Core-first Approach Research Articles

Synthesis of Linear and Star-Shaped Poly[4-(diphenylamino)benzyl methacrylate]s by Group Transfer Polymerization and Their Electrical Memory Device Applications

Hole-transporting triphenylamine (TPA)-based polymers, the linear poly[4-(diphenylamino)benzyl methacrylate] (PTPMA) and the three-armed star-shaped poly[4-(diphenylamino)benzyl methacrylate] (N(PTPMA)3), have been synthesized by group transfer polymerization (GTP). For the synthesis of N(PTPMA)3, the core-first approach coupled with the GTP was adopted using a newly designed silyl ketene acetal initiator with three initiating points. The polymerization results showed that the polymer molecular weights were precisely controlled by the monomer/initiator ratio. The new hole-transporting TPA-based polymers could be blended with electron-accepting [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) for electrical memory device applications. The experimental results showed that pristine PTPMA and N(PTPMA)3 exhibited dynamic-random-access-memory (DRAM) volatile electrical behavior, but their PCBM composite-based devices changed to the write-once-read-many times (WORM) nonvolatile memory characteristic or conducto...

Synthesis, characterization, and rheological properties of multiarm stars with poly(glycidol) core and poly(methyl methacrylate) arms by AGET ATRP

Well-defined multiarm star block copolymers poly(glycidol)-b-poly(methyl methacrylate) (PGOHBr-b-PMMAx) with an average number of PMMA arms of 85, 55, and 45 have been prepared. The core-first approach has been selected as the methodology using atom transfer radical polymerization (ATRP) of MMA from an activated hyperbranched poly(glycidol) as the core. These activated hyperbranched macroinitiators were prepared by esterification of hyperbranched poly(glycidol) (PGOH) with 2-bromoisobutyryl bromide. The effect of monomer/initiator ratio, catalyst concentration, time, temperature, and solvent on the growing of the arms has been studied in detail in order to optimize the process and to diminish the radical-radical coupling. The final products and intermediates were characterized by means of size exclusion chromatography (SEC), nuclear magnetic resonance (NMR) and Fourier transform-infrared (FTIR) spectroscopy. The length of PMMA arms was determined by SEC after cleavage of ester bond linked to PGOH core. Glass transition temperature (Tg), thermal stability and rheological properties of the multiarm star copolymers were also studied. Finally, tapping mode atomic force microscopy (TMAFM) allowed the clear visualization of nano-sized particles (80–200 nm) corresponding to individual star molecules. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Double stimuli-responsive behavior of linear and star-shaped poly( N,N-diethylaminoethyl methacrylate) in aqueous solution

We report on the synthesis and characterization of linear and star-shaped poly( N,N-diethylaminoethyl methacrylate) (PDEA). The synthesis was accomplished by Atom Transfer Radical Polymerization (ATRP) via a core-first approach using sugar-based multifunctional initiators. The investigation of the solution properties in water shows that PDEA is both pH- and temperature-responsive, analogous to the behavior of poly( N, N-dimethylaminoethyl methacrylate) (PDMA). In literature, PDEA is frequently referred to as being only pH-sensitive. The critical pH values for the aggregation are close to the apparent p K a values in all cases, i.e. a high charge density is necessary to keep the polymers soluble. The cloud points show a strong dependence on the pH value of the solution but no dependence on either molecular weight or architecture. Thus, the two polymers differ only quantitatively, as PDEA has cloud points about 40 K lower than PDMA and critical pH values which are 1.5–2 units lower than PDMA.

One‐pot synthesis of star‐shaped aliphatic polyesters with hyperbranched cores and their characterization with size exclusion chromatography

Star-shaped polymers, bearing a strictly defined number of linear arms, provide useful model structures for studies of branched macromolecular structures. In the dynamically growing area of biocompatible and biodegradable polymers, the synthesis and characterization of starshaped aliphatic polyesters are particularly advanced. Among others, some of our recent articles report the synthesis and characterization of 1-, 2-, 3-, 4-, 5-, 6-, 8, 13-, and 32-arm star-shaped poly(L-lactide)s by size exclusion chromatography (SEC), liquid chromatography under critical conditions (LC–CC), two-dimensional LC–CC/SEC, NMR, and fluorescence spectroscopy. The method perhaps most commonly used for starshaped polymer synthesis is based on the core-first approach, in which a multifunctional reagent (the core) plays the role of the (co)initiator and/or transfer agent. Another possibility in this area, originally developed by Rempp et al. for a styrene/divinylbenzene (DVB) system, was followed more recently by Lapienis and Penczek for an epoxide/diepoxide system. The principle of this approach is based on a two-step synthesis with the one-pot, arms-first coremethod. In the first step, a linear living polymer (the precursor) is prepared (e.g., polystyrene or polyepoxide). Then, in the second step, the resulting precursor initiates the polymerization of a tetrafunctional monomer (e.g., DVB or diepoxide), giving a highly branched macromolecular structure (the core). This communication reports the successful synthesis of highly branched, star-shaped aliphatic polyesters from various lactone and lactide/bislactone systems—e-caprolactone (CL)/5,50-bis(oxepan-2-one) (bis-CL), L,L-lactide (LA)/ bis-CL, CL/1,6-dioxaspiro[4,4]nonane-2,7-dione) (bis-BL), and LA/bis-BL—with aluminum isopropoxide [Al(OPr)3] as the initiator. Experimental evidence of the formation of starlike structures is provided by SEC and matrixassisted laser desorption/ionization time-of-flight (MALDITOF) analysis.

