Chain End Structure Research Articles

Polyisobutylenes with Controlled Molecular Weight and Chain-End Structure: Synthesis and Actual Applications.

The polymerization of isobutylene allows us to obtain a wide spectrum of polyisobutylenes (PIBs) which differ in their molecular weight characteristics and the chemical structure of chain-end groups. The bulk of the PIBs manufactured worldwide are highly reactive polyisobutylenes (HRPIBs) with -C(Me)=CH2 end-groups and low-molecular weights (Mn < 5 kDa). HRPIBs are feedstocks that are in high demand in the manufacturing of additives for fuels and oils, adhesives, detergents, and other fine chemicals. In addition, HRPIBs and CMe2Cl-terminated PIBs are intensively studied with the aim of finding biomedical applications and for the purpose of developing new materials. Both chain control (molecular weight and dispersity) and chemoselectivity (formation of exo-olefinic or -CMe2Cl groups) should be achieved during polymerization. This review highlights the fundamental issues in the mechanisms of isobutylene polymerization and PIB analysis, examines actual catalytic approaches to PIBs, and describes recent studies on the functionalization and applications of HRPIBs and halogen-terminated PIBs.

The benzyl ethyl trithiocarbonate mediated control synthesis of a block copolymer containing N-vinyl pyrrolidone by RAFT methodology: Influence of polymer composition on cell cytotoxicity and cell viability

Benzyl ethyl trithiocarbonate (BET) mediated reversible addition–fragmentation chain transfer (RAFT) polymerization was carried out in dry 1,4-dioxane medium at 85 °C to synthesize Poly(n-Butyl Acrylate) (PBA) with pre-determined molecular weight (Mn 7999), narrow polydispersity (PDI 1.13) and precise chain end structure. The end chain functionality of macro chain transfer agent (CTA) was proved by homo-chain and hetero-chain extension polymerization experiment to get the poly (n-Butyl Acrylate) (Mn 17902) and poly (n-Butyl Acrylate)-b-poly(N-Vinyl Pyrrolidone) di-block copolymer (Mn 16029) respectively. Resulting polymers were characterized by 1H NMR, FTIR, GPC, TGA-DTA, DTG, DSC. The kinetic investigations confirmed the pseudo-first-order kinetic of the homopolymerization and linear evolution of the molar mass w.r.t. conversion within the range of polydispersity (PDI) (1.11 –1.20) in dry 1, 4-dioxane medium. The cyto-toxicities of PBA Macro CTA and PBA-b-PNVP polymers were evaluated on RAW264.7 mouse macrophage cells. The cellular viability was found to be 60.31% and 77.83% at 250 μM concentration, respectively for the two polymers. The viability was found more at high concentrations of polymers. The mechanism of cell death associated with these polymers was further evaluated in terms of apoptosis and necrosis on MCF-7 cell lines. Both the polymers induced apoptotic-like changes in MCF-7 cells.

Chain-End Functionalization of Living Polyisobutylene via an End-Quenching Comonomer That Terminates by Indanyl Ring Formation

A method for chain end functionalization of polyisobutylene (PIB) has been developed whereby living PIB is end-capped with the comonomer, 4-(4-allyloxyphenyl)-2-methyl-1-butene (AMB), at full isobutylene conversion. Addition is readily limited to just over one comonomer unit per chain end through the use of a low [AMB]/[chain end] ratio. When difunctional (telechelic) living PIB was end-capped using [AMB]/[chain end] = 2 at −70 °C, NMR showed that between 1.08 and 1.22 comonomer units on average were added per PIB chain end, depending on solvent polarity. Chain end structure and mechanism of formation were investigated by 1H, 13C APT, 2D gradient HSQC, and 2D HMBC NMR. tert-Chloride chain ends formed via PIB+Ti2Cl9– or PIB-AMB+Ti2Cl9– ion pair collapse were not observed. Olefinic chain ends formed via β-proton loss also were not detected; instead, a cyclic end group structure was observed. Upon addition of AMB the resulting carbocation tends to undergo terminative chain transfer consisting of alkylation o...

