Control of End Groups in Anionic Polymerizations Using Phosphazene Bases and Protic Precursors As Initiating System (XH-Bu^tP₄ Approach): Application to the Ring-Opening Polymerization of Cyclopropane-1,1-Dicarboxylates

Abstract

A synthetic method involving the in situ generation of an anionic initiator Xobtained by reaction of its conjugate acid precursor XH with Bu t P 4 phosphazene base was tested as a possible way to easily and better control the end groups of polymers derived from the anionic ring-opening polymerization of cyclopropane-1,1-dicarboxylates. Several types of precursors were investigated, including thiols, alcohols, a carbazole, and a malonate. In all but one cases, a living polymerization mechanism could be observed, which was exploited to control the nature of the terminal end groups by reaction of the propagating malonate carbanion R-C(COOPr) 2with alkylating agents. It was also demonstrated that toluene was a much better solvent than the traditional one used in these reactions (THF) as a larger range of available temperatures was available despite an almost identical solvent influence on the polymerization. Exploitation of this feature provided access to higher degrees of polymerization than previously possible.

Design of Optimized Reaction Conditions for the Efficient Living Anionic Polymerization of Cyclopropane‐1,1‐Dicarboxylates

“…The obtained results matched quite satisfacto rily those described previously, with high yields, low polydisper sities and numberaverage molecular weights corresponding to the theoretical values calculated from the [M] 0 /[I] 0 ratio. [7] The internal reproducibility was also quite good (compare duplicate entries 1 and 2).…”

“…In the initial optimization experiments (entries 1 and 2 cor responding to identical conditions), rather drastic conditions were used to purge the monomer from dissolved gases, using three vacuum/nitrogen cycles followed by three freeze/thaw purge cycles as described in the original publication. [7] In addi tion, all reagents were added subsequently under a glove box filled with argon. The obtained results matched quite satisfacto rily those described previously, with high yields, low polydisper sities and numberaverage molecular weights corresponding to the theoretical values calculated from the [M] 0 /[I] 0 ratio.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] The ringopening polymeri zation of cyclopropanes was not a new concept at the time we started investigating the issue, but previous attempts had always led to oligomers, with broad to very broad molecular weight dis tributions. [14] For the first time, with the advent of our activated monomer approach, efficient procedures to polymerize livingly threemembered ring carbon monomers became available, yielding carbonchain polymers substituted on every third atom alongside the backbone (see Scheme 1A for an example).…”

“…In addition, high temperatures, up to 120-140 °C, can be used and further modi fications of the obtained macrocarbanion with electrophiles are possible due to the excellent thermal stability and wellknown nucleophilicity of malonate carbanions, the propagating species involved in these polymerizations. [3,7,14] The prototypical monomers that have been efficiently polymerized thus far in this series of activated cyclopropanes are based on cyclopropyl rings 1 geminally substituted by two electronwithdrawing groups, typically esters (Scheme 1) but also nitriles. Monomers with two propyl ester groups (either as n or ipropyl) have been particularly investigated, as both the monomers and their homopolymers are soluble in a wide variety of solvents, in contrast to methyl or ethyl ester polymers whose solubility in organic solvents com patible with mandatory anionic polymerization conditions is low, due to their high susceptibility to crystallize.…”

“…One set involves simple alkali ions, [1][2][3][4] while the other one implicates the large tBuP 4 H + phosphazenium cation derived from the protonation of tBuP 4 superbase (see Scheme 3 for a mechanism). [6,7] Both systems have their pros and cons. The first one is much cheaper, involves relatively straightforward workup processes, but the polymerization is slow, has to be carried out at high tempera tures (typically over 100 °C), which are not necessarily compat ible with some functional groups on the ester, and is practi cally limited to degrees of polymerization of 20-30.…”