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This work investigates, for the first time, cinchona alkaloids such as cinchonidine (CD) and β-isocupreidine (ICD) consisting of both a chiral nucleophilic amine catalyst site and an electrophilic hydroxy moiety, as bifunctional, stereoselective organocatalysts for the ring-opening polymerization (ROP) of l-lactide (l-LA) and rac-lactide (rac-LA). While the ROP of l-LA by CD is ineffective, the ROP of l-LA by ICD proceeds without noticeable epimerization, thus affording polylactide (PLA) with a quantitative isotacticity. The measured number-average molecular weight (Mn) is much higher than the expected Mndue to sluggish initiation by ICD, but when benzyl alcohol is added as an external protic initiator, the ROP is highly efficient and proceeds to high conversions without undesired transesterification reactions, thus producing PLA with a controlled Mnand a narrow molecular weight distribution of 1.12. More significantly, the ROP of rac-LA by ICD/alcohol affords crystalline isotactic-rich, stereogradient PLA that exhibits multiple melting-transition temperatures as a result of a partial kinetic resolution polymerization that preferentially polymerizes l-LA and kinetically resolves d-LA. Overall, this study uncovers the first kinetic resolution polymerization of rac-LA by an organocatalyst. [ABSTRACT FROM AUTHOR]

This contribution reports the first examples of stereoregular (isotactic, it-, and syndiotactic, st-) methacrylate-POSS (polyhedral oligomeric silsesquioxane) hybrid polymers and their derived nanostructured assemblies. The polymerization of methyl methacrylate (MMA) by isospecific and syndiospecific living metallocene catalysts, when end-capped with methacrylisobutyl POSS (MA-POSS) or simultaneously copolymerized with MA-POSS at ambient temperature, readily produces highly stereoregular (94% it- and st-) MA-POSS end-capped PMMA (PMMA-POSS) or statistical copolymers PMMA-co-P(MA-POSS). The MA-POSS incorporation in the it-copolymers ranges from a low 2.6 mol % (20 wt %) to a high, maximum 24 mol % (75 wt %), whereas the incorporation in the st-copolymers is relatively lower with the same comonomer feed ratio due to the formation of a crystalline inclusion complex in which the POSS nanocages are encapsulated within the helical st-PMMA cavity. The it-copolymers with high POSS contents (>20 mol %) show evidence for interchain association through POSS aggregation and display a ∼20 °C Tgenhancement over the pristine it-PMMA. MA-POSS end-capped diastereomeric PMMAs exhibit versatile nanostructured assemblies, including micelle-like core−shell nanostructures (Rhup to 186 nm derived from the 5.7 nm it-PMMA-POSS unimer) through POSS inorganic collapse in selective solvents, helical stereocomplexes (Rh= 11 nm) between two or three polymer chains through organic self-organization of the diastereomeric PMMAs, and large core−shell nanospheres (Rhup to 636 nm) through synergistic organic PMMA self-organization and inorganic POSS nanocluster aggregation. Thermal annealing of the it-/st-PMMA-POSS blend in a 1:2 ratio also generates the crystalline stereocomplex with a high Tmof 213 °C. [ABSTRACT FROM AUTHOR]

Highly active, bifunctional propagating species that contain both nucleophilic and electrophilic catalyst sites as well as bring about efficient living/controlled (meth)acrylate polymerization at ambient temperature via a bimolecular-activated monomer propagation mechanism have been generated by instantaneous oxidative activation of ketene silyl acetal initiators with a catalytic amount (0.05 mol % relative to monomer) of the trityl activator. Studies of structure−reactivity relationships for this novel polymerization system have included effects on the polymerization activity and the degree of control by structures of initiator(alkyl, silyl, germyl, and stannyl acetals), catalyst(variations in alkyl and silyl groups of the silyl acetals), activator(trityl salts of anions differing in their stability, size, coordination, and chirality), and monomer(methacrylates vs acrylates). Of several valuable findings obtained from this study, a most interesting and significant result is that there exhibits remarkable selectivity of the silyl group structure of the acetal initiator (and thus the derived silyl cation catalyst) for monomer structure: initiators having small silyl groups, such as dimethylketene methyl trimethylsilyl acetal, promote highly active and efficient polymerization of methacrylates, but they are poor initiators for polymerization of less sterically hindered, active α-H bearing acrylate monomers. On the other hand, initiators incorporating bulky silyl groups, such as the triisobutylsilyl derivative, exhibit low activity toward methacrylate polymerization but exceptionally high activity(completed reaction in 1 min), efficiency(achieved quantitative initiator efficiency), and degree of control(regulated low to high Mn(>10 5) with narrow molecular weight distributions) for acrylate polymerization at ambient temperature in polar noncoordinating (CH 2Cl 2), aromatic (toluene), or aliphatic (cyclohexane) solvents. [ABSTRACT FROM AUTHOR]