RAFT‐mediated emulsion polymerization of chloroprene and impact of chain‐end structure on chloroprene rubbers

ABSTRACTThe reversible addition‐fragmentation chain transfer (RAFT) polymerization of chloroprene (CP) in an emulsion system using a dithiocarbamate‐type RAFT agent was studied. The controlled RAFT‐mediated emulsion polymerization was achieved by the appropriate combination of a RAFT agent and nonionic surfactant (polyoxyethylene phenyl ether) using a water‐soluble initiator (VA‐044) at 35 °C. An almost linear first‐order kinetic plot was observed until relatively high conversion (&gt;80%) with molecular weights between 22,300 and 33,100 and relatively narrow molecular weight distributions (Mw/Mn ≦ 1.5) were achieved. The amount of the emulsifier used and the pH of the system were found to affect the controlled character, polymerization rate, and induction period, which are related to the size of the emulsion particles. Large‐scale RAFT‐mediated emulsion polymerization was also employed to afford industrially applicable poly(CP) (Mw &gt; 25 × 104, resulting product &gt; 2300 g). The vulcanized CP rubber obtained from the RAFT‐synthesized poly(CP) exhibited better physical properties, particularly tensile modulus and compression set, which may be due to the presence of the reactive end groups and the absence of low‐molecular‐weight products. We also evaluated the impact of the chain‐end structure on the mechanical and physical properties of these industrially important CP rubbers with carbon black. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46008.

New Vistas on the Anionic Polymerization of Styrene in Non-Polar Solvents by Means of Density Functional Theory.

The elementary processes of anionic styrene polymerization in the gas phase and in cyclohexane were studied using M062X (a recently developed density functional theory (DFT) method) combined with the 6-31+G(d) basis sets, in order to clarify the complicated phenomena caused by the association of the active chain-ends and elucidate the details of the polymerization mechanism. Three types of HSt2Li (a model structure of polystyryllithium chain-ends) were obtained; the well-known first structure in which Li is coordinated to the side chain, the second structure in which Li is coordinated to the phenyl ring, (both without the penultimate unit coordination), and the third structure in which Li is coordinated to both the chain-end unit and the penultimate styrene unit. Although the third HSt2Li is the most stable as expected, the free energy for the transition state of its reaction with styrene is higher than those for the other two transition states due to its steric hindrance. The free energy for the transition state of the reaction of the second HSt2Li with styrene is the lowest, suggesting that the route through it is the predominant reaction path. The penultimate unit effect, slower addition of styrene to HSt2Li than to HStLi, is attributed to coordination of the penultimate styrene units of the polystyryllithium dimer (one of the starting materials) to its Li atoms. The calculated enthalpy for the reaction barrier of the second HSt2Li with styrene in cyclohexane was found to agree with the observed apparent activation energy in benzene.

Radical Trap‐Assisted Atom Transfer Radical Coupling of Diblock Copolymers as a Method of Forming Triblock Copolymers

The formation of ABA‐type triblock copolymers is studied as a function of chain end structure of the second block (benzylic alkyl bromides compared to α‐bromo esters) and the inclusion of the radical trap 2‐methyl‐2‐nitrosopropane (MNP) in the coupling reaction. In the case of coupling poly(methyl methacrylate)‐block‐polystyrene (PMMA‐b‐PSBr), both traditional atom transfer radical coupling (ATRC) reactions and analogous radical trap‐assisted ATRC (RTA‐ATRC) reactions lead to ABA‐type triblock copolymers. Contrastively, coupling of polystyrene‐block‐poly(methyl acrylate) precursors is unsuccessful in ATRC reactions lacking the nitroso radical trap, yet forming high amounts of triblock in analogous RTA‐ATRC reactions, consistent with lower KATRP values of the chain end α‐bromo ester. Synthesis of the triblocks is also attempted using coupling reactions of dibrominated PS and monobrominated poly(methyl acrylate), relying on selective coupling to be successful. In the presence of the radical trap MNP, substantial coupling is observed with gel permeation chromatography data indicating the formation of the triblock. Traditional ATRC reactions performed in an analogous manner do not produce the triblock to an appreciable extent, with lowered extents of coupling overall. image

HCl·Et2O-Catalyzed Metal-Free RAFT Cationic Polymerization: One-Pot Transformation from Metal-Free Living Cationic Polymerization to RAFT Radical Polymerization1

The metal-free RAFT cationic polymerization (MRCP) of vinyl ethers (VEs) mediated by HCl·Et2O and 1-isobutoxyethyl ethanedithioate (IDTA) as a RAFT cationogen was demonstrated. The IDTA was efficiently catalyzed and the cationic polymerization was initiated by the HCl·Et2O, allowing good control over the molecular weight, polydispersity, and chain end structure of the resulting polyVEs. The propagation step is considered to involve a reversible addition–fragmentation chain transfer (RAFT) to the growing carbocationic species. Thus, the resulting polyVEs can exhibit high number-average end functionality at the RAFT terminal group, depending on the [IDTA]0/[HCl]0 ratio. The polymers obtained by this process could be used as macro-chain transfer agents for the RAFT radical polymerization of radically polymerizable monomers such as (meth)acrylates and styrenes to synthesize novel block copolymers. Significantly, this MRCP system allows a one-pot transformation from MRCP to RAFT radical polymerization as a res...

