Liquid crystalline block copolymers by sequential cationic or promoted cationic and free-radical polymerizations

The scope for the study of the synthesis and properties of liquid crystalline (LC) block copolymers is briefly outlined. While there are many approaches to the synthesis of LC block copolymers, the use of azo macroinitiators is very versatile and allows one to produce diverse block copolymer architectures. Azo macroinitiators are prepared by cationic or promoted cationic polymerization of tetrahydrofuran (1) or cyclohexene oxide (2), and are then used to initiate the free-radical polymerization of various methacrylates 3,4 or acrylates 5–9 containing mesogenic azobenzene or biphenyl units thereby yielding block copolymers. The AB or ABA block copolymers are microphase-separated and form smectic and/or nematic mesophases similar to the respective LC homopolymers.     

Photo-switchable azobenzene-functionalized block copolymers from atom transfer radical polymerizations

Diblock copolymers composed of monomers of tert-butyl acrylate and a side-chain azobenzenecontaining monomer, 4-[(E)-(4-nitrophenyl)diazenyl]phenyl prop-2-enoate were synthesized using atom transfer radical polymerization technique. Experimental strategy involved synthesis of block of tert-butyl acrylate macroinitiator followed by addition of second block of azobenzene-containing monomer to prepare desired block-copolymer. GPC analysis indicated narrow molecular weight distributions with degree of polymerization found in good agreement with targeted value. Prepared block copolymers of varying chain lengths can potentially be used to obtain morphologies that can find useful applications for biomedical applications including intriguing photo-switchable drug delivery systems.     

Novel segmented copolymers by combination of controlled ionic and radical polymerizations

Transformation of different living and non‐living polymerization mechanisms to controlled/“living” atom transfer radical polymerization (ATRP) in order to prepare block and graft copolymers is described. The synthesis and characterization of macroinitiators and the resulting segmented copolymers is discussed.     

Block copolymers of styrene and p-acetoxystyrene with polyisobutylene by combination of living carbocationic and atom transfer radical polymerizations

Successful combination of quasiliving carbocationic (QLCP) and atom transfer radical polymerizations (ATRP) was carried out by initiating ATRP with polyisobutylene (PIB) macroinitiators obtained by QLCP. It has been found that 1-chloro-1-phenylethyl-telechelic PIBs with M̄_n of 7 800 and 30 700 are efficient macroinitiators for ATRP of styrene in bulk and in xylene solution and for p-acetoxystyrene (pAcOSt) in xylene. Size exclusion chromatography (SEC) traces clearly indicated quantitative initiation and the formation of the desired polystyrene-block-polyisobutylene-block-polystyrene (PSt-PIB-PSt) and PpAcOSt-PIB-PpAcOSt triblock copolymers. Experiments also revealed absence of thermal self-initiation of styrene under ATRP conditions.     

Anionic amphiphilic end‐linked conetworks by the combination of quasiliving carbocationic and group transfer polymerizations

A series of amphiphilic end‐linked conetworks was synthesized by the combination of two “quasiliving” polymerization techniques, quasiliving carbocationic (QLCCP) and group transfer polymerizations (GTP). The hydrophobic monomer was polyisobutylene methacrylate synthesized by the QLCCP of isobutylene and subsequent terminal modification reactions. The hydrophilic monomer was methacrylic acid (MAA) introduced via the polymerization of 2‐tetrahydropyranyl methacrylate followed by acid hydrolysis after (co)network formation. The conetwork syntheses were performed by sequential monomer/crosslinker additions under GTP conditions. All the precursors and the extractables from the conetworks were characterized by gel permeation chromatography and ¹H NMR. The resulting polymer conetworks were investigated in terms of their degree of swelling (DS) in aqueous media and in tetrahydrofuran (THF) over the whole range of ionization of the MAA units and in n‐hexane for uncharged conetworks. The DSs in water increased with the degree of ionization (DI) of the MAA units and the hydrophilic content in the conetwork, whereas the DSs in THF increased with the reduction of the DI of the MAA units. The effective pK of the MAA units in the conetworks increased from 8.4 to 10.5 with decreasing MAA content. These findings can facilitate the design of similar unique conetworks with adjustable swelling behavior and composition‐dependent pK values. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4289–4301, 2009     

Preparation of polyethylene block copolymers by a combination of postmetallocene catalysis of ethylene polymerization and atom transfer radical polymerization

