Selective Chain Transfer Research Articles

Syndiotactic Poly(4-methyl-1-pentene)-Based Stereoregular Diblock Copolymers: Synthesis and Self-Assembly Studies.

Syndiotactic poly(4-methyl-1-pentene) (sP4M1P)-based stereoregular diblock copolymers, namely sP4M1P-b-polystyrene and sP4M1P-b-polymethylmethacrylate, were prepared from an α-bromoester-capped sP4M1P macroinitiator, which was chain extended with styrene and methyl methacrylate, respectively, via the atom transfer radical polymerization reaction. The α-bromoester-capped sP4M1P was generated by the esterification of hydroxyl-capped sP4M1P with α-bromoisobutyryl bromide. The hydroxyl-capped sP4M1P was synthesized by inducing a selective chain transfer reaction to aluminum during the syndiospecific polymerization of 4-methyl-1-pentene in the presence of a syndiospecific metallocene catalyst. As stereoregular diblock copolymers are difficult to prepare using existing methods, the current study offers an effective process for the preparation of sP4M1P-based stereoregular diblock copolymers. These copolymers were found to have well-defined architectures and they can undergo molecular self-assembly into ordered nanostructures, as evidenced by small-angle X-ray scattering analyses.

Preparation of small and photoresponsive polymer nanoparticles by intramolecular crosslinking of reactive star azo-polymers

Four-arm star azo-polymers with molecular weight control are prepared using the high selectivity of olefin cross-metathesis between acrylates and terminal olefins in the presence of multifunctional acrylates as the selective chain transfer agent. These reactive star azo-polymers are readily converted, by two different intramolecular crosslinking reactions of the end acrylate groups in dilute solution, to the new types of polymeric nanoparticles that exhibit distinct photoresponsive performance. Furthermore, the size and morphology of two types of polymeric nanoparticles show interesting and circular transformations when azo chromophore in nanoparticles underwent trans–cis–trans or cis–trans–cis isomerization cycles.

Syndiospecific polymerization of styrene with C1-symmetric [OSNO]-type bridged bis(phenolate) titanium (IV) complexes

Two C1-symmetric [OSNO]-type ethanediyl-bridged imine-thiobis(phenolate) titanium (IV) complexes of [Ti(OC6H2-6-tBu-4-tBu)2{CHN(CH2)2SCH2}Cl2] and [Ti(OC6H2-6-tBu-4-tBu)2{CHN(CH2)2S}Cl2] were synthesized by reacting the corresponding ethanediyl-bridged imine-thiobis(phenols) [(HOC6H2-6-tBu-4-tBu)2 {CHN(CH2)2SCH2}] and [(HOC6H2-6-tBu-4-tBu)2{CHN(CH2)2S}] with the titanium precursor TiCl4. The resultant complexes and intermediates were characterized by elemental analysis (EA) and nuclear magnetic resonance (NMR). Upon activation with triisobutyl aluminum modified methylalumoxane (MMAO), the [OSNO]-type titanium complexes can polymerize styrene into syndiotactic polystyrene (sPS) with high syndiotacticity (rrr > 95%). Electron paramagnetic resonance (EPR) analysis on [OSNO]-type titanium (IV) complexes in the presence of MMAO in toluene revealed that cationic titanium (III) complexes are the active species for syndiospecific polymerization of styrene. Furthermore, it was found from the end group analysis of the polymerization products by NMR that the selective chain transfer to MMAO took place during syndiospecific polymerization of styrene.

