Make or break: Mg(ii)- and Zn(ii)-catalen complexes for PLA production and recycling of commodity polyesters

Abstract

A series of Mg(ii) and Zn(ii) catalen complexes have been prepared for PLA formation and recycling.

Chemically recyclable polymer materials: polymerization and depolymerization cycles

“…[164][165][166] Different types of catalysts can achieve this strategy, such as imino monophenolate Zn complexes, Zn(OAc) 2 , TBD, Mg n Bu 2 , DMAP, Sn(Oct) 2 , Zn(HMDS) 2 , etc. [167][168][169][170][171][172][173][174][175][176][177][178][179] Furthermore, the tetramethyl glycolide (TMG), a cyclic dimer of hydroxyisobutyric acid (HIBA), was also explored as a degradable polymer by Nishida. 180 A high molecular weight PTMG (M n up to 90 kg mol −1 ) was obtained by t BuLi catalyzed anionic ring-opening polymerization.…”

Simple Zn(ii) complexes for the production and degradation of polyesters

“…The PLA produced by Zn(1-9) 2 was essentially atactic (P r ¼ 0.51-0.60) and this is in keeping with other zinc monophenolate complexes under these conditions. 56,69,71 When the lactide to initiator ratio was increased, ([LA] : [I] : [BnOH] ¼ 3000 : 1 : 10) the activity differences between the complexes became more apparent (Table 3). Ethylene bridge complexes, Zn(1-3) 2 , took much longer to reach high conversion (t ¼ 30-60 min) than the phenylene bridge equivalents.…”

“…Isolated yields between 42% and 55% were observed, and this is consistent with previous literature examples using zinc catalysts. 71,72 The most active catalyst {Zn(2) 2 : t ¼ 1.5 h, Y BHET ¼ 0.16 g (48%) was tested with thin-lm PET; a proxy for manufacturing waste. As expected, the reaction time was signicantly reduced (t ¼ 0.75 h) and a percentage yield of 51% was attained, again in keeping with previous results by Payne et al 71,72 In the reaction with Zn(2) 2 , colourless BHET was obtained aer recrystallisation and this is important when considering the quality of PET if it were repolymerized (Fig.…”

“…66 More recently, Jones and co-workers have reported several homoleptic zinc monophenolate complexes that are active for the degradation of PLA into alkyl lactates at mild conditions. 56,[67][68][69][70][71] An imine-monophenolate complex with a propylene diamine linker was able to achieve a high yield of methyl lactate (Y Me-La ¼ 89%) aer just 30 minutes at 50 C. 56 This far exceeds the activity shown by the ethylenediamine-based analogues which required several hours to reach high conversion at comparable temperatures. 70 The most effective propylenediamine catalyst was tested for the degradation of post-consumer PLA products into ethyl lactate.…”

“…67 Although activity was lower than with the clear PLA cup used for the methanolysis experiments, good yields of ethyl lactate were observed and the variation in activity was attributed to additives and the ease of dissolution of each product. More recent work has demonstrated an ethylene diamine-based ligand with a catam (N-H) moiety that far exceeds the activity of the direct imine-based counterpart, achieving 85% Me-LA yields aer 30 minutes at 50 C. 70,72 The aforementioned ligand is a building block of the tetradentate catalen system, zinc complexes of which were also active for PLA degradation to methyl lactate at moderate yields aer 8 hours at 80 C. 71 Recent examples have also achieved PLA alcoholysis at ambient conditions and with no solvent, in some cases demonstrating full PLA conversion aer 1 hour. 73,74 Organocatalysts, such as DMAP, 75 TBD, 76 TMC 77 and ionic liquids, [78][79][80] have also been reported but typically require high catalyst loading and can be limited by issues relating to corrosivity, toxicity and high cost.…”