Chain‐growth polycondensation via the substituent effect: Investigation of the monomer structure on synthesis of poly(N‐octyl‐benzamide)

Abstract

A systematic study of the behavior of different leaving groups on a variety of ester‐based monomers was performed for the chain‐growth polycondensation synthesis of poly(N‐octyl benzamide). Linear and branched alkane esters were compared with their phenyl analogs using both computational and experimental methods. Kinetic experiments along with qualitative solubility observations were used, with the aid of nuclear magnetic resonance spectroscopy and gel‐permeation chromatography, to determine progress of the reaction, molecular weights, and molecular weight distributions. It was found that the reactivity of the monomer's ester group depends more on the stability of the leaving alkoxide than the electrophilicity of the carbonyl carbon, which contradicts previous literature. The order of reactivity increases for the alkyl esters with decreasing steric hindrance and decreasing pKa of the substituent. For the phenyl ester derivatives, the more electron withdrawing character of a para substituent increases the reactivity of the ester group, due to the higher resonance stabilization of the leaving phenoxide anion, not due to an increase in the electrophilicity of the carbonyl carbon.

Chain‐growth polycondensation via the substituent effect: Investigation in to the role of initiator and base on the synthesis of poly(N‐octyl benzamide)

“…[20] In an effort to better understand the effect of monomer structure on the kinetics of the CGC process and on the polymers produced, we recently investigated the polymerization of a variety of N-octyl benzamidebased monomers with different ester substituents. [8] The main conclusion from this work was that the structure of the monomers ester-leaving group dictates the reactivity of the monomer in CGC and the solubility of the polymerization by-products. One of the most interesting observations from this previous study was that, while the phenyl ester monomers demonstrated faster reactivity and increased solubility of the lithium phenoxide byproducts, they were also more difficult to control during polymerization, which resulted in broader molecular weight distributions and multimodal GPC traces.…”

“…The methyl ester monomer M-OAB was chosen as a control, since it has been extensively used in previous studies. [8,18,[20][21][22][23][24] The polymerization of M-OAB with no initiator and 1 M equivalent of LiHMDS yielded no measurable polymer formation over a 12 hr period at either 0 or −20 C, indicating that, at these temperatures, self-initiation is not present for the commonly used M-OAB monomer. However, our previous studies have demonstrated that the methyl ester monomer has low reactivity at these temperatures, even in the presence of initiator, due to the basicity of the methoxide-leaving group.…”

“…DMA-P is synthesized using a previously reported procedure with the following conditions [8] : 4-(phenoxycarbonyl) benzoic acid (1.1 g, 4.5 mmol), SOCl 2 (11 ml, 150 mmol), dimethylamine (2.73 ml, 5.5 mmol), triethylamine (0.7 ml, 5.0 mmol), and DCM (20 ml). Product: as white crystals; mp 111-112 C, (0.86 g, yield 70%).…”

“…[2] Previous work has demonstrated that well-defined aromatic polyamides can be produced using the CGC method with various phenyl 4-octylaminobenzoate monomers, a strong nonnucleophilic lithium amide base to prepare the amide anion on the monomer, and an initiating species containing an activated ester. [8,18,19] Within this work, there has been a preference toward methyl ester-based monomers for CGC, as these monomers yield low-boiling point methanol as the by-product after the aqueous workup, which is claimed to make the overall process more convenient. [10] However, previous work by our group using the methyl ester monomer M-OAB to produce polymer brushes from surface-immobilized initiators, demonstrated that even though polymer brushes could be formed, issues with by-product solubility and slow kinetics restricted the thickness of the polymer brushes.…”

“…As such, it is evident that an initiator with an ester group that is more reactive than the propagating polymer chain is required to produce well-defined polymers via the CGC process. This concept was particularly evident in the recent work from our group that examined the effect of monomer structure on the kinetics of CGC polymerization in the preparation of poly(N-octyl benzamide) [8] This work demonstrated that the reactivity of substituted phenyl 4-octylaminobenzoate monomers dramatically increased, when the monomer contained an electron-withdrawing phenyl ester group as the leaving group in the nucleophilic acyl substitution reaction to add monomer to the polymer chain end. However, when polymerizing these reactive monomers, it was observed that control over the polymerization was decreased when using conventional initiators for CGC systems.…”