Stereoblock Copolymers Research Articles

An efficient solid‐state polycondensation method for synthesizing stereocomplexed poly(lactic acid)s with high molecular weight

AbstractSimultaneous solid‐state polycondensation (SSP) of the powdery prepolymers of poly(L‐lactic acid) (PLLA) and poly(D‐lactic acid) (PDLA) can produce entire stereocomplexed poly(lactic acid)s (sc‐PLA) with high molecular weight and can be an alternative synthetic route to sc‐PLA. Ordinary melt polycondensations of L‐ and D‐lactic acids gave the PLLA and PDLA prepolymers having medium molecular weight which were pulverized for blending in 1:1 ratio. The resultant powder blends were then subjected to SSP at 130–160 °C for 30 h under a reduced pressure of 0.5 Torr. Some of the products thus obtained attained a molecular weight (Mw) as high as 200 kDa, consisting of stereoblock copolymer of PLLA and PDLA. A small amount of the stereocomplex should be formed in the boundaries of the partially melted PLLA and PDLA where the hetero‐chain connection is induced to generate the blocky components. The resultant SSP products showed predominant stereocomplexation after their melt‐processing in the presence of the stereoblock components in spite of containing a small amount of racemic sequences in the homo‐chiral PLLA and PDLA chains. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3714–3722, 2008

Thermoassociating Poly(N-isopropylacrylamide) A−B−A Stereoblock Copolymers

Properties of aqueous solutions of A−B−A stereoblock copolymers with an isotactic poly(N-isopropylacrylamide) (PNIPAM) as the other block (either A or B) and atactic PNIPAM as the other block were studied. Introduction of isotacticity lowered the solubility of the polymers in water at room temperature, but the copolymers were found to dissolve easily at 5 °C. Calorimetric scans of the polymer solutions did not reveal other transitions than the typical endotherm close to the cloud point of PNIPAM (32 °C). The transition temperature did not change with polymers with different lengths of the isotactic blocks, but the enthalpy change was significantly lowered compared with atactic PNIPAM. Polymers containing isotactic A blocks were found to exhibit thermothickening property, with enhanced viscosity at room temperature. The polymers formed hydrogels at room temperature at polymer concentrations above 45 g/L, while solutions of polymer containing an isotactic B block did not exhibit enhanced viscosity. Thermothickening was found to occur at lower temperature and lead to more elastic hydrogels with polymers with longer isotactic A block.

Hierarchical Assembly of Nanostructured Organosilicate Networks via Stereocomplexation of Block Copolymers

The effect of the stereochemistry of polylactide (PLA)-based block copolymers on templated inorganic nanostructures has been investigated from the self-assembly of a stereoisomer pair/organosilicate mixture followed by organosilicate vitrification and copolymer thermolysis. Isomeric PLA homopolymers, block copolymers, and a stereoblock copolymer were prepared by ring-opening polymerization of D-, L-, or rac-lactide using an organocatalytic catalyst. Both differential scanning calorimetry and atomic force microscopy showed the formation of a stereocomplex between enantiomeric stereoisomers, that is, block copolymer/block copolymer and block copolymer/homopolymer mixtures as well as a stereoblock copolymer. The unique noncovalent interactions driven by stereocomplexation of D- and L-lactide provided supramolecular structures with a hierarchical order as characterized by distinctive vertical and horizontal growth of toroidal nanostructured inorganic features. This study demonstrates the potential of hierarchically assembling suprastructures that bridge the nano- to mesoscale feature sizes in the design of tunable functional nanomaterials suitable for future applications of microelectronics, material science, and bioengineering.

