π-AllylPdCl-Based Initiating Systems for Polymerization of Alkyl Diazoacetates: Initiation and Termination Mechanism Based on Analysis of Polymer Chain End Structures

Abstract

Polymerization of ethyl and benzyl diazoacetates (EDA and BDA) initiated with π-allylPdCl-based systems [π-allylPdCl/NaBPh4, π-allylPdCl/NaBArF 4 (ArF = 3,5-{CF3}2C6H3), and π-allylPdCl] is described. Initiation efficiencies of the π-allylPdCl-based systems are much higher than those of the previously reported (NHC)­Pd/borate (NHC = N-heterocyclic carbene) systems, and the new systems are capable of polymerizing the alkyl diazoacetates at low temperatures (0 ∼ −20 °C), where the (NHC)­Pd/borate systems cannot initiate the polymerization. MALDI–TOF–MS analyses of the polymers obtained from EDA provide information for the chain-end structures of the polymers, based on which initiation and termination mechanisms are proposed. Interestingly, EDA polymerization by the π-allylPdCl-based systems in the presence of alcohols (EtOH, nPrOH, and nBuOH) or water was found to afford RO- or HO-initiated polymers as major products, as confirmed by MALDI–TOF–MS analyses.

Synthesis of polymers with densely-grafted oligo(ethylene glycol)s by Pd-initiated polymerization of oxyethylene-containing diazoacetates

“…At a concentration of 2.5 wt%, the cloud points gradually decreased with decreasing M n in the studied molecular weight range. 1 and S1-S5 †) suggests that the products obtained here are atactic polymers, 3,20 and thus, dependence of tacticity on cloud points is not clear for these polymers. It is known that the cloud point is generally decreased by hydrophobic end groups and the effect is more enhanced for shorter polymer chains, because the hydrophobic end groups allow the formation of some aggregates even below the transition temperature, which reduces the entropy of mixing of the polymer chain to bring about a lower phase transition temperature.…”

“…The results are summarized in Table 1. In our previous publication, 20 we observed that, on the assumption that the SEC-estimated M n s are correct, the average number of polymers produced per Pd (N p ) calculated from M n and polymer yield exceeded 1.0 in the polymerization of ethyl diazoacetate using (η 3 -C 3 H 5 )PdClbased initiating systems, and the N p value became higher with the increase of the monomer feed ratio, suggesting that some kinds of chain transfers occur during the polymerization. The addition of NaBPh 4 as a cocatalyst was effective in increasing the polymer yield and molecular weight (entry 2, yield = 51%, M n = 3600), as is the case with the polymerization of hydroxy-containing diazoacetates.…”

“…Fig. 20 Although the above-mentioned chain transfer should generate polymers with end groups other than the phenyl group, the predominant phenyl group should work as a hydrophobic end group, resulting in lower phase separation temperatures with decreasing M n at 2.5 wt%. We can observe three types of M n -dependence on cloud points depending on the polymer concentration.…”

Polymerization of Hydroxy-Containing Diazoacetates: Synthesis of Hydroxy-Containing “Poly(substituted methylene)s” by Palladium-Mediated Polymerization and Poly(ester–ether)s by Polycondensation through O–H Insertion Reaction

“…11 Based on this result of the chain-end structure analysis, we have proposed the mechanism described in Scheme 2A, where Pd-OR was generated as a major initiating species during the period for initiator preparation to yield polymers, and a part of the RO-initiated polymers could also be formed by chain transfer of propagating chains with ROH followed by initiation with the resulting Pd-OR species. Herein, we carried out polymerization in the presence of an equimolar amount of an alcohol with a monomer as a model reaction for the polymerization of hydroxy-containing monomers.…”

Aqueous Phase Separation Behavior of Highly Syndiotactic, High Molecular Weight Polymers with Densely Packed Hydroxy-Containing Side Groups

“…By changing the monomer/catalyst ratio to 25:1 ( Table 1 , entry 6), we were able to synthesize polymers with a molecular weight almost half of the M w obtained when using a 50:1 ratio ( Table 1 , entry 5). Our group 40 and the group of Ihara 26 have independently reported that in the presence of alcohols or water Rh- and Pd-catalyzed polymerization of diazoacetates proceeds to give polymers. Ihara showed that when using [(η 3 -C 3 H 5 )PdCl] as the catalyst, direct polymerization of the unprotected hydroxyl-containing diazoacetate M 5 is possible and forms the same polymers as when using the protected monomer s M 5 .…”