Synthesis of Star Polymers using RAFT Polymerization: What is Possible?

Various pathways to generate star polymers using reversible addition–fragmentation transfer (RAFT) are discussed. Similar to other polymerization techniques, star polymers can be generated using arm-first and core-first approaches. Unique to the RAFT process is the subdivision of the core-first approach into the R-group and Z-group approaches, depending on the attachment of the RAFT agent to the multifunctional core. The mechanism of the R- and Z-group approaches are discussed in detail and it is shown that both techniques have to overcome difficulties arising from termination reactions. Termination reactions were found to broaden the molecular weight. However, these side reactions can be limited by careful design of the synthesis. Considerations include RAFT and radical concentration, number of arms, type of RAFT agent and monomer. Despite obvious challenges, the RAFT process is highly versatile, allowing the synthesis of novel polymer architectures such as poly(vinyl acetate) and poly(vinyl pyrrolidone) star polymers.

Synthesis of functionalized multiarm poly(ethylene oxide) stars

Six arm poly(ethylene oxide) (PEO) stars carrying either 6 pyridyl or 12 hydroxyl groups at their periphery were synthesized using an arm-first approach. To this end, two novel α,ω-heterodifunctional PEO's, namely α-ketal,ω-hydroxy and α-pyridyl,ω-hydroxy PEO's were synthesized and, after the deprotonation of their hydroxyls, deactivated onto hexachlorocyclophosphazene which served to build the core. Quaternization of the stars containing six outer pyridyl groups created positive charges at their periphery while the acidic treatment of stars carrying terminal ketal rings afforded six arm PEO stars with 12 peripheral hydroxyls. The latter compound was subsequently deprotonated and used to polymerize ethylene oxide by a classical core-first approach. This resulted in the formation of highly branched PEO's also referred to as dendrimer-like PEO stars which consisted of a first generation of six PEO arms and a second generation of 12 hydroxy-ended PEO branches.

Synthesis of Water-Soluble Star-Block and Dendrimer-like Copolymers Based on Poly(ethylene oxide) and Poly(acrylic acid)

Star-block copolymers (PEO3-b-PAA3) and dendrimer-like copolymers (PEO3-b-PAA6) consisting of three inner poly(ethylene oxide) (PEO) arms and either three or six peripheral poly(acrylic acid) (PAA) blocks were derived by a core-first approach. To this end, the OH end groups of three-arm PEO stars prepared anionically were derivatized into either three or six bromo-ester functions that served to grow the poly(tert-butyl acrylate) (PtBA) blocks by atom transfer radical polymerization (ATRP) in a controlled fashion. This could be achieved at 80 °C in toluene in the presence of CuBr/pentamethyldiethylenetriamine (CuBr/PMDETA) as the catalyst system. Characterization by size exclusion chromatography and NMR of star-block copolymers (PEO3-b-PtBA3) and dendrimer-like copolymers (PEO3-b-PtBA6) confirmed their well-defined character. Subsequent treatment with trifluoroacetic acid selectively hydrolyzed the PtBA blocks, leading to the targeted PEO3-b-PAA3 and PEO3-b-PAA6 compounds. Alternatively, an arm-first metho...

Stars and dendrimer-like architectures by the divergent method using controlled radical polymerization

Using novel multihalide compounds based on 4-tert-butylcalix[4,6,8]arenes as initiators, molecularly well-defined polystyrenes (PS) and poly[alkyl-(meth)acrylates] could be obtained by atom transfer radical polymerization (ATRP). This core-first approach and therefore the very same initiators also served to derive star block copolymers. Dendrimer-like architectures based on poly(ethylene oxide) (PEO) and PS were prepared upon combination of anionic polymerization for the inner PEO part and ATRP for the outer PS shell.

Multi-arm star block copolymers based on ɛ-caprolactone with hyperbranched polyglycerol core

Multi-arm star block copolymers based on ε-caprolactone have been prepared by a core-first approach using propoxylated hyperbranched polyglycerol P(G52PO3) as a polyfunctional initiator. Various monomer/initiator ratios were employed in the Sn-catalyzed polymerization in order to vary the length of the caprolactone arms (DPn(arm) = 10 to 50). The molecular weights of the block copolymers were in good aggreement with the calculated values. Careful NMR analysis revealed that the monomer/initiator ratio not only controls the arm length, but also influences the degree of functionalization. Poly(ε-CL) stars with 52 arms and absolute molecular weights between 69 200 and 360 400 g/mol have been prepared. SEC measurements showed that the narrow polydispersity of the core molecule (Mw/Mn = 1.24) could be maintained upon block copolymerization. The resulting star polymers exhibited polydispersities between 1.21 and 1.33.

Synthesis of PMMA star polymers with siloxane cores via group-transfer polymerization using silyl ketene acetal functionalized cyclic siloxanes

Four-arm, poly(methyl methacrylate) star polymers having cyclic siloxane cores were prepared by group-transfer polymerization (GTP). In this core-first approach, a cyclic hydromethylsiloxane tetramer was hydrosilated with a terminal olefin bearing a silyl ketene acetal, and methyl methacrylate polymerization was initiated in THF using acetate ion as a catalyst. Star polymers having between 20 and 150 methyl methacrylate repeat units per arm were prepared, and had polydispersities of ca. 1.2-1.3. The polymerizations were quantitative