A–B–A stereoblock copolymers of N‐isopropylacrylamide

AbstractA–B–A stereoblock polymers with atactic poly(N‐isopropylacrylamide) (PNIPAM) as a hydrophilic block (either A or B) and a non‐water‐soluble block consisting of isotactic PNIPAM were synthesized using reversible addition fragmentation chain transfer (RAFT) polymerizations. Yttrium trifluoromethanesulfonate was used in the tacticity control, and bifunctional S,S′‐bis(α,α′‐dimethyl‐α″‐acetic acid)‐trithiocarbonate (BDAT) was utilized as a RAFT agent. Chain structures of the A–B–A stereoblock copolymers were determined using 1H NMR, SEC, and MALDI‐TOF mass spectrometry. BDAT proved to be an efficient RAFT agent in the controlled synthesis of stereoregular PNIPAM, and both atactic and isotactic PNIPAM were successfully used as macro RAFT agents. The glass transition temperatures (Tg) of the resulting polymers were measured by differential scanning calorimetry. We found that the Tg of isotactic PNIPAM is molecular weight dependent and varies in the present case between 115 and 158 °C. Stereoblock copolymers show only one Tg, indicating the miscibility of the blocks. Correspondingly, the Tg may be varied by varying the mutual lengths of the A and B blocks. The phase separation of aqueous solutions upon increasing temperature is strongly affected by the isotactic blocks. At a fixed concentration (5 mg/mL), an increase of the isotacticity of the stereoblock copolymers decreases the demixing temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 38–46, 2008

Propylene polymerizations with a binary metallocene system—Chain shuttling caused by trimethylaluminium between active catalyst centers

AbstractPropylene was polymerized at varying trimethylaluminium (TMA) concentration with a homogeneous binary metallocene catalyst system activated by methylaluminoxane (MAO) in an attempt to better understand interactions between active catalyst sites and to clarify the role of the TMA as a chain shuttling agent. TMA‐free polymerization conditions were obtained by chemical treatment of MAO solution with 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT). A binary catalyst system consisting of catalyst precursors diphenylmethyl(cyclopentadienyl)(9‐fluorenyl)zirconium dichloride (1) producing high Mw syndiotactic polypropylene and rac‐dimethylsilylbis(4‐tert‐butyl‐2‐methyl‐cyclopentadienyl)zirconium dichloride (2) producing low Mw isotactic polypropylene was investigated. At the studied polymerization conditions, chain shuttling between the active catalysts caused by TMA was confirmed. The chain shuttling reactions caused changes in catalyst activity, molecular weights, melting behavior, and polymer microstructure. We propose that TMA is capable to transfer a growing polymer chain from catalyst 2 to catalyst 1, and a stereoblock copolymer is formed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1364–1376, 2007

Stereoblock copolymers and tacticity control in controlled/living radical polymerization.

Three controlled/living radical polymerization processes, atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP), were investigated for the polymerization of N,N-dimethylacrylamide in the presence of Lewis acids known to enhance isotacticity, such as yttrium trifluoromethanesulfonate (Y(OTf)(3)) and ytterbium trifluoromethanesulfonate (Yb(OTf)(3)). Poly(N,N-dimethylacrylamide) with controlled molecular weight, low polydispersity (M(w)/M(n) < 1.2), and a high proportion of meso dyads ( approximately 85%) was prepared by ATRP (with initiating system methyl 2-chloropropionate/CuCl/Me(6)TREN) and RAFT (with cumyl dithiobenzoate transfer agent) in the presence of Y(OTf)(3). The combination of NMP (using N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, SG1) and a Lewis acid complexation technique led to less precise control over chain architecture and microstructure ( approximately 65% meso dyads), as compared to RAFT/Y(OTf)(3) or ATRP/Y(OTf)(3). The latter two systems were used for the first one-pot synthesis of stereoblock copolymers by radical polymerization. Well-defined stereoblock copolymers, atactic-b-isotactic poly(N,N-dimethylacrylamides), were obtained by adding Y(OTf)(3) at a given time to either RAFT or ATRP polymerizations, initially started without the presence of the Lewis acid.

Multiple melting behavior of poly(epichlorohydrin) enantiomorphs

Differential scanning calorimetry (DSC) studies demonstrate that the multiple melting behavior observed in both R and S enantiomorphs of poly(epichlorohydrin), their equimolar blend and the stereoblock copolymer form follows two distinctly different patterns, depending on the crystallization conditions. Isothermal crystallization at large undercoolings results in primary crystallites which during the heating scan undergo a process of melting/recrystallization and final melting, evidenced by a triple melting peak endotherm with the shape and position of the highest melting peak strongly dependent on scanning rate. By comparison, isothermal crystallization at small undercoolings yields primary and secondary crystallized material which melts with a double peak endotherm, the shape of which depends strongly on the crystallization time, with no indication of reorganization during the scan. These melting behavior observations support previous suggestions about the role of enantiomorphism in the crystallization of the stereoblock copolymer. Characteristic slow overall rates of crystallization of poly(epichlorohydrin) make this polymer an ideal subject for the study and refinement of multiple melting in polymers.

