Abstract
To synthesize well-defined poly (α-amino acid)s (PAAs), ring opening polymerizations (ROP) of cyclic monomers of α-amino acid N-carboxyanhydrides (NCAs) and N-thiocarboxyanhydrides (NTAs) are most widely used. In this mini-review, we summarize the mechanism details of the monomer preparation and ROP. The present study used density functional theory calculations to reveal the mechanisms together with experimental phenomena in the past decades. Detailed discussion includes normal amine mechanism and the selectivity of the initiators bearing various nucleophilic groups.
Highlights
Poly(α-amino acid)s (PAAs) are promising biomaterials that have been employed in drug delivery, gene engineering, and self-assembly fields in recent years (Deming, 2016; Gangloff et al, 2016; Tao et al, 2018a; Birke et al, 2018)
In amine-mediated ring opening polymerization (ROP) of NCA and NTA, integrated understanding of normal amine mechanism (NAM) has been established by DFT calculations including three-step framework, NAM-TMS mechanism, the effect of N-substitution in initiator or monomer ring, and the α,ω-functionalized initiator
Besides NAM, two problems have still not been solved in the ROP of NCA and NTA
Summary
Poly(α-amino acid)s (PAAs) are promising biomaterials that have been employed in drug delivery, gene engineering, and self-assembly fields in recent years (Deming, 2016; Gangloff et al, 2016; Tao et al, 2018a; Birke et al, 2018). The ring opening polymerization (ROP) of cyclic monomers is still the most promising synthetic route toward PAAs. Due to the existence of multiple active sites located on NCA or in the NTA ring, including carbonyl groups at 2- and 5-positions for nucleophilic attack and acidic protons of 3-NH and 4-CH groups, various initiators have been carried out for polymerization including amines (Kricheldorf, 1987), trimethylsilyl (TMS) containing compounds, (Lu and Cheng, 2007, 2008; Yuan et al, 2016; Baumgartner et al, 2017; Yuan et al, 2018), salts (Szwarc, 1965; Wu et al, 2018), (activated) alcohols (Zhao et al, 2019; 2020), N-heterocyclic carbene (Guo and Zhang, 2009; Falivene et al, 2016; Falivene and Cavallo, 2017), transition metal catalysts (Deming, 1997), rare earth complexes (Ling et al, 2012; Peng et al, 2012; Tao et al, 2014), and so on. Initiators in NAM usually bear at least one labile proton, as
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