Stereoblock Poly Research Articles

Isotactic-b-syndiotactic stereoblock poly(methyl methacrylate) by chiral metallocene/Lewis acid hybrid catalysts.

Stereoblock polymerization with chiral ansa-metallocene/strong Lewis acid hybrid catalysts capable of switching stereospecificity of the methyl methacrylate polymerization produces the highly stereoregular isotactic-b-syndiotactic stereoblock poly(methyl methacrylate).

Stereoselective ring‐opening polymerization of rac‐lactide with a single‐site, racemic aluminum alkoxide catalyst: Synthesis of stereoblock poly(lactic acid)

Stereoselective ring‐opening polymerization of rac‐lactide with a single‐site, racemic aluminum alkoxide catalyst: Synthesis of stereoblock poly(lactic acid)

Stereoselective ring-opening polymerization ofrac-lactide with a single-site, racemic aluminum alkoxide catalyst: Synthesis of stereoblock poly(lactic acid)

The polymerization of racemic lactide with a racemic aluminum alkoxide catalyst is reported. Microstructural analysis of the polymer produced with 1 H NMR spectroscopy revealed that an isotactic stereoblock poly(lactic acid) formed, where each enantiomerically pure block contained an average of 11 lactide monomer units. The melting point of this polymer, 179°C, was higher than that of the enantiomerically pure polymer, consistent with the cocrystallization of the enantiomeric blocks of the polymer. The mechanism of the polymer formation is currently unknown, although a polymer exchange pathway, where living chain ends switch between metal centers to produce diastereomeric active species, is proposed.

Simulations on crystallization in stereoblock poly(propylene). Idealized structures showing the effects of atactic block length

In typical polymerizations of propene, the stereochemical structures of the resulting polymer depend on the replication probability (for meso placements) in two ways. Decrease in this probability decreases the overall isotactic content, but also decreases the average lengths of the isotactic sequences. New catalysts, however, can now decrease isotactic content without necessarily decreasing the average lengths of the isotactic sequences, by either internally rotating between two forms of the active site or by having two alternative sites available for occupation by the growing polymer chain. The present investigation takes parallel arrays of such chains having idealized stereoblock structures and looks for matches in structure that would lead to the formation of crystallites. Counting the numbers of units in such matches relative to the total number then gives the percent crystallinity, and it is also possible in this way to estimate melting points, and other related thermodynamic and molecular properties. Of particular interest here is the dependence of such properties on the lengths of the atactic sequences separating isotactic blocks of constant length.

Simulations on crystallization in stereoblock poly(propylene). Idealized structures showing the effects of atactic block length

In typical polymerizations of propene, the stereochemical structures of the resulting polymer depend on the replication probability (for meso placements) in two ways. Decrease in this probability decreases the overall isotactic content, but also decreases the average lengths of the isotactic sequences. New catalysts, however, can now decrease isotactic content without necessarily decreasing the average lengths of the isotactic sequences, by either internally rotating between two forms of the active site or by having two alternative sites available for occupation by the growing polymer chain. The present investigation takes parallel arrays of such chains having idealized stereoblock structures and looks for matches in structure that would lead to the formation of crystallites. Counting the numbers of units in such matches relative to the total number then gives the percent crystallinity, and it is also possible in this way to estimate melting points, and other related thermodynamic and molecular properties. Of particular interest here is the dependence of such properties on the lengths of the atactic sequences separating isotactic blocks of constant length.

Intra- and Intermolecular Stereocomplex Formation of Uniform Stereoblock Poly(methyl methacrylate)

Intra- and Intermolecular Stereocomplex Formation of Uniform Stereoblock Poly(methyl methacrylate)

Preparation of Uniform Stereoblock Poly(methyl methacrylate)

Preparation of Uniform Stereoblock Poly(methyl methacrylate)

Stereospecific polymerization with metallocenes: Macromolecular design via catalysis

AbstractForty years after Natta's discoveries of stereospecific olefin polymerization with Ziegler catalysts, new catalysts are causing a renaissance in stereospecific olefin polymerization. Metallocene Ziegler‐Natta catalysts are unprecedented in their ability to polymerize α‐olefins to a variety of polymer microstructures.1 Isotactic, syndiotactic, atactic and stereoblock poly(α‐olefins) have been produced using catalysts derived from group 4 metallocene catalyst precursors.