Preparation of uniform stereoregular polymer, stereoblock polymer and copolymer of methacrylate, and their stereocomplex formation

AbstractIsotactic (it‐) and syndiotactic (st‐) uniform poly(methyl methacrylate)s (PMMAs) were prepared by stereospecific living polymerizations followed by supercritical fluid chromatographic separation. The it‐and st‐uniform PMMAs were used to study the mechanism of stereo‐complex formation in solution with the aid of gel permeation chromatography and 1H NMR spectroscopy. The it‐ and st‐uniform PMMAs with hydroxy end‐group were prepared and reacted with sebacoyl dichloride to form uniform stereoblock PMMA. The stereoblock PMMA was found to form intra‐ and intermolecular stereocomplexes in acetone. A uniform stereoblock copolymer, it‐PMMA‐block‐st‐poly(ethyl methacrylate), was prepared and its stereocomplex formation was also studied.

Homogeneous Binary Zirconocenium Catalyst Systems for Propylene Polymerization. 1. Isotactic/Atactic Interfacial Compatibilized Polymers Having Thermoplastic Elastomeric Properties

Polypropylene with thermoplastic elastomeric properties was synthesized by homopolymerization of propylene using two metallocene catalysts of different stereospecificities: rac-ethylenebis(1-η5-indenyl)zirconium dichloride or rac-dimethylsilylenebis(1-η5-indenyl)zirconium dichloride as isospecific catalyst precursors and ethylenebis(9-η5-fluorenyl)zirconium dichloride as an aspecific precursor. The catalysts formed by activation of the precursors with triphenylcarbenium tetrakis(pentafluorophenyl)borate and triisobutylaluminum exhibit a very high activity of 5 × 107 g of PP/([Zr][C3H6] h). Products ranging from tough plastomers to weak elastomers were obtained by varying the ratio of the two types of precursors. The polymers containing less than half of the isotactic fraction display excellent thermoplastic elastomeric properties, which is attributable to an interconnected crystalline superstructure in an amorphous continuum compatibilized and bound together by a stereoblock copolymer at the interface against macrophase separation. Materials exhibiting elastic recoveries of 97−98%, which are relatively independent of elongation up to 500%, were obtained.

Material Design in Poly(Lactic Acid) Systems: Block Copolymers, Star Homo- and Copolymers, and Stereocomplexes

Abstract This paper gives a general overview of several approaches we have investigated for designing new PLA-based polymers with a broad range of properties and improved processability. These approaches include: copolymerization (block and stereoblock copolymers), microstructure and architecture control, and stereocomplexation. Multiblock copolymers with alternating “soft” and “hard” segments, synthesized over a broad range of chemical compositions, show properties ranging from hard plastics to elastomers. Stereoblock copolymers with alternating amorphous and semicrystalline PLA blocks combine the advantages of PLA homopolymers (crystallinity) and random copolymers (processability). Independent control of polymer architecture and microstructure allows for the synthesis of star polymers with various arm morphologies. A new method for stereocomplex formation between L-PLA and D-PLA, which combines in-situ polymerization with stereocomplexation, is also described. For the synthesis of these new materials we...

Synthesis of stereoblock copolymers via ligand exchange in living stereoselective ring opening metathesis polymerisation of 2,3-bis(trifluoromethyl)bicyclo[2.2.1]hepta-2,5-diene

Poly{2,3-bis(trifluoromethyl)bicyclo[2.2.1]hepta-2,5-diene} is synthesised as a stereoblock copolymer containing cis-and trans-vinylene blocks via ligand exchange in living stereoselective ring opening metathesis polymerisation initiated by a well defined Schrock type initiator.

Synthesis of a Stereoblock Polyketone through Ancillary Ligand Exchange

We report a new strategy for synthesis of stereoblock polymers involving ancillary ligand exchange during a living transition-metal-catalyzed polymerization. We have also demonstrated cooperativity between chain-end and site control mechanisms through ligand exchange. On the basis of these results, stereoblock copolymer synthesis was approached by first growing an isotactic segment with I-2, then switching on syndiotactic growth by displacing the bisoxazoline ligand from the Pd(II) catalyst with bipyridine. 18 refs., 1 fig.

