Synthesis of Tris-Phosphazene Bases with Triazine as Core and Their Applications for Efficient Ring-Opening Alternating Copolymerization of Epoxide and Anhydride: Notable Effect of Basicity and Molecular Size

Abstract

Phosphazenes as organocatalysts for the synthesis of polymers have evolved to powerful tools, and their catalytic performances highly depend on the basicity and molecular structure (size and shape). Therefore, designing phosphazenes with tunable basicity and molecular structure is greatly promising for the development of organocatalysts with improved catalytic properties, for example, high activity and selectivity. In this contribution, 2,4,6-tris[tri(dimethylamino)iminophosphorane]-1,3,5-triazine (C 3 N 3 -Me-P 3 ) and 2,4,6-tris[tri(1-pyrrolidinyl)iminophosphorane]-1,3,5-triazine (C 3 N 3 -Py-P 3 ) containing a 1,3,5-triazine-core were designed and synthesized. NMR spectroscopy analysis and single-crystal X-ray diffractions reveal that C 3 N 3 -Me-P 3 and C 3 N 3 -Py-P 3 , particularly the latter, show relatively low basicity, similar as t-BuP 1 , but have a bulky molecular size, similar as t-BuP 4 . C 3 N 3 -Me-P 3 and C 3 N 3 -Py-P 3 were successfully employed as organocatalysts for the ring-opening alternating copolymerization (ROAC) of anhydrides and epoxides with high activity. The produced polyesters were characterized using NMR spectroscopy, GPC and MALDI TOF, revealing perfectly alternating sequence, controlled molar mass and low dispersity and suggesting highly controlled ROAC reactions. Thus, well-defined triblock polyester P(PA-alt-CHO)-b-P(PA-alt-PO)-b-P(PA-alt-CHO) was facilely synthesized by one-pot reaction via sequential addition of two different epoxides.

Organocatalytic Ring‐Opening Alternating Copolymerization of Epoxides and Cyclic Anhydrides by a Simple Organobase/Urea Binary Catalyst

“…These results suggested the presence of phthalic acid as an impurity acted as a difunctional initiator even if the PA has been intensively purified, which was consistent with previous reports. [ 11a,14 ] Besides, the tendency of intramolecular transesterification and/or back‐biting side reactions increased when a large DP was targeted as evidenced by the presence of cyclic P(PA‐ alt ‐CHO) (Figure S9, Supporting Information), which agreed well with the tailings of SEC curves in the low molecular weight region (Figure 3). Both the polymer chains initiated by phthalic acid as well as the intramolecular transesterification accounted for the lower molecular weights than the targeted values and broadened distributions of obtained P(PA‐ alt ‐CHO).…”

“…The ROAC of PA with CHO was then conducted at various feeding molar ratio to obtain P(PA-alt-CHO) with different molecular weights. As shown in Figure 3, the molecular weight of P(PA-alt-CHO) displays a linear increase from 2.5 to 4.0 to 4.5 to 5.4 to 6.4 kDa as the feeding molar ratio of PA/CHO/BDM increases from 50/100/1 to 75/150/1 to 100/200/1 to 125/250/1 to 150/300/1, respectively (run [12][13][14][15][16]. The SEC traces display narrow distributions, but tailings are observed when the feeding molar ratio of PA/CHO/BDM is larger than 125.…”

“…[ 11a,c ] Our group recently developed two new phosphazene bases possessing similar basicity but bulkier molecular size relative to t ‐BuP 1 , and demonstrated their high selectivity for the ROAC of PA with various epoxides. [ 14 ] Despite the good control over the ROAC achieved with the reported phosphazene bases, low catalytic activity still remains as a problem. The catalytic activity can be improved by using phosphazene bases with stronger basicity, but usually accompanies with undesirable self‐propagation of epoxides as well as macromolecular transesterification.…”

Switchable Polymerization Catalysis Using a Tin(II) Catalyst and Commercial Monomers to Toughen Poly(l-lactide)

“…PLLA is industrially produced by controlled l -lactide ring-opening polymerization (ROP), catalyzed by Sn­(II) alkoxides generated in situ from Sn­(II) precursors and alcohols. , Polyesters are also controllably prepared by epoxide and anhydride ring-opening copolymerization (ROCOP) which is thermodynamically feasible for many functional monomers. , Importantly, such monomers are common in current polymer manufacturing and have acceptable costs. Efficient epoxide/anhydride ROCOP catalysts are known, − and relevant here, Phomphrai and co-workers recently reported a tin­(II) catalyst for cyclohexene oxide/phthalic anhydride ROCOP yielding a poly­(ester- ran -ether) ABB structure . Switchable polymerization catalysis enchains mixtures of epoxides, anhydrides, and lactones using one catalyst in a single reactor process. ,− It is applicable to many monomers, organometallic, inorganic, or organocatalysts, − but until now, commercially relevant tin­(II) catalysts were unexplored.…”

Synthesis of Diverse Polycarbonates by Organocatalytic Copolymerization of CO2 and Epoxides: From High Pressure and Temperature to Ambient Conditions

“…[36] Recently,three phosphazenes (CTPB,C 3 N 3 -Py-P 3 and C 3 N 3 -Me-P 3 as shown in Scheme 1) with tunable basicity and structures have been successfully developed in our group and showed high efficiencyf or ROPs of various cyclic monomers. [55][56][57][58][59][60][61][62][63] In this study,t hese phosphazenes in combination with TEB were tested as binary organocatalytic pair for copolymerization of CO 2 with different epoxides (Schemes 1a nd 2) under various conditions.…”