Kinetic Principles of Living Anionic Polymerization of Isocyanates

Abstract

Polyisocyanates are an important class of synthetic helical polymers which have attracted much attraction in diverse fields of multidisciplinary chemistry. Many fascinating principles and practical appzed protein chains. The present thesis provides findings of the kinetic studies of the living anionic polymerization of isocyanates related to principles that determine the reaction mechanism, livingness, and compositional arrangement. Chapter 1 reviews the background knowledge related to the living anionic polymerization of isocyanates. Living anionic polymerization of vinyl monomers using carbanionic initiators was founded starting from a work of Michael Szwarc in 1956 and solidified by a number of findings of him and other researchers. These contributions have become the crucial foundation of the basic concept of anionic polymerization of isocyanates. Therefore, general theories of the living anionic polymerization of vinyl monomers are introduced first to help understand many aspects of the anionic polymerization of isocyanates. Then, the previous studies on the classical (non-living) and advanced (living) anionic polymerization of isocyanates are reviewed in sequence to elucidate the methodologies to accomplish the livingness of the anionic polymerization. At the end of this chapter, the necessity of establishment of kinetic principles of living anionic polymerization of isocyanates is finally discussed in terms of its consequence for the construction of high-MW polyisocyanates with a broad spectrum of tailored compositions and functionalities as protein-mimic materials. Chapter 2 identifies the kinetic principle of living anionic polymerization of n-hexyl isocyanate (HIC) using highly reactive sodium diphenylmethane (NaDPM) initiator in THF at −98 °C under 10–6 Torr. Incorporation of sodium tetraphenylborate (NaBPh4; [NaBPh4]0/[NaDPM]0 = 5) to the initiator helped to suppress the formation of unstable free amidate anions through a common-ion effect led to termination-free propagation by amidate ion pairs. At [HIC]0/[NaDPM]0 = 50.8/101/201, the initiation of NaDPM early reached ∼100% efficiencies during propagation, which led to the yield of poly(n-hexyl isocyanate)s (PHICs) with predictable molecular weights (Mn,theo = 6.50/12.7/24.7 kDa; Mn = 6.50/12.7/26.1 kDa) and low dispersities (Đ = 1.06/1.07/1.15). Within the conditions, the rate of propagation accorded with a first-order dependence on [NaDPM]0, indicating that the propagating amidate ion pairs are intrinsically unimeric (nonassociated). Chapter 3 deals with the propagation-inspired initiation of sodium N-phenethyl-3-phenylpropanamide (NaPEPPA), an aliphatic sodium amidate, for the living anionic polymerization of isocyanates in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10–6 Torr. This initiator was compared with sodium benzanilide (NaBA), an aromatic sodium amidate, in the living anionic homopolymerization of HIC ([NaBPh4]0/[NaBA]0 = 5). Only NaPEPPA attained the initiation efficiencies close to unity at the early stage of propagation due to fast and quantitative initiation. The rate of propagation reaction exhibited a first-order dependence on the initial molar concentration of NaPEPPA. The anionic polymerization with [HIC]0/[NaPEPPA]0 = 38.9/85.1/203 led to PHICs with predictable MWs and low dispersities (Mn,theo = 5.12/10.7/24.7 kDa; Mn = 5.22/11.1/27.4 kDa; Đ = 1.11/1.10/1.06). NaPEPPA was also used to initiate the living anionic polymerization of two functional isocyanates, furfuryl isocyanate (FIC) and allyl isocyanate (AIC), yielding well-defined poly(furfuryl isocyanate) (PFIC) and poly(allyl isocyanate) (PAIC), respectivly. The rate laws of propagation reaction for FIC, AIC, and HIC were determined from individual kinetic profiles. Chapter 4 verifies new mechanism of living anionic polymerization of isocyanate monomers initiated by a dimerically self-associative sodium diphenylamide (NaDPA) through the kinetic study and density functional theory calculations. The NaDPA-initiated anionic polymerization of HIC proceeded in a living manner in the presence of NaBPh4 ([NaBPh4]0/[NaDPA]0 = 5) in THF at −98 °C under 10−6 Torr. The kinetic profiles revealed the one-half initiation efficiency of NaDPA and one-half-order propagation reaction rate on [NaDPA]0. A mechanism, initiator-transfer anionic polymerization (ITAP), is proposed basing on NaDPA in a dual role, with half of the NaDPAs initiating polymerization while by the other half protecting the chain end by a reversible 1:1 cross-association/dissociation. The anionic polymerization with varying [HIC]0/[NaDPA]0 ratio yielded PHICs with MWs nearly twice the theoretical values and with low dispersities. Chapter 5 describes the kinetic behaviors of the living anionic copolymerization of binary mixtures of FIC, AIC, and HIC initiated by NaPEPPA that have different β-carbon substituents in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10–6 Torr. The rate constants of the self- and cross-propagation reactions were determined, and their comparison showed the electrophilicity order of isocyanate as FIC > AIC > HIC. The kinetic analysis revealed that the anionic copolymerization of FIC:HIC, AIC:HIC and FIC:AIC generated three different sequence-controlled polyisocyanate copolymers, poly(furfuryl isocyanate-block-n-hexyl isocyanate) (P(FIC-b-HIC)), poly(allyl isocyanate-tapered block-n-hexyl isocyanate) (P(AIC-tb-HIC)) and poly(furfuryl isocyanate-gradient-allyl isocyanate) (P(FIC-grad-HIC)), respectively. The homogeneous sequential anionic copolymerization of the binary isocyanate mixtures generated the polyisocyanate copolymers having repeated monomer sequences. A series connection of P(FIC-b-HIC), P(AIC-tb-HIC) and P(FIC-grad-HIC) segments in the copolymer was attempted by heterogeneous sequential anionic copolymerization of the binary isocyanate mixtures. The variation of the comonomer addition order allowed the resulting polyisocyanate copolymers to be encoded by the three combined monomer sequences.