RAFT Polymerization of Phenyl Vinyl Sulfide Using Trithiocarbonate Mediating Agents and Synthesis of Block Copolymers

SummaryReversible addition‐fragmentation chain transfer (RAFT) polymerization of phenyl vinyl sulfide (PVS), in which the thioether moiety is directly connected to the vinyl group, was investigated using trithiocarbonates as chain transfer agents (CTAs). A trithiocarbonate with a 2‐cyano‐2‐propyl group in the R moiety was found to be effective in achieving controlled radical polymerization of the S‐vinyl monomer. Reasonable control of the polymerization of PVS was indicated by the formation of a narrow dispersity polymer, molecular weight controlled by the monomer/CTA molar ratio, a linear increase in molecular weight with the conversion, and preserved chain end structure (determined by 1H NMR). Well‐defined block copolymers were synthesized by RAFT polymerization of PVS using polystyrene macro‐CTA.

Thermal degradation mechanism of poly(hexamethylene carbonate)

Thermal degradation behaviors of poly(hexamethylene carbonate) (PHC) were investigated by means of thermogravimetric analysis (TGA), proton nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). Six types of PHC samples with different end groups and molecular weights were synthesized to systematically investigate the thermal degradation mechanism. Thermal degradation behaviors of the PHC samples were examined under both isothermal and non-isothermal conditions. The PHC samples showed distinct thermal degradation behaviors from other aliphatic polycarbonates (poly(propylene carbonate), poly(trimethylene carbonate) and poly(butylene carbonate)). The results indicated that the chain-end structure makes a slight effect on the thermal stability of PHC regardless of the molecular weight. During the non-isothermal degradation of PHC, four main reactions were illustrated: unzipping, intramolecular transesterification, β-H transfer and decarboxylation reactions. Intramolecular transesterification reaction dominantly occurs below 300 °C accompanying with unzipping reaction which can only be induced by hydroxyl end group, and releasing cyclic hexamethylene carbonate monomer and dimer. Above 300 °C, the four degradation reactions take place simultaneously.

Synthesis and characterization of nano micelles of poly(N-acrylamidohexanoic acid)-b-poly(N-vinylcaprolactam) via RAFT process: Solubilizing and releasing of hydrophobic molecules

RAFT polymerization at 70 °C in 1,4-dioxane with a trithiocarbonate as a CTA is used to synthesize well-defined poly(N-acrylamidohexanoic acid) with pre-determined molecular weight, narrow molar mass dispersity and precise chain end structure. Amphiphilic block copolymers of N-acrylamidohexanoic acid with N-vinylcaprolactam is synthesized and characterized by 1H NMR, FTIR and GPC. The micelles formed by poly(N-acrylamidohexanoic acid)-b-poly(N-vinylcaprolactam) block copolymer in aqueous solution are characterized by UV-visible spectroscopy of 1-pyrenecarboxaldehyde (1-PyCHO) probe, light scattering and TEM analysis. The critical micelle concentration value of the block copolymers, determined by UV–visible spectroscopy increased with the increase in the chain length of PVCL block. 1-PyCHO is encapsulated into the core of micelles via dialysis method. Loading of 5.3 wt % is observed and the micelles containing 1-PyCHO exhibit a sustained release profile. The results indicate that the PNAHA-b-PVCL micelles may be a potential drug delivery carrier for hydrophobic molecules.

Zero-valent metal catalyzed removal of thiocarbonylthio end groups from RAFT-made polystyrene: producing controllable bimodal molecular weight distribution

This work presents a facile method for removing thiocarbonylthio end groups from RAFT-made polystyrene (PS) under the catalysis of Cu(0) or Fe(0). In Cu(0)-catalyzed cases, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) was able to promote the removal of thiocarbonylthio end groups with the production of a distinct bimodal molecular weight distribution (MWD). The chain end structure of the resultant polymer was carefully identified by the matrix-assisted laser desorption ionization time-of-flight mass spectrum. However, in the Fe(0)-catalyzed system, TEMPO and ligands were not needed for the removal of thiocarbonylthio end groups with a more efficient removal. This work provides an alternative approach for removing thiocarbonylthio end groups of RAFT-made PS as well as producing thiocarbonylthio-free PS with controllable bimodal MWD.