Stereocomplex formation of isotactic poly(methyl methacrylate) with a wide variety of syndiotactic polymethacrylates

Syndiotactic (st–) polymers of methacrylates with primary and secondary ester groups, prepared by the syndiotactic-specific living polymerization with t-C4H9Li/R3Al, were found to form stereocomplexes with isotactic (it–) poly(methyl methacrylate) (PMMA) by annealing in the solid state or by mixing in certain solvents such as acetone and toluene. Melting points of the complexes depend on the structure of the ester group and can be changed in a wide range of temperature. st–Polymers of tertiary esters did not form the complex. Effects of anneal conditions, molecular weight, and tacticity on the melting point of the complex were studied in some detail for the combination of st–poly(benzyl methacrylate) and it–PMMA. st–Random copolymers of MMA with several alkyl methacrylates also formed stereocomplexes with it–PMMA, whose melting point could be changed continuously by changing the composition in a certain range of temperature. st–Block copolymers of PMMA and poly(benzyl methacrylate) formed stereocomplexes with it–PMMA which showed two melting points, provided the block lengths are long enough for the two types of the com plexes to form independently. Stereoblock PMMA, it–PMMA–block–st–PMMA, and stereoblock copolymer, it–PMMA–block–st–poly(butyl methacrylate), were found to form stereocomplexes more easily than the corresponding mixtures. The stereoregular uniform PMMAs were used for elucidating the process of stereocomplex formation and its stoichiometry by means of gelpermeation chromatography (GPC). The preliminary results clearly indicated that the complexation occurs mainly in 1:1 stoichiometry in the beginning, while a small fraction of 1:2 (it–: st–) complex was also formed concomitantly. By similar GPC experiments using a series of uniform PMMAs, the minimum length of PMMA chains for the complex formation was found to be in the range of degrees of polymerization from 42 to 46.

Fractionation of polypropylene according to molecular weight and tacticity

AbstractDifferent extraction techniques were applied to the fractionation of isotactic polypropylene. In solvent gradient extraction experiments two different solvent/non‐solvent systems were studied. Investigation of the fractions obtained by GPC, DSC and FTIR indicated that the fractionation proceeded according to molecular weight when ethylene glycol monobutyl ether/diethylene glycol monobutyl ether was used as a solvent/non‐solvent system at 159–161°C. The fractions covered a broad molecular weight range, had narrow molecular weight distribution and showed only small variations in their isotacticity. Xylene/ethylene glycol monoethyl ether combination at 121–128°C separated polypropylene in fractions mainly according to stereoregularity, the separation being less dependent on molecular weight. A large number of fractions with varying stereoregularity could be produced. Successive extraction with solvents of increasing boiling temperature gave fractions differing both in molecular weight and isotacticity, and was the only method to separate the stereoblock copolymer. Comparison of heptane extractables and xylene solubles methods showed differences in the structure and molecular weight of heptane and xylene soluble fractions and heptane and xylene insoluble fractions, respectively.

Synthesis of stereoblock poly(methyl methacrylate) via transformation of isotactic‐specific living polymer anion to syndiotactic‐specific anion

AbstractPolymerization of methyl methacrylate (MMA) by isotactic poly(methyl methacrylate) (PMMA) living anion, prepared with t‐C4H9MgBr in toluene at −60°C, was carried out at −78°C in the presence of trialkylaluminiums (R3Al; R = CH3, C2H5, and n‐C4H9) to obtain a stereoblock PMMA, isotactic PMMA‐block‐syndiotactic PMMA. Among the R3Als, (CH3)3Al gave most effectively the steroblock PMMA with narrow molecular weight distribution. The fraction of rr triad in the syndiotactic PMMA block increased with increase in the ratio of Al/Mg and reached about 76% at a ratio of Al/Mg ≥ 6. The method was also used to prepare stereoblock copolymer comprising isotactic PMMA block and syndiotactic block of butyl methacrylate. Stereocomplex formation and solution viscosity of the stereoblock PMMA were also studied.

