Alkali and Alkaline Earth Metal Complexes as Versatile Catalysts for Ring-Opening Polymerization of Cyclic Esters.

Biodegradable polyesters such as poly(ϵ-caprolactone) (PCL) and poly(lactic acid) (PLA) have been considered for use in several areas, such as drug delivery devices, sutures, tissue engineering, and GBR membranes, due to its bio-renewability, biodegradability, and biocompatibility. Several synthetic techniques for the preparation of polyesters have been reported in the literature, amongst which the ring-opening polymerization (ROP) of cyclic esters is the most efficient. A convenient approach to access iso-selective PLAs is polymerization of racemic lactide (rac-LA), which shows excellent stereoregularity without the need for costly chiral auxiliaries or ligands. In this personal account, we review a series of methods that have been practiced to the synthesis of biodegradable polyesters from various cyclic monomers using alkali and alkaline earth metal complexes as efficient catalysts.

Ring Opening Polymerization and Copolymerization of Cyclic Esters Catalyzed by Group 2 Metal Complexes Supported by Functionalized P-N Ligands.

We report the preparation of alkali and alkaline earth (Ae) metal complexes supported by 2-picolylamino-diphenylphosphane chalcogenide [(Ph2P(=E)NHCH2(C5H4N)] [E = S (1-H); Se (2-H)] ligands. The treatment of the protic ligand, 1-H or 2-H, with alkali metal hexamethyldisilazides at room temperature afforded the corresponding alkali metal salts [M(THF)2(Ph2P(=E)NCH2(C5H4N)] [M = Li, E = S (3a), Se (3b)] and [{M(THF) n(Ph2P(=E)NCH2(C5H4N)}2] [M = Na, E = S (4a), Se (4b); M = K, E = Se (5b)] in good yield. The homoleptic Ae metal complexes [κ2-(Ph2P(=Se)NCH2(C5H4N)Mg(THF)] (6b) and [κ3-{(Ph2P(=Se)NCH2(C5H4N)}2M(THF) n] (M = Ca (7b), Sr (8b), Ba (9b)] were synthesized by the one-pot reaction of 2-H with [KN(SiMe3)2] and MI2 in a 2:2:1 molar ratio at room temperature. The molecular structures of the protic-ligands 1-H and 2-H, as well as complexes 3a,b-5a,b and 6b-9b were established using single-crystal X-ray analysis. The Ae metal complexes 6b-9b were tested for ring-opening polymerization (ROP) of racemic lactide ( rac-LA) and copolymerization of rac-LA and ε-caprolactone (ε-CL) at room temperature. In the ROP of rac-LA, the calcium complex 7b exhibited high isoselectivity, with Pi = 0.89, whereas both the barium and strontium complexes showed lower isoselectivity with Pi = 0.78-0.62. In the copolymerization of rac-LA and ε-CL, both barium and strontium complexes proved to be efficient precatalysts for the formation of the block copolymer rac-LA-CL, but the reactivity of 9b was found to be better than that of 8b. All the polymers were fully characterized using differential scanning calorimetry, thermogravimetric analysis, and gel permeation chromatography analyses. Kinetic studies on the ROP reaction of LA confirmed that the rate of polymerization followed the order Ba ≫ Sr ≈ Ca.

Highly Active and Iso-Selective Catalysts for the Ring-Opening Polymerization of Cyclic Esters using Group†2 Metal Initiators.

A series of alkali and alkaline earth (Ae) metal complexes bearing 1,2-phenylene(bis-diphenylphosphinothioic/selenoic amine) [{Ph2 P(E)NH}2 C6 H4 ] (E=S (1-H2); Se (2-H2) ligands are reported. Alkali metal complexes [{Ph2 P(S)N}2 C6 H4 ]Na(THF)4 (3 a) [{Ph2 P(Se)N}2 C6 H4 ]Na(THF)4 (3 b), and [{Ph2 P(Se)N}2 C6 H4 ]K(THF)5 (4 b) were obtained in good yield by treating protic ligands 1-H2 or 2-H2 with metal hexamethyldisilazides [MN(SiMe3 )2 ] (M=Na or K) at ambient temperature. The Ae metal complexes formulated as [{Ph2 P(E)N}2 C6 H4 ]M(THF)3 [E=S, M=Ca (5 a), Sr (6 a), Ba (7 a); E=Se, M=Ca (5 b), Sr (6 b), Ba (7 b)] can be synthesized by using two routes. The molecular structures of the free ligand 1-H2 and metal complexes 5 a,b-7 a,b in their solid states were established. Complexes 3 a and 3 b are isostructural; however, in complex 4 b, an attachment different from ligand 2 was observed. The complexes 5 a,b-7 a,b are isostructural and each metal ion exhibits a distorted pentagonal bipyramidal geometry around it. All Ae metal complexes 5 a,b-7 a,b were tested for the ring-opening polymerization (ROP) of racemic lactide (rac-LA) and ϵ-caprolactone (ϵ-CL) at room temperature. Calcium complexes 5 a and 5 b show excellent iso-selectivity, with Pi values of 0.78-0.87 at 298 K and with a high degree of polymerization control, whereas the corresponding strontium complexes 6 a and 6 b exhibit moderate iso-selectivity, and barium complexes 7 a and 7 b yield only atactic polylactides (PLAs). In all cases, the catalyst initiates the ROP catalytic cycle in the absence of any external initiator. Kinetic studies of the polymerization reactions indicate the relative order of polymerization rate increases with increase in the size of the metal ion: Ba>Sr>Ca.

