Additive-controlled Switching from Abnormal to Normal Unstoichiometric Suzuki–Miyaura Polycondensation for Poly(biphenylenevinylene)

Abstract

SuzukiMiyaura polycondensation of substituted 4,4¤-stilbenediboronic acid ester with an excess of substituted 4,4¤-dibromostilbene in the presence of 5 mol % of t-Bu 3 P-ligated palladium (Pd) precatalyst afforded high-molecular-weight poly(biphenylenevinylene) (PBPV) with boronic acid ester moieties at both ends, but the same polymerization in the presence of unsubstituted stilbene gave low-molecular-weight PBPV with bromines at both ends.In the polycondensation of AA monomer and BB monomer, the use of exactly equimolar amounts of the two monomers is crucial for obtaining a high-molecular-weight polymer, according to the Flory principle.1 If excess AA monomer reacts with BB monomer, a low-molecular-weight polymer with the A functional group at both ends is mainly obtained (designated as "normal unstoichiometric polycondensation"). However, some polycondensations yield a high-molecular-weight polymer with the B functional group at both ends, even if excess AA monomer is used (designated as "abnormal unstoichiometric polycondensation"); 2 examples include the polycondensation of diphenol and dihalomethane, yielding polyformal, 3 and the superacidcatalyzed polycondensation of aliphatic 1,2-diketones and nonactivated aromatic hydrocarbons. 4 This behavior is attributed to enhancement of the reactivity of one B group in the BB monomer when the other B group reacts with the AA monomer.Transition-metal-catalyzed polycondensation can also occur in an abnormal unstoichiometric manner through intramolecular catalyst transfer on the monomer. Nomura found that the Pdcatalyzed TsujiTrost reaction of the malonic acid ester with difunctional allyl acetate afforded a high-molecular-weight polymer even when an excess of the latter monomer was used. 5 We have recently reported abnormal unstoichiometric Suzuki Miyaura polycondensation with a t-Bu 3 P-ligated Pd catalyst, which undergoes intramolecular transfer on the dibromoarene monomer, to yield π-conjugated aromatic polymers with boronate at both ends.6 Higashihara reported similar polymerization behavior in a Stille coupling reaction. 7In these polycondensations, the occurrence of either normal or abnormal unstoichiometric polycondensation is dependent on the nature of the monomers and catalysts, and therefore the possibility that both normal and abnormal unstoichiometric polycondensations of the same monomers might be achieved simply by changing the polymerization conditions is an intriguing one. Such switching of the polymerization mode would allow us to control the molecular weight and the chain end groups of polymers obtained by polycondensation of AA and BB monomers. In this context, it occurred to us that Suzuki Miyaura coupling polymerization using tetraalkoxy-substituted 4,4¤-dibromostilbene would enable such switching, because the t-Bu 3 P-ligated Pd catalyst walks from one phenyl ring to the other through the carboncarbon double bond (C=C) in model reactions, while the intramolecular catalyst transfer is suppressed by trapping the catalyst by addition of alkenes...

Self-Accelerating Click Reaction in Step Polymerization

“…By eliminating the equal reactivity of functional groups, several stoichiometric imbalance-promoted homogeneous step polymerization methods with AA and BB type monomers have been developed to produce the high molecular weight polymers. − One common feature of these methods lies in the fact that they use the self-accelerating reaction as the condensation reaction. In this approach, the reaction of the first functional group like B in BB monomers with A functional group significantly enhances the reactivity of the remaining B group.…”

“…The activated B group reacts with A group having a much larger reaction rate constant compared to the original B group in BB monomers. In this case, the usage of excess molar amount of BB monomers in a certain degree does not deteriorate the molecular weight of the resultant polymers but significantly increases it as a matter of fact. − Because the self-accelerating reaction property eliminates the effect of excess BB monomers on the second step of condensation reaction between A and activated B groups, the excess BB monomers can be used to enhance the efficiency of the first rate-determining step of condensation reaction between A group and BB monomers. In addition, the usage of excess BB monomers can assist A group to achieve a quantitative conversion during the polymerization.…”

Polymer Synthesis Based on Self‐Accelerating 1,3‐Dipolar Cycloaddition Click Reactions†

“…To date, several stoichiometric imbalance‐promoted homogeneous step polymerization methods have been developed based on varied self‐accelerating reactions. They include palladium‐catalyzed allylation [ 53 ] and etherification polymerization, [ 54 ] palladium‐catalyzed Suzuki‐Miyaura and Stille coupling polycondensations, [ 13,55‐56 ] rhodium‐catalyzed oxidative polycoupling of internal diynes and phenylpyrazole [ 14 ] or arylboronic acids, [ 57 ] polycondensation of Friedel−Crafts acylation [ 17,58 ] or hydroxyalkylation reaction, [ 59‐61 ] poly‐Radziszewski reaction, [ 16 ] polycondensations of dibromomethane and 4,4’‐thiobisbenzenethiol [ 62 ] or bisphenol A, [ 63 ] and polycondensation of bisphenol A with 2,2‐dichloro‐1,3‐benzodioxole. [ 64 ] These stoichiometric imbalance‐ promoted homogeneous step polymerization methods have been summarized in recent review papers.…”

Catalyst‐Transfer Suzuki–Miyaura Condensation Polymerization of Stilbene Monomer: Different Polymerization Behavior Depending on Halide and Aryl Group of ArPd(tBu3P)X Initiator

“…Elucidation of the structural requirements for Pd catalyst transfer on CC allowed us to conduct unstoichiometric Suzuki–Miyaura coupling polymerization of stilbenediboronic acid ester and excess alkoxy‐substituted dibromostilbene, affording high‐molecular‐weight poly(biphenylenevinylene) (PBPV) with boronate at both ends; this polymerization behavior is accounted for by intramolecular transfer of the Pd catalyst on dibromostilbene . However, we have not studied CTCP of alkoxy‐substituted stilbene AB‐type monomer through intramolecular catalyst transfer from the Pd initiator to the polymer end.…”