Radical Ring-Opening Polymerization of Phosphorus Heterocycles: Computational Design of Suitable Phosphetane Monomers
Michelle L. Coote A B , Jennifer L. Hodgson A , Elizabeth H. Krenske A , Mansoor Namazian A and S. Bruce Wild AA Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia.
B Corresponding author. Email: mcoote@rsc.anu.edu.au
Australian Journal of Chemistry 60(10) 744-753 https://doi.org/10.1071/CH07121
Submitted: 25 April 2007 Accepted: 6 July 2007 Published: 9 October 2007
Abstract
High-level ab initio calculations have been used to determine the propensities of various phosphetanes towards radical ring-opening polymerization. At the G3(MP2)-RAD level of theory, the propagation rate constants of 1-methylphosphetane (7.5 × 104 L mol–1 s–1), 1-phenylphosphetane (4.6 × 105 L mol–1 s–1), cis,cis-2,4-dichloro-1-phenylphosphetane (3.8 × 107 L mol–1 s–1), cis,cis-2,4-difluoro-1-phenylphosphetane (3.0 × 107 L mol–1 s–1), and 1-phenyl-3-oxaphosphetane (4.0 × 106 L mol–1 s–1) are very high, rendering them unsuitable for copolymerization with common alkenes. In contrast, the propagation rate constants of 1-tert-butylphosphetane (1.7 × 103 L mol–1 s–1) and cis,cis-2,4-dimethyl-1-phenylphosphetane (9.2 × 102 L mol–1 s–1) indicate that either incorporation of a t-butyl substituent at phosphorus or alkylation at the 2- and/or 4-positions will produce monomers with more compatible reactivities for copolymerization with alkenes. In the case of 1-tert-butylphosphetane, however, homolytic substitution of the propagating radical with the t-butyl substituent at P will be competitive with the propagation step and could affect the microstructure of the polymer. The borane adduct and the oxide of 1-phenylphosphetane were both found to be unreactive towards radical ring-opening. The calculations suggest that, for chiral phosphetanes, the ring-opening reaction is enantioselective at phosphorus and the resulting polymer will be isotactic.
Acknowledgement
We gratefully acknowledge generous allocations of computing from the Australian Partnership for Advanced Computing and the Australian National University Supercomputing Facility, and provision of an Australian Research Council Discovery grant (to MLC and SBW).
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* Propagation rate coefficients calculated at the G3(MP2)-RAD//B3-LYP/6-31G(d) level of theory using standard transition state theory in conjunction with the rigid rotor/harmonic oscillator approximation. The k11 value for phenyl phosphetane homopropagation differs slightly from that in Table 1 as the latter used the more accurate variational transition state theory. The k22 value for X = Cl was taken from Ref. [5].
