A. Dworak, THE ROLE OF CATIONIC AND COVALENT ACTIVE-CENTERS IN THE POLYMERIZATION OF 2-METHYL-2-OXAZOLINE INITIATED WITH BENZYL BROMIDE, Macromolecular chemistry and physics, 199(9), 1998, pp. 1843-1849
It was shown that in the polymerization of 2-methyl-2-oxazoline initia
ted with benzyl bromide in nitrobenzene the equilibrium constant of th
e interconversion of cationic and covalent active centers could be mea
sured over a wide range of temperatures, thus making the thermodynamic
parameters of this reaction available. The ''temperature jump'' metho
d was used to measure the rate constants and the activation parameters
of the reversible transformation of the ionic active centers into cov
alent ones. The apparent rate constant of the chain growth in this pol
ymerization was measured, and individual rate constants of chain growt
h on ionic and covalent centers were approximated using systems in whi
ch one kind of active centers strongly prevails. The obtained data all
owed for a complete description of the individual reactions and for an
estimation of the contribution of these reactions to chain growth. Be
cause of the higher reactivity of the ionic species and higher rate of
ionization without monomer participation (thus, through intramolecula
r ionization), there are no conditions at which the covalent species m
ay prevail in the chain growth. Even if there is a large excess of cov
alent species, propagation proceeds on the ionic ones. The effective m
onomer concentration is approximately 10(3) mol/L, indicating that at
1 mol/L of monomer the internal ionization proceeds 10(3) times faster
than the external ionization (with monomer).