Syndiotactic Polyallyltrimethylsilane-Based Stereoregular Diblock Copolymers: Syntheses and Self-Assembled Nanostructures

Structurally well-defined stereoregular diblock copolymers composed of syndiotactic polyallyltrimethylsilane (sPATMS) and poly(methyl methacrylate) (PMMA) were prepared by using an α-bromoester-terminated sPATMS macroinitiator, which was chain extended by MMA using a cuprous halide-based atom transfer radical polymerization (ATRP) system. The α-bromoester-terminated sPATMS macroinitiator was prepared via the esterfication of hydroxyl-capped sPATMS with α-bromoisobutyryl bromide. The hydroxyl-capped sPATMS was generated via a selective chain transfer reaction to triethylaluminum (TEA) during the syndiospecific polymerization of allyltrimethylsilane (ATMS) conducted in the presence of syndiospecific ansa-metallocene catalysts. The proposed synthetic route not only offers the high-yield production of stereoregular sPATMS-b-PMMA but also provides the linking of the stereoregular block (sPATMS) with PMMA through a controlled/living radical polymerization process. Moreover, the proposed method offers effective control over the block chain length, the molecular weight distribution (Mw/Mn) and the stereoregularity of sPATMS block. Thus, the self-assembly of the resultant diblock copolymers produces well-ordered nanostructures from microphase separation, as evidenced by transmission electron microscopy and small-angle X-ray scattering results.

End‐functionalization of isotactic polypropylene via consecutive chain transfer reaction to norbornadiene and hydrogen

AbstractStructurally well‐defined end functionalized isotactic polypropylene (iPP) is prepared by conducting a selective chain transfer reaction during the isospecific polymerization of propylene in the presence of norbornadiene (NBD) and hydrogen using rac‐Me2Si(2‐Me‐4‐Ph‐Ind)2 ZrCl2/MAO as the catalyst. The production of NBD‐capped iPP involves a unique consecutive chain transfer reaction, first to NBD and then to hydrogen, for situating the incorporated NBD at the iPP chain end. The NBD end group of NBD‐capped iPP can be converted into other reactive functional group through functional group transformation reactions. The resulting functional group end‐capped iPP can be used for the construction of stereoregular block copolymers (e.g., iPP‐b‐PMMA and iPP‐b‐PS) through postpolymeriztion reactions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Stereoregular Diblock Copolymers of Syndiotactic Polystyrene Derivatives and Polylactide: Syntheses and Self-Assembled Nanostructures

Structurally well-defined stereoregular diblock copolymers, syndiotactic poly(4-methylstyrene)-b-poly(L-lactide) (sPMS-b-PLLA), were prepared by the controlled ring-opening polymerization of an aluminum alkoxide end-capped sPMS macroinitiator with l-lactide. The aluminum alkoxide end-capped sPMS macroinitiator was generated via the reaction between hydroxyl-capped sPMS and triethylaluminum. The hydroxyl-capped sPMS was prepared from the hydroboration of ethenyl-capped sPMS. The ethenyl-capped sPMS was generated by the desilylation of dimethylphenylvinylsilane-capped sPMS prepared via a unique vinylsilane-inducing selective chain transfer reaction during the syndiospecific polymerization of 4-methylstyrene conducted in the presence of dimethylphenylvinylsilane using Cp*Ti(OMe)3/MAO as catalyst. The proposed synthetic method can be used for the preparation of a broad variety of end-functionalized syndiotactic polystyrene derivatives (syndiotactic polystyrenes) end-capped with various end groups and facilitates the synthesis of syndiotactic polystyrenes-based stereoregular BCPs. The method offers effective control over the stereoregularity of polystyrenes and facilitates linking between blocks to provide stereoregular BCPs, which are capable of generating well-ordered nanostructures resulting from the self-assembly of stereoregular BCPs, as evidenced by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM).

Syndiospecific polymerization of styrene in the presence of cyclic olefins: A new approach for molecular weight, molecular weight distribution, and stereoregularity controls for syndiotactic polystyrene