Crafting sustainable solutions: architecting biodegradable copolymers through controlled ring-opening copolymerization.

Polylactic acid (PLA) is a biodegradable and biocompatible polymer with versatile applications in packaging and medicine. It is derived from lactic acid and thus represents an eco-friendly option sourced from renewable raw materials. Despite its advantages, PLA exhibits few drawbacks, such as brittleness and relatively high melting and glass transition temperatures. However, these limitations can be addressed through copolymerization with other monomers like ε-caprolactone (ε-CL), resulting in a composite material with improved physical properties. This paper comprehensively reviews achievements in PLA-PCL copolymerization using organometallic catalysts, discussing scientific findings and various copolymer architectures obtained, including random or block configurations. It also demonstrates various sustainable catalysts for achieving the required microstructure under mild reaction conditions without the aid of any external initiator.

Unprecedented ROP of quinazolinones to polyacylamidines using a cesium catalyst.

Unprecedented ring-opening polymerization of quinazolinones to produce novel polyacylamidines, led by a unique cooperation between a cesium metal center and imino-phosphanamidinate ligand, was developed. Morphological studies revealed the formation of a unique macromolecular assembly producing nanofibers in the absence of a templating agent with excellent control of molecular weights and polydispersity index.

Highly efficient and well-controlled ROP and copolymerization of cyclic esters using a cesium complex.

A cesium imino-phosphanamidinate, [{NHIDippP(Ph)NDipp}Cs], enabling efficient ring-opening (co)polymerization of rac-LA and ε-CL is disclosed. Owing to the highly controlled polymerization, precise di-block copolymers (PLA-b-PCL) with different block lengths can be produced by a simple one-pot reaction. NMR, GPC, DSC and microscopic analyses confirm the production of di-block copolymers with crystalline properties.

Frequency and risk factors stratification of hypertension among the rural population of Bangladesh.

Background and Objectives: Hypertension is one of the major risk factors of premature morbidity and mortality in our daily clinical practice. Various studies carried out in the urban settings, but there is scarcity of epidemiological data regarding hypertension among the rural people in Bangladesh. Therefore, this current study has been designed to find out the frequency and risk factors stratification of hypertension among the rural people in Jashore, Bangladesh. Materials and Methods: A cross-sectional study recruited 1812 participants above 18 years attending on national hypertensive week of 2019 in Bagherpara and Keshabpur upazila (subdistrict) health complex in Jashore, Bangladesh. 2020 International Society of Hypertension Global Hypertension Practice Guidelines had been demonstrated to classify hypertension. Results: Out of the total study population, the frequency of hypertension was 20.6% (Grade 1 and Grade 2 hypertensive patients 15.8% and 4.9%, respectively), and high normal blood pressure was 9.0%. The mean age of the study population, Grade 1 hypertensive and Grade 2 hypertensive cohorts were 42 ± 16, 49 ± 15 and 51 ± 14 years, respectively, with a male and female ratio was 1:2. Progressive rise of mean systolic and diastolic blood pressure were noticed with increasing age. Age (P: <0.001), sex (P: 0.004), occupation (P: <0.001), BMI (P: <0.001), family (P: <0.001) and past history (P: <0.001) of hypertension, sedentary life style (P: 0.004), additional salt intake (P: <0.001) and smoking (P: 0.011) were significantly associated with hypertension following bivariate analysis. Multivariate logistic regression analysis revealed that age after 50 years (AOR = 1.866, 95% CI: 1.210-2.876), positive past history of hypertension (AOR = 3.493, 95% CI: 2.676-4.558), additional salt intake (AOR = 0.591, 95% CI: 0.453-0.770) and obesity (AOR = 3.389, 95% CI: 1.830-6.274) were significantly associated with developing hypertension. Conclusion: High frequency of hypertension was found among the rural population in Bangladesh where presence with a lot of significantly associated risk factors. The data would be helpful for the health policymakers dealing noncommunicable diseases to reach the sustainable goal and mitigate morbidity and mortality of cardiovascular diseases in Bangladesh.

Alkaline Earth Metal-Mediated Highly Iso-selective Ring-Opening Polymerization of rac-Lactide.

Alkaline earth (Ae) metal complexes of the aminophosphine borane ligand are highly active and iso-selective catalysts for the ring-opening polymerization (ROP) of rac-lactide (LA). The polymerization reactions are well controlled and produce polylactides with molecular weights that are precise and narrowly distributed. Kinetic studies reveal that the ROP of rac-LA catalyzed by all Ae metal complexes had a first-order dependency on LA concentration as well as catalyst concentration. A plausible reaction mechanism for Ae metal complex-mediated ROP of rac-LA is discussed, based on controlled kinetic experiments and molecular chain mobility.

Functionalized vinylsilanes via highly efficient and recyclable Pt-nanoparticle catalysed hydrosilylation of alkynes.

A mild, selective and facile synthesis of vinylsilanes via a recyclable platinum nanoparticle catalysed hydrosilylation of alkynes is reported. Various functionalized alkynes are selectively hydrosilylated to furnish functional ß-E vinylsilanes in high yields. The catalytic effectiveness, ease of catalyst recovery and recyclability of the polysiloxane stabilized Pt-nanoparticle catalyst are the major achievements of this work. Detailed in situ characterization using Electron Microscopy and controlled poisoning experiments supports the participation of Pt-nanoparticles as active catalysts.