FREE-RADICAL POLYMERIZATION FOR NARROW-POLYDISPERSITY RESINS - SEMIEMPIRICAL MOLECULAR-ORBITAL CALCULATIONS AS A CRITERION FOR SELECTING STABLE FREE-RADICAL REVERSIBLE TERMINATORS
Pm. Kazmaier et al., FREE-RADICAL POLYMERIZATION FOR NARROW-POLYDISPERSITY RESINS - SEMIEMPIRICAL MOLECULAR-ORBITAL CALCULATIONS AS A CRITERION FOR SELECTING STABLE FREE-RADICAL REVERSIBLE TERMINATORS, Macromolecules, 28(6), 1995, pp. 1841-1846
Semiempirical molecular orbital calculations at the AM1 and PM3 levels
have been used to model stable free-radical-mediated living polymeriz
ation reactions. These calculations predict that in the living free-ra
dical polymerization, the reversible terminator of the growing polymer
radical must have a calculated bond dissociation enthalpy of less tha
n 35 kcal/mol in order to achieve reasonable rates of chain propagatio
n. Furthermore, the calculations also correctly predict that reversibl
e terminators such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) hav
e an endothermic enthalpy of reaction with styrene monomer and, theref
ore, are not able to initiate new chains, a prerequisite for narrow po
lydispersity in these systems. Calculations have been performed on 21
possible reversible terminator structures. One of these reversible ter
minators, di-tert-butyl nitroxide, was predicted to have a lower bond
dissociation enthalpy than the benchmark TEMPO reversible terminator.
Experiments have confirmed that under comparable reaction conditions,
the di-tert-butyl nitroxide-mediated reaction proceeds to completion m
ore rapidly than the comparable TEMPO reaction, consistent with the pr
edictions of the molecular orbital calculations.