Naumann Stefan, Thomas Anthony W, Dove Andrew P
Department of Chemistry, University of Warwick, CV4 7AL Coventry, United Kingdom.
ACS Macro Lett. 2016 Jan 19;5(1):134-138. doi: 10.1021/acsmacrolett.5b00873. Epub 2016 Jan 6.
In this work, the activity of N-heterocyclic olefins (NHOs), a newly emerging class of organopolymerization catalyst, is investigated to affect the metal-free polymerization of lactones and trimethylene carbonate (TMC). A decisive structure-activity relationship is revealed. While catalysts of the simplest type bearing an exocyclic ═CH moiety polymerize l-lactide (l-LA) and δ-valerolactone (δ-VL) in a non-living and non-quantitative manner, the introduction of methyl substituents on the exocyclic carbon radically changes this behavior. 2-Isopropylidene-1,3,4,5-tetramethylimidazoline is found to be highly active for a range of monomers such as l-LA, δ-VL, ε-caprolactone (ε-CL), and TMC, with quantitative conversion occurring within seconds with catalyst loadings of just 0.2 mol %. The high activity of this NHO further enables the ring-opening polymerization (ROP) of the macrolactone ω-pentadecalactone (PDL). However, this broad applicability is offset by a lack of control over the polymerizations, including side reactions as a consequence of its strong basicity. To overcome this, a saturated, imidazolinium-derived analogue was synthesized and subsequently demonstrated to possess a harnessed reactivity which enables it to polymerize both l-LA and TMC in a controlled manner ( < 1.2). NMR spectroscopic and MALDI-ToF MS experiments highlight the differences in polymerization pathways for 2-methylene-1,3,4,5-tetramethylimidazoline, in which the exocyclic carbon is not substituted, in contrast to 2-isopropylidene-1,3,4,5-tetramethylimidazoline, with the former operating via its nucleophilicity and the latter acting as a base with enolizable δ-VL.
在本研究中，我们考察了新型有机聚合催化剂N-杂环烯烃（NHOs）对内酯和碳酸三亚甲基酯（TMC）的无金属聚合反应的活性。揭示了决定性的构效关系。虽然带有环外═CH部分的最简单类型的催化剂以非活性和非定量的方式使L-丙交酯（L-LA）和δ-戊内酯（δ-VL）聚合，但在环外碳上引入甲基取代基会彻底改变这种行为。发现2-异亚丙基-1,3,4,5-四甲基咪唑啉对一系列单体如L-LA、δ-VL、ε-己内酯（ε-CL）和TMC具有高活性，在仅0.2 mol%的催化剂负载量下，数秒内即可实现定量转化。这种NHO的高活性还使得大环内酯ω-十五内酯（PDL）能够进行开环聚合（ROP）。然而，这种广泛的适用性被聚合反应缺乏控制所抵消，包括由于其强碱性导致的副反应。为了克服这一问题，合成了一种饱和的、源自咪唑啉鎓的类似物，随后证明其具有可控的反应活性，能够以可控方式（<1.2）使L-LA和TMC聚合。核磁共振光谱和基质辅助激光解吸电离飞行时间质谱实验突出了2-亚甲基-1,3,4,5-四甲基咪唑啉（其环外碳未被取代）与2-异亚丙基-1,3,4,5-四甲基咪唑啉聚合途径的差异，前者通过亲核性起作用，后者对可烯醇化的δ-VL起碱的作用。
