Alder reactions of isoprene. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, to the Thomas F. and Kate Miller Jeffress Memorial Trust, and to the Grants-In-Aid program for faculty of Virginia Commonwealth University for the partial support of this research.
Structure of the Fluorene-Benzene Aromatic Dimer from Rotational Coherence Spectroscopy P. W. Joireman, L. L. Connell, S. M. Ohline, and P. M. Feker**+ Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1549 (Received: March 26, 1991; In Final Form: May 9, 1991) Rotational coherence spectroscopy has been used to measure rotational constants for four isotopomers of the ammaticammatic dimer fluorentbenzene. Possibilities for the vibrationally averaged dimer geometry have been deduced from the measured values. In the geometries the benzene moiety is above ths five-membered ring of fluorene, displaced away from the apex of that ring, and tilted into the plane of the fluorene. The geometries are discussed in terms of the intermolecular forces between the species and the theoretical and experimental results on other aromatic dimer structures. Introduction Spectroscopic studies of binary molecular complexes have proved to very useful in efforts to characterize intermolecular forces.' Such studies have been especially successful in expanding and refining knowledge of hydrogen-bondingand moleculerare
'NSF Presidential Young Investigator 1987-92. 0022-365419112095-4935502.50/0
gas atom interactions. In large part, this success can be attributed to the fact that rotationally resolved spectra leading to structural information have been obtained for numerous hydrogen-bonded (1) For reviews, see: (a) Strucrure a d Dynamics of Weakry Bound Complexes; Weber, A., Ed.;Reidel: Dortrecht, 1987. (b) Legon, A. C.; Millen, D. J . Chem. Rev. 1986.86,635.
0 1991 American Chemical Society