Synthesis of a stereoblock poly(methyl methacrylate)

AbstractThe sequential block copolymerization of diphenylmethyl methacrylate (DMA) and trityl methacrylate (TrMA), initiated by diphenyl methyl lithium in THF at ‐78°C followed by hydrolysis and methylation with diazomethane, leads to the formation of an isotacticsyndiotactic stereoblock poly(methylmethacrylate) (PMMA) of narrow molecular weight and block length distribution. Complete conversion of DMA is shown to be essential, since the presence of small quantities of this monomer during the polymerization of TrMA significantly lowers the isotactic content of this block. Polymerization times of at least 12 h were found to be necessary for complete polymerization of the TrMA.

Isotactic and stereoblock poly(methyl methacrylates) produced by butylmagnesium initiators

The use of tert-, sec-, iso- and normal-butylmagnesium bromides and chlorides in toluene, tetrahydrofuran and mixed solvents to prepare Bernoullian, stereoblock and isotactic poly(methyl methacrylate). PMMA, and the factors controlling the stereochemistry are described. By careful control of solvent composition, highly isotactic polymer can be prepared with all the butylmagnesium bromides, but none of the corresponding dibutylmagnesiums. The halide content does not influence stereospecificity when it is in excess of the Grignard stoichiometry but it influences the mol. wt distribution and retards the polymerization. The mol. wt distributions are usually polymodal and dependent on the same factors that control stereospecificity. The effect of THF concentration on stereospecificity operates in a manner quite distinct from its effect on complex concentration and rate. The concentration of residual THF in toluene-rich solution determines the type of initiation and propagating species that prevail. Structures are proposed for those responsible for stereospecific initiation and propagation. It is concluded that tert-butylmagnesium bromide is the most reliable and robust initiator for preparing isotactic PMMA.

Kinetics and mechanism of polymerization yielding stereoblock poly(methyl methacrylate) with VCl4‐Al(C2H5)3 catalyst system

AbstractMethyl methacrylate was polymerized at 40°C with the VCl4–AlEt3 catalyst system in n‐hexane. The rate of polymerization was proportional to the catalyst and monomer concentration at Al/V ratio of 2, indicating a coordinate anionic mechanism of polymerization. NMR spectra were further used to confirm the mechanism of polymerization and stability of active sites responsible for isotacticity.

Solvent effect on NMR spectra of poly(methyl methacrylate)

AbstractThe NMR spectra of stereoblock poly(methyl methacrylate) in several solvents were measured. It is concluded from the following experimental results that the solute–solvent complexes are formed in benzene solution: the chemical shifts measured in C6H6 go to a lower field than do those in CDCl3, except those of the ester methyl group, which splits into three resonances, and the shifts in the aromatic solvents are so different from those in the aliphatic solvents that Buckingham's theory cannot be applied to the results. The analysis of the temperature dependence of the chemical shifts of PMMA in benzene solution gave the heat of formation of the complex: ΔH = 2.8 ± 0.5 kcal./mole.

Identification of a stereoblock poly(methyl methacrylate) sample with a stereocomplex

AbstractAcid hydrolysis of a stereoblock poly(methyl methacrylate) sample leads to a mixture of isotactic and syndiotactic poly(methacrylic acid) which can be separated by electrophoresis. The experiment confirms the stereochemical identity between the so‐called “stereoblock” poly(methyl methacrylate) and the stereocomplex which syndiotactic and isotactic poly(methyl methacrylate) form in the ratio 2:1. A possible mechanism of replica polymerization is suggested to account for this effect.