Cationic copolymerization of racemic -β-butyrolactone with L,L-lactide: One-pot synthesis of block copolymers

Cationic copolymerization of racemic-β-butyrolactone (β-BL) with l,l-lactide (LA) initiated by alcohol and catalyzed by trifluoromethanesulfonic acid proceeding by activated monomer (AM) mechanism was investigated. Although both comonomers were present from the beginning in the reaction mixture, polymerization proceeded in sequential manner, with poly-BL formed at the first stage acting as a macroinitiator for the subsequent polymerization of LA. Such course of copolymerization was confirmed by following the consumption of both comonomers throughout the process as well as by observing the changes of growing chain-end structure using 1H NMR. 13C NMR analysis and thermogravimetry revealed the block structure of resulting copolymers. The proposed mechanism of copolymerization was confirmed by the studies of changes of 1H NMR chemical shift of acidic proton in the course of copolymerization, providing an indication that indeed protonated species and hydroxyl groups are present throughout the process, as required for AM mechanism. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4873–4884

RAFT Polymerization of S-Vinyl Sulfide Derivatives and Synthesis of Block Copolymers Having Two Distinct Optoelectronic Functionalities

Well-defined sulfur-containing polymers and block copolymers were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization of S-vinyl sulfide derivatives, in which the thioether moiety is directly connected to the vinyl group. We initially investigated RAFT polymerization of four different S-vinyl sulfide derivatives, phenyl vinyl sulfide (PVS), 4-chlorophenyl vinyl sulfide (CPVS), 2,4-dichlorophenyl vinyl sulfide (DCPVS), and 4-bromophenyl vinyl sulfide (BPVS). Three xanthate-type chain transfer agents (CTAs), a dithiocarbamate-type CTA, and a dithioester-type CTA were compared for these RAFT polymerizations. Reasonable control of the polymerization of PVS was confirmed by an observed linear increase in the molecular weight with the conversion, feasibility to control molecular weight based on the ratio of monomer consumed to the amount of CTA used, chain end structure determined by 1H NMR, and chain extension experiment. The RAFT polymerization of the bromo-substituted monome...

Atom transfer radical polymerization as a tool for making poly(N-acryloylglycinamide) with molar mass independent UCST-type transitions in water and electrolytes

Atom transfer radical polymerization (ATRP) of N-acryloylglycinamide has been used as a tool to make poly(NAGA) showing UCST-type phase transitions in water and electrolytes independent of molar mass and end-groups. We hypothesized that similarity in the structure of polymer chain ends with the repeat unit of poly(NAGA) could help in eliminating the effect of molar mass dependence of the cloud point especially in the low molar mass region. The monomer-like end-groups were introduced by choosing an appropriate initiator for ATRP like chloropropionamide (CPA). The catalyst system CuCl/CuCl2 and tris[2-(dimethylamino)ethyl]-amine (Me6TREN) as a ligand provided controlled polymerization of NAGA in DMSO at 45 °C with UCST-type transitions retained in water and electrolytes without being influenced by chain ends/molar mass and high concentration of salts like NaCl and Na2SO4.

Controlled polymerization of methyl acrylate for high‐molecular‐weight polymers by pentafluorophenylbis(triflyl)methane‐promoted group transfer polymerization using triisopropylsilyl ketene acetal

AbstractThe group transfer polymerization (GTP) of methyl acrylate (MA) was studied using pentafluorophenylbis(triflyl)methane (C6F5CHTf2) as the organocatalyst and 1‐trimethylsiloxy‐, 1‐triethylsiloxy‐, and 1‐triisopropylsiloxy‐1‐methoxy‐2‐methyl‐1‐propene (MTSMe, MTSEt, and MTSiPr, respectively) as the initiators. The C6F5CHTf2‐promoted GTP of MA using MTSiPr proceeded in a living nature to produce poly(methyl acrylate)s (PMAs) with controlled molecular weights and narrow molecular weight distributions, which allowed the synthesis of high‐molecular‐weight PMA with the number‐average molecular weight (Mn(SEC)) of up to 108,000 and the polydispersity (Mw/Mn) of 1.07. The matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry measurement revealed that the obtained PMA possessed the chain end structure that originated from MTSiPr, showing that the C6F5CHTf2‐promoted GTP of MA proceeded without any side reactions. In addition, the kinetic study and the postpolymerization experiment supported the living manner of the polymerization. Moreover, the block copolymerization of MA and n‐butyl acrylate (nBA) smoothly proceeded to afford the well‐defined PMA‐block‐poly(n‐butyl acrylate) (PnBA). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Thermal degradation kinetics of thermoresponsive poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) copolymers prepared via RAFT polymerization