Copolymerization of β-butyrolactone and β-benzyl malolactone with aluminoxane catalysts

Poly (β-hydroxylbutyrate-co-benzyl β-malolactonate), (P-HB-co-BML) copolymers, were prepared by the ring-opening copolymerizations of β-butyrolactone (BL) and benzyl β-malolactone (BML) with several alkyl aluminoxane catalysts, including ethylaluminoxane (EAO), methylaluminoxane (MAO), and isobutylaluminoxane (IBAO). The copolymers had very broad molecular weight distributions with weight average molecular weights, Mw, greater than 200,000. The products were fractionated by solubility into acetone-soluble and insoluble copolymers. The former were random, atactic copolymers while the latter were stereoblock copolymers. Random, atactic copolyesters of these two monomers were also obtained with the diethylzinc-water, ZnEt2-H2O (0.6:1), and the (5,10,15,20-tetraphenylporphinato)aluminium chloride, TPPAlCl, catalysts. The copolymerization reactions with the latter two catalysts gave much higher yields (up to 98%) and better control of both the copolymer compositions and the molecular weight distributions, but these copolymers had Mw values of only 20,000 or less. The copolymer tacticities and comonomer sequences were determined by 13C NMR spectroscopy.

Stereo block copolymers of L‐ and D‐lactides

AbstractSequential diblock copolymers composed of L‐ and D‐lactic acid residues were synthesized through a living ring‐opening polymerization of L‐ and D‐lactide initiated by aluminium tris(2‐propanolate). The composition of the block copolymers was varied by changing the reaction conditions and monomer over initiator ratio and confirmed by 1H NMR analysis, molecular weight determination and optical rotation measurements. Molecular weights ranged from 1,3 to 2,0 · 104 with 1,2 &lt; M̄w/M̄n &lt; 1,4. Stereocomplex formation in all block copolymers was determined using differential scanning calorimetry showing melting temperatures of about 205°C.

Block copolymers of α‐olefins prepared from macromolecules with long chain lifetimes

AbstractIt has been shown that certain Ziegler‐type catalysts, i.e., mixtures of TiCl4 with lithiumaluminium alkyls or VOCl3 with aluminum chloroalkyls, produce “living” macromolecules having lifetimes of the order of several days or more in the homo‐ and copolymerization of α‐olefins. The long‐lived macromolecules made it possible to prepare stereoblock copolymers of α‐olefins consisting of blocks of random copolymers alternating with blocks of homopolymers with a certain steric isometry and with a wide range of physical properties. Some of the relationships existing between the molecular structure and the physical properties of these model block copolymers have been studied and compared with random copolymers and homopolymers of α‐olefins. Crystallizable elastomers composed of amorphous random copolymer blocks of ethylene and propylene or ethylene and butene‐1, alternating with homopolymer blocks of ethylene, propylene, or butene‐1 are discussed. In particular, the homogeneity of the products, the stress–elongation behavior, the x‐ray diffraction patterns in the unstretched and stretched states, and the physical properties of the reinforced and vulcanized products of the crystallizable stereoblock rubbery copolymers are presented.

Physical and mechanical properties of polypropylene fractions

A large-scale fractionation of polypropylene according to molecular weight as well as to crystallinity has been carried out. The fractions obtained differed widely in properties. A fraction of low crystallinity, known as a ‘stereoblock copolymer’, possessed rubberlike properties. Evaluation of the fractions and of mixtures thereof showed that yield stress, dynamic modulus, drop in dynamic modulus at the transition point, mechanical damping and hardness depend only on the degree of crystallinity, whereas impact strength and elongation at break also depend on molecular weight and molecular weight distribution. A high impact strength requires a high molecular weight and a narrow molecular weight distribution. A correlation was found between tensile impact strength and loss factor at the glass transition point. The morphological structure of the various fractions is described.

Stereospecific polymerization of alkyl thiolacrylates. Part II – 3. Infrared spectrum of poly‐tert‐butyl thiolacrylate

Abstracttert‐Butyl thiolacrylate was readily polymerized by various anionic catalysts to give crystalline polymers. The polymers formed were markedly different from each other in elasticity according to the nature of the catalyst used. The elasticity continuously decreased in the following order of catalysts: LiAlEt3Bu, CaZnEt4, n‐BuLi, SrZnEt4 and PhMgBr. The crystalline polymer prepared by phenylmagnesium bromide was resinous in contrast to the elastic polymer formed by lithium aluminum tetraalkyl catalyst. Gradual change in the DEBYE‐SCHERRER's rings as well as the infrared spectra of these crystalline polymers was also observed in the above order. It was suggested that these crystalline polymers were probably stereoblock copolymers or polymer blends consisting of two stereo‐components, the proportion of which changed continuously according to the mentioned order.