Copolymerization of Vinyl Acetate and Maleic Anhydride
The Journal of Physical Chemistty, Vol. 83, No. 15, 1979
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Conformational and Nuclear Magnetic Resonance Studies of the Charge Transfer Complex between Vinyl Acetate and Maleic Anhydride. Reactivity of this Complex in Radical Alternating Copolymerization Denis Ghesquiere, Service de chimie physique, C.E.N. Saciay, B.P. No. 2, F 91 190 Gif Sur Yvette, France
Roger Arnaud, Laboratoire de chimie g6n6rale, Universit.4 Scientifique et Mgdicale de Grenoble, B.P. No. 53, Centre de tri, F 38041 Grenobie, Cedex, France
and Claude Caze* Laboratoire de chimie macromoi6culaire, Universit6 des Sciences et Techniques de Liiie, B.P. No. 36, F 59650 Vileneuve D’ Ascq, France (Received June 6, 1978; Revised Manuscript Received April 5, 1979)
The association constant for the charge transfer complex between maleic anhydride and vinyl acetate was obtained from ‘H NMR measurements. The most probable conformation of the complex was investigated with PCILO calculations which showed that the most stable geometry is a “sandwich” form. The electronic structure of the complex was deduced from CNDO/2 calculations. The calculated charge distributions and the observed variation of I3C NMR shifts agree with each other. The relative reactivity of each isolated monomeric molecule, and of the complex in radical alternating copolymerization, has been discussed qualitatively in terms of frontier orbitals.
Introduction The alternating copolymerization1 of vinyl acetate (VA) and maleic anhydride (MA) involves the participation of a charge transfer complex between c o m ~ n o m e r s . ~The -~ particular reactivity of the ~ o m p l e xis~not , ~ yet explained. Therefore, we undertook a NMR study (‘H and 13C) and theoretical calculations on the VA-MA system. Starting from these experimental and theoretical results, we proposed a qualitative interpretation of the reactivity of the complex.
In order to study the conformation of the complex we investigated the following various structures: (i) The donor and the acceptor molecules were located on parallel planes (sandwich model). The coordinate axis and the relative position of the donor (VA) and acceptor (MA) molecules are illustrated in Figure 2. The midpoint P of the C=C double bond of VA was taken as the origin of coordinates and in the computation the middle M of MA C=C double bond was moved in the three dimensional space, varying the X , Y , and 2 coordinates. The relative position of the molecules was defined by the values of X , Y , 2, and the angle a (shown in Figure 2) which is the projection on the XPY plane of the angle between the plane containing P and M and perpendicular to XPY plane. (ii) The molecules of the complex were considered in the same plane. Figure 3 shows two conformations investigated here. The complex stabilization energy is given by
Experimental Section Materials. Vinyl acetate was purified according to a procedure described in the literature.6 Maleic anhydride was twice recrystallized from a purified chloroform solution, dried under reduced pressure, and then distilled under 10-1torr pressure just before use. All solvents were purified by using standard procedures. NMR Spectra. Fourier transform NMR spectra were EST = Ec - E, (1) recorded at 250 MHz for ‘H (Cameca TSN 250) and at 25.2 MHz under complete noise decoupling of protons for 13C where E , is the energy of isolated donor and acceptor (Varian XL 100) with Me4Si as internal reference at 25 “C. molecules and Ec the energy of the complex. Calculation and Choice of Model. For conformational studies, we used the PCILO method (perturbative conResults and Discussion figuration interaction with localized orbitals) in its ‘H NMR Determination of the Equilibrium Constant. standard f ~ r m . ~For - ~conformational problems, such a The equilibrium constant K for charge transfer complex method is more suitable and gives better values for the formation between vinyl acetate and maleic anhydride is energy than classical SCF calculations in a much shorter defined by eq 2 , where C refers to the charge transfer calculation time. Recently this procedure has been applied MA+VApC successfully to molecular associations studies: hydrogen bonded complexeslOJ1and charge transfer c ~ m p l e x e s . ~ ~ J ~ K = [Cl/([MAl[VAI) (2) The complex between maleic anhydride and vinyl acetate can be considered as a single molecule. Figure 1 complex, K was evaluated by ‘H NMR, according to the gives the well-known planar geometry of MAI4 and VA15J6 method of Hanna et al.I7 molecules. Intramolecular distances and angles were kept With the assumption of rapid exchange between comconstant in all the following calculations. plexed and free states of MA molecules, one finds that the 0022-3654/79/2083-2029$0 1.OO/O
0 1979 American Chemical Society
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The Journal of Physical Chemistty, Vol. 83, No. 15, 1979
D. Ghesquiere, R. Arnaud, and C. Care
TABLE I: Variation of the 'C Chemical Shift between the Complex and Uncomplexed VA Moleculea solvent
c,
concn, mol L-l
none
VA= VA= VA= VA=
CJA, CDC1,
7.3 5.8 6.0 2.0
MA= MA= MA= MA=
3.3 5.0 1.5 1.2
t 0.44 -t0.69 t 0.47 t 0.60
c 2
c 4
C6
-0.12
t0.50
t 0.05
-0.17
t 0.80 t 0.56 t 0.30
t 0.10
-0.06 -0.21
+ 0.06
t 0.09
are calculated by difference between the resonance in ppm of the complex and uncomplexed molecule in the same solvent with Me4Si as internal reference and at 25 C. For notation see Figure 1. a A6
H
H
l/Aobs ( 1 O 2 H Z ~ ' )
Flgure 4. l/[VA], vs. l/Aobd for MA - VA complexation (MA proton) in C6Hj2at 250 MHz, t = 25 "C.
C&l,*3
where 6:td, 6FA,and 6FAare the shifts of the MA protons in a given observation, in the free form, and in the pure complex, respectively. If the initial concentrations are chosen as [MA],
