1070-4272/05/7807-1137+2005 Pleiades Publishing, Inc.
Russian Journal of Applied Chemistry, Vol. 78, No. 7, 2005, pp. 1137!1144. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 7,
2005, pp. 1157!1164.
Original Russian Text Copyright C 2005 by Kalnin’sh, Semenov.
AND POLYMERIC MATERIALS
Excited Biradicals in Thermal Dimerization
and Polymerization of Bicyclobutanes
K. K. Kalnin’sh and S. G. Semenov
Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia
St. Petersburg State University, St. Petersburg, Russia
Received April 21, 2005
Abstract-The structure and electronic parameters of 1-cyanobicyclobutane and 1-cyano-3-vinylbicyclo-
butane were determined by RHF, ROHF, and GVB (TCSCF)/6-31G* quantum-chemical calculations.
Theoretical study of the polymerization mechanism
at the level of elementary steps gives an insight into
the regioselectivity, chemical structure of the prod-
ucts, and reaction rates. Not only well-understood
polymerization reactions (i.e., anionic polymerization
of 1,3-butadiene or isoprene ) but also other little
known reactions are of interest in this respect. 1-Cya-
nobicyclobutane (I) and 1-cyano-3-vinylbicyclobu-
tane (II) have unusual steric structure and are highly
reactive. These are colorless compounds stable at
380oC in the presence of a free-radical inhibitor [2, 3].
Compound I in the liquid state or in a solution
dimerizes on heating to +80oC. At ~150oC, the trimer
of this compound is formed . Carbonitrile I poly-
merizes at room temperature or on mild heating in
the presence of a radical initiator, with the polymer
skeleton consisting mainly of four-membered carbon
monocycles [2, 3]. Carbonitrile I can copolymerize
with acrylonitrile, styrene, methyl methacrylate, vinyl
chloride, isobutene, sulfur dioxide, and other mono-
mers. The products are high-melting thermally stable
polymers soluble in polar solvents .
This study is based on the theoretical concept on
electronically excited biradicals or their complexes
formed in thermal reactions occurring on molecular
collisions . Generally, neutral or charged excited
radicals (biradicals) (thermoexciplexes or thermo-
excimers) of various chemical structures should be
considered. The main feature of these species is low
energy (<1 eV) relative to that of free reagents.
These states are populated at ordinary temperatures.
The final reaction step is recombination of the radicals
to form stable molecular products. Previously, with
perfluorinated olefins as example, we showed  that
electronically excited biradical quasi-degenerate states
are involved in thermal dimerization. The ex-
perimental heats of formation of the intermediates
were correlated with the calculated excitation energies
of the olefins.
The existence of these electronic states and the
possibility of their thermal population are indicated by
instability of bicyclobutanes in the absence of radical
initiators at temperatures higher than 380oC. In this
study, ab initio quantum-chemical calculations of the
molecular geometries of I and II in the ground and
lowest excited states were performed with the aim to
determine the mechanism of dimerization and poly-
merization of these compounds.
Calculation procedures. Quantum-chemical cal-
culation of the electronic and steric structure, vibra-
tional spectra, and relative energies of the compounds
in the ground (S
) and triplet (T
) electronic states
were performed by restricted Hartree3Fock methods
(RHF/6-31G* and ROHF/6-31G*) using GAMESS
software  (version pcGAMESS 6.2 ). In the case
of lowest excited singlet electronic states (S
used the generalized two-configuration version of the
valence bond procedure (TCSCF GVB/6-31G*). The
geometric parameters of the triplet states were calcu-
lated by energy minimization at the ROHF level with-
out symmetry restrictions. The geometry of the singlet
excited states was assumed to be identical to that of
states and then was optimized at the GVB
level by minimization of total energies at fixed popu-
lations of the active natural MO n
= 1. It should
be noted that the two-configuration method of gen-
eralized valence bonds, used to calculate the singlet
excited state, corresponds to the ROHF calculation of
the triplet state.
The valence V
of each atom A in a molecule or
free radical is a sum of the indices of the chemical