Asymmetric coordination polymerization of 13 acrylamide and methacrylate monomers of four different classes has been investigated using chiral ansa-zirconocenium ester enolate catalyst ( S,S)-(EBI)Zr +(THF)[OC(O iPr)CMe 2][MeB(C 6F 5) 3] −[( S,S)- 1), EBI = C 2H 4(η 5-Ind) 2] and its enantiomer ( R,R)- 1. This polymerization system is built upon four advanced features of polymerization including living, stereospecific, coordination, and asymmetric core elements, thus efficiently converting prochiral N,N-diaryl acrylamides at ambient temperature to optically active, stereoregular polymers with solution-stable, single-handed helical secondary structures. Kinetic studies show that the polymerization of N,N-diaryl acrylamides by 1proceeds via a monometallic, coordination-conjugate addition mechanism. Investigation into polymer chain-length effects on optical activity of the chiral polymers reveals two opposite trends, depending on the polymer secondary structure (i.e., helical vs random coil conformation). Examination of the polymerization scope shows that the formation of optically active poly(acrylamide)s due to solution-stable helical conformations with an excess of one-handed helicity is dictated by the sterics and rigidity of the monomer repeat units; while diaryl acrylamides can readily achieve such conformations, unsymmetrically substituted diaryl acrylamides give the chiral polymers with much higher optical activity than the symmetrically substituted ones. It is also possible for N,N-dialkyl acrylamides to lead to chiral helical polymers. Extensive asymmetric block copolymerization studies of MMA with acrylamides and other methacrylates have also been carried out, producing optically active, high-molecular-weight methacrylate- b-acrylamide stereoblock copolymers in which the acrylamide block can be either helical or nonhelical; in sharp contrast, all high-molecular-weight methacrylate- b-methacrylate di- or triblock copolymers produced by the enantiomeric catalysts 1are optically inactive. [ABSTRACT FROM AUTHOR]

Oxidative activation of group transfer polymerization initiator silyl ketene acetals with a catalytic amount of the olefin polymerization activator Ph3CB(C6F5)4directly affords the first methyl methacrylate addition product, the highly active propagating species that contains both the nucleophilic and electrophilic catalyst sites to promote the controlled methacrylate polymerization via cooperative catalysis. [ABSTRACT FROM AUTHOR]

This contribution reports the synthesis and characterization of stereochemically controlled, as well as crystalline stereocomplex, P(MMA)‐clay nanocomposites using metallocene complexes and alane‐intercalated clay activators. The ligand elimination and exchange reactions involving Lewis acids E(C6F5)3 (E = Al, B) and an organically modified montmorillonite clay were employed to synthesize the alane‐intercalated clay activators. When combined with dimethyl metallocenes of various symmetries, these clay activators brought about efficient MMA polymerizations leading to in situ polymerized, stereochemically controlled P(MMA)‐intercalated clay nanocomposites. The most noticeable thermal property enhancement observed for the clay nanocomposite P(MMA), when compared with the pristine P(MMA) having similar molecular weight and stereomicrostructure, has a considerable increase in Tg (≥10 °C). Mixing of dilute THF solutions of two diastereomeric nanocomposites in a 1:2 isotactic to syndiotactic ratio, followed by reprecipitation or crystallization procedures, yielded unique double‐stranded helical stereocomplex P(MMA)‐clay nanocomposites with a predominantly exfoliated clay morphology. Remarkably, the resulting crystalline stereocomplex P(MMA) matrix is resistant to the boiling‐THF extraction and its clay nanocomposites exhibit high Tm of 201 to 210 °C. Furthermore, the stereocomplex P(MMA)‐clay nanocomposite shows a one‐step, narrow decomposition temperature window and a single, high maximum rate decomposition temperature of 377 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2581–2592, 2007 [ABSTRACT FROM AUTHOR]