Reversible addition-fragmentation chain transfer polymerization at 70 °C in N,N-dimethylformamide was used to prepare poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) copolymers in various compositions to afford well-defined polymers with pre-determined molecular weight, narrow molecular weight distribution, and precise chain end structure. The copolymer compositions were determined by 1H NMR spectroscopy. The reactivity ratios of N-isopropylacrylamide (NIPAM) and N,N-dimethylacrylamide (DMA) were calculated as rNIPAM = 0.838 and rDMA = 1.105, respectively, by the extended Kelen–Tudos method at high conversions. The lower critical solution temperature of PNIPAM can be altered by changing the DMA content in the copolymer chain. Differential scanning calorimetry and thermogravimetric analysis at different heating rates were carried out on these copolymers to understand the nature of thermal degradation and to determine its kinetics. Different kinetic models were applied to estimate various parameters like the activation energy, the order, and the frequency factor. These studies are important to understand the solid state polymer degradation of N-alkyl substituted polymers, which show great potential in the preparation of miscible polymer blends due to their ability to interact through hydrogen bonding.

Cationic methacrylate polymers containing chiral amino acid moieties: controlled synthesis via RAFT polymerization

Two methacrylate containing amino acid based chiral monomers, Boc-L-alanine methacryloyloxyethyl ester (Boc-Ala-HEMA) and Boc-L-phenylalanine methacryloyloxyethyl ester (Boc-Phe-HEMA), were polymerized by the reversible addition–fragmentation chain transfer (RAFT) process to afford well-defined amino acid based polymers with controlled molecular weight, narrow molecular weight distribution, and precise chain end structure. Linear pseudo-first-order kinetics and number average molecular weight dependence on conversion were observed for the RAFT polymerizations. Di-block copolymers were prepared by RAFT polymerization of methyl methacrylate using poly(Boc-L-alanine methacryloyloxyethyl ester) or poly(Boc-L-phenylalanine methacryloyloxyethyl ester) as macro-chain transfer agents. Subsequent deprotection of the Boc group produced architectures with a primary amine moiety at the side chain, which showed pH responsiveness. The specific rotations of both the chiral monomers and corresponding homo- and block copolymers were studied and alteration of specific rotation due to transformation of monomer to polymer was noticed. In addition, the surface charge of various polymeric architectures was studied using dynamic light scattering (DLS) measurements. The self-assembly of block copolymer films was investigated by atomic force microscopy (AFM). These pH-responsive cationic polymers can potentially be extended for the delivery of small interfering RNA.

Quantitative Analysis of the Effect of Azo Initiators on the Structure of α-Polymer Chain Ends in Degenerative Chain-Transfer-Mediated Living Radical Polymerization Reactions

The effects of azo initiators on the structures of the α-polymer chain ends in organotellurium-, organostibine-, organobismuthine-, and organoiodine-mediated living radical polymerizations (TERP, SBRP, BIRP, and IRP, respectively) and reversible addition–fragmentation chain transfer radical polymerization (RAFT) were quantitatively analyzed for the first time. These polymerization methods predominantly, or exclusively, follow the degenerative chain transfer (DT) mechanism for the activation and deactivation of dormant and radical species. An external radical species is required to initiate the polymerization under DT conditions, and the influx of radical species decreases the α-end-group fidelity. The effect was examined for polymerizations of styrene (St), n-butyl acrylate (BA), N-vinylpyrrolidone (NVP), methyl methacrylate (MMA), and isoprene (Ip) in the presence of different chain transfer agents (CTAs) and azo initiators, such as 2,2′-azobis(isobutyronitrile) (AIBN), 1,1′-azobis(cyclohexane-1-carbonit...

Synthesis of Novel Polyolefin Stabilizer Having HALS Modified Chain-End Structure and its Photo-Stabilizing Effect

Synthesis of Novel Polyolefin Stabilizer Having HALS Modified Chain-End Structure and its Photo-Stabilizing Effect

Vanadium alkoxide catalyzed polymerization of vinyl chloride

The polymerization of vinyl chloride (VC) with vanadium complex/alkylaluminum catalyst was investigated. In the case of polymerization with vanadium oxytriethoxide (VO(OEt)3), poly(vinyl chloride) was obtained in a good yield. The effect of cocatalyst, solvent, and cocatalyst/precatalyst ratio was observed. The structure of the polymer obtained with VO(OEt)3/i-Bu3Al catalyst consisted of regular head-to-tail sequence and isobutyl chain-end structure. VO(OEt)3/alkylaluminum catalyst was able to copolymerize VC with styrene, 1-butene, methyl methacrylate, and methyl acrylate. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011