A new approach for facilitating microstructural controls for syndiotactic polystyrene (sPS), in which, styrene polymerization is conducted in the presence of cyclic olefins and hydrogen, is proposed. Detailed structural analyses revealed that cyclic olefins are not incorporated into the polystyrene main chain; instead, they are capable of interrupting the chain propagation processes by binding onto the active catalyst to form a cyclic-olefin-coordinated active site. Thus, in the presence of hydrogen, chain transfer by hydrogen addition occurs selectively, which leads to the generation of drastically lower molecular weight sPS with a narrower range of molecular weight distribution. Chain end structural analyses of the resulting polymers revealed that styrene polymerization under theses conditions involves a selective chain transfer pathway for providing styrene polymers with uniform chain end structures. A unique method for inducing a selective chain transfer reaction by using non-incorporated cyclic olefins to regulate the chain reaction mechanism is demonstrated. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Hard nanocomposite comprising stereoregular syndiotactic polypropylene with chemically bonded gold nanoparticles

AbstractAn effective method to synthesize directly a hard composite material containing uniformly dispersed nanogold particles chemically bonded with a stereospecific, crystalline polymer matrix has been developed. Syndiotactic polypropylene was synthesized and functionalized to have a hydroxyl terminal group (sPPOH) via a metallocene catalysis with a selective chain transfer. Next, sPPOH was activated to react with ethylene sulfide forming the thiol‐terminated polymer, sPPSH. sPPSH was then chemically bonded to gold nanoparticles (AuNPs) formed in situ via a reduction of HAuCl4. The bonding between thiol and AuNP stabilized the AuNPs and led to the formation of sPPAuNPs composite containing uniformly‐dispersed AuNPs of a 19–40 nm size without noticeable aggregation. Furthermore, the chemical bonding of AuNPs has afforded sPPAuNPs a thermal degradation temperature (TD) 49.4 °C higher than the pristine sPP or sPPOH and 25.7 °C higher than sPPSH without any adverse effect on the crystalline temperature and melting temperature. In addition, the characteristic UV‐Vis absorption wavelength of sPPAuNPs remains the same at various temperatures, thus indicating the independence of optical property on temperature as well as the good thermal stability of the sPPAuNPs composite. 1H NMR, 13C NMR, FESEM, STEM, XPS, TGA and DSC were used to investigate the molecular structure, morphology and thermal properties of the resulting sPPAuNPs nanocomposite. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

End‐functionalization of syndiotactic polystyrene by vinylsilane inducing selective chain transfer reactions

AbstractStructurally well‐defined end‐functionalized syndiotactic polystyrene (sPS) can be prepared by conducting a simultaneous selective chain transfer reaction during the syndiospecific polymerization of styrene in the presence of vinylsilanes. The production of vinylsilane end‐capped sPS involves a unique selective chain transfer pathway via the incorporation of a terminal vinylsilane unit at the polymer chain end by 2,1‐insertion. This unusual insertion pattern situates the bulky silyl functional group at a closer β‐position from the active catalyst center, thus deactivating the propagating chain by a steric jam between the vinylsilane end group and the active catalyst. Subsequently, chain releasing by hydrogen addition (in the presence of H2) or by β‐elimination (in the absence of H2) can take place, which leads to the production of end‐functionalized sPS with precise controls of stereoregularity and of the location of functionality. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1690–1698, 2010

Stereoregular Diblock Copolymers of Syndiotactic Polypropylene and Polyesters: Syntheses and Self-Assembled Nanostructures

Structurally well-defined stereoregular diblock copolymers composing of syndiotactic polypropylene (sPP) and polyester have been prepared using controlled ring-opening copolymerization of cyclic esters, including e-caprolactone and DL-lactide, mediated by aluminum alkoxide end-capped sPP as the macroinitiator. The aluminum alkoxide end-capped sPP was generated by the in situ activation of hydroxyl-capped sPP with triethylaluminum (TEA). The hydroxy-capped sPP was prepared via the selective chain transfer reaction to TEA while inducing the syndiospecific polymerization of propylene in the presence of ansa-metallocene catalysts. The synthetic method not only offers high-yield production of the stereoregular block copolymers but also provides successful linking of the stereoregular polyolefins with an abundant variety of polymers as the second block. Moreover, the synthetic method offers effective control over the block chain length and molecular weight distribution; thus, the microphase separation of the resultant diblock copolymers generated well-ordered nanostructures as revealed by small-angle X-ray scattering.

リビングカチオン重合 １．リビングカチオン重合の夜明け

This special lecture, divided into three parts, provides a general overview of living cationic polymerization of vinyl monomers including vinyl ethers, isobutene, and styrenes. The first installment of a three-part series highlights critical researches in the “pre-living&rdquo: era on controlled initiation, selective chain transfer (oligomerization), and polymerization by long-lived species. The first two major breakthroughs are also described briefly in the last section of this article.

Isotactic Polypropylene-Based Stereoregular Diblock Copolymers: Syntheses and Self-Assembly

The preparation of polyolefin-based stereoregular diblock copolymers (BCPs) by postpolymerization of hydroxy-capped isotactic polypropylene (iPP) led to the successful generation of a series of structurally well-defined iPP-based stereoregular BCPs for self-organizing into various nanostructures. The hydroxy-capped iPP prepolymer was synthesized by using ansa-metallocene catalysts to mediate the selective chain transfer to alkylaluminums during isospecific polymerization of propylene, leading to the production of alkylaluminum end-capped iPP as the preliminary product. Subsequently, in-situ O2/H2O2 treatment of the resulting aluminum-capped iPP led to the successful preparation of hydroxy-capped iPP. The terminal hydroxyl end group of the iPP-based prepolymer was allowed to undergo tosylation reaction that led to the production of tosyl group end-capped iPP with a quantitative functional group conversion ratio. Subsequently, coupling the tosyl group end-capped iPP with living anionic poly(2-vinylpyridine)...

Alternating Copolymerization of Ethylene and Propylene: Evidence for Selective Chain Transfer to Ethylene

C1-symmetric ansa-metallocenes Me2E(Ind)(Flu)ZrCl2 (E = Si (1) and C (2)), upon activation with methylaluminoxane (MAO), produce atactic alternating ethylene−propylene copolymers (up to 92% EP dyads) with low molecular weight (less than 20K). Quantitative 13C NMR analysis of copolymer chain ends produced with 2/MAO indicates that the chain initiation/termination process is highly selective to generate a vinylidene as the unsaturated end group (terminating end) and an ethyl group as the saturated end group (initiating end). The high selectivity for sec-butyl end groups reveals a highly selective chain-initiation process where ethylene insertion is followed by propylene insertion at the initiating end. Chain transfer from a propylene-terminated polymer chain to a coordinated ethylene monomer is proposed to explain this selectivity. The facile cross-monomer chain-transfer process explains the much lower molecular weights obtained for the EP copolymers relative to the ethylene or propylene homopolymers.

Selective protein-protein interactions direct channeling of intermediates between polyketide synthase modules.

Polyketide synthases (PKSs) have represented fertile targets for rational manipulation via protein engineering ever since their modular architecture was first recognized. However, the mechanistic principles by which biosynthetic intermediates are sequentially channeled between modules remain poorly understood. Here we demonstrate the importance of complementarity in a remarkably simple, repetitive structural motif within these megasynthases that has been implicated to affect intermodular chain transfer [Gokhale, R. S., et al. (1999) Science 284, 482]. The C- and N-terminal ends of adjacent PKS polypeptides are capped by short peptides of 20-40 residues. Mismatched sequences abolish intermodular chain transfer without affecting the activity of individual modules, whereas matched sequences can facilitate the channeling of intermediates between ordinarily nonconsecutive modules. Thus, in addition to substrate-PKS interactions and domain-domain interactions, these short interpolypeptide sequences represent a third determinant of selective chain transfer that must be taken into consideration in the protein engineering of PKSs. Preliminary biophysical studies on synthetic peptide mimics of these linkers suggest that they may adopt coiled-coil conformations.

Selective Chain Transfer Reactions in Metallocene Catalyzed Copolymerization of Ethylene with Allylbenzene

Selective Chain Transfer Reactions in Metallocene Catalyzed Copolymerization of Ethylene with Allylbenzene