The synthesis of block copolymers consisting of a polyethylene segment and either a poly(meth)acrylate or polystyrene segment was accomplished through the combination of postmetallocene-mediated ethylene polymerization and subsequent atom transfer radical polymerization. A vinyl-terminated polyethylene (number-average molecular weight = 1800, weight-average molecular weight/number-average molecular weight =1.70) was synthesized by the polymerization of ethylene with a phenoxyimine zirconium complex as a catalyst activated with methylalumoxane (MAO). This polyethylene was efficiently converted into an atom transfer radical polymerization macroinitiator by the addition of α-bromoisobutyric acid to the vinyl chain end, and the polyethylene macroinitiator was used for the atom transfer radical polymerization of n-butyl acrylate, methyl methacrylate, or styrene; this resulted in defined polyethylene-b-poly(n-butyl acrylate), polyethylene-b-poly(methyl methacrylate), and polyethylene-b-polystyrene block copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 496–504, 2004     

Amphiphilic block copolymers by a combination of anionic polymerization and selective post-polymerization functionalization

Anionic polymerization is the oldest known living/controlled polymerization methodology that leads to well defined macromolecules. It has been also used, with considerable success, for the synthesis of amphiphilic block copolymers (AmBC), a class of functional copolymers having interesting self-assembling properties and high potential for applications in various technological fields. The use of mild and effective post-polymerization functionalization/chemical modification reactions on block copolymers has substantially increased the synthetic capabilities of anionic polymerization methodologies, toward the creation of a variety of AmBC. In this feature article we review work done on these directions in the last ten years. Some perspectives and future work on this particular field of polymer science are also discussed.     

Formation of polymer vesicles by liquid crystal amphiphilic block copolymers

We report the formation of polymer vesicles (or polymersomes) by a new class of amphiphilic block copolymers in which the hydrophobic block is a side-on nematic liquid crystal polymer. Two series of these block copolymers, named PEG-b-PA444 and PEG-b-PMAazo444, with different hydrophilic/hydrophobic ratios were synthesized and characterized in detail. Polymersomes and nanotubes were formed by adding water into a solution of copolymers in dioxane. Polymersomes in water were finally obtained by dialyzing the resulting mixture against water. These self-assemblies have been studied by classical TEM and cryo-TEM. For the PEG-b-PA444 series, polymersomes were observed for hydrophilic/hydrophobic ratios ranging from 40/60 to 19/81. For PEG-b-PMAazo444 series, polymersomes were observed for hydrophilic/hydrophobic ratios ranging from 26/74 to 18/82. For a PEG-b-PA444 sample with hydrophilic/hydrophobic ratio equal to 25/75, a tubular morphology with tube diameter of typically 100 nm and tube length of up to 10 mum was also observed together with polymersomes during addition of water into the polymer solution in dioxane.     

Synthesis of PMMA and PMMA block copolymers at elevated temperatures by phosphor ylide‐mediated polymerizations

The MMA polymerization initiated by (1‐naphthyl) triphenylphosphonium triphenylmethyl anion(TPM,NTPP) or the NTPP salt of the methylisobutyrate anion (MIB,NTPP) in THF at temperatures varying from 25 to 70°C appears to be a living polymerization as indicatred by a linear M_n vs. MMA conversion plot and by the narrow MW distributions. As indicated by the proton spectrum of MIB, NTPP in THF d₈ the predominant polymerization intermediate appears to be the ylide formed exclusively by addition of the MIB anion to the 4‐carbon of the 1‐naphthyl group. This ylide shows upfield shifts for the 3‐proton of the cyclohexadienyl ring and of the 6‐ and 8‐protons of the remaining aromatic ring. The rates of the NTPP mediated polymerizations are reduced by factors of around 10⁴‐10⁵ compared to that mediated by the corresponding ylides formed by addition of MIB to the phenyl ring of tetraphenylphosphonium ion. The reductions in rate in the presence of NTPP make it possible to carry out MMA polymerizations under conditions not normally accessible.     

Synthesis of semifluorinated block copolymers by atom transfer radical polymerization

Two sets of styrene‐based semifluorinated block copolymers, one with a perfluoroether pendant group and another with a perfluoroalkyl group, were synthesized by atom transfer radical polymerization. Microphase separation of the block copolymers was established by small‐angle X‐ray scattering and differential scanning calorimetry (DSC). DSC measurements also showed that the perfluoroether‐based polymer had a low glass‐transition temperature (?44 °C). Contact‐angle measurements indicated that the semifluorinated block copolymers had low surface energies (ca. 13 mJ/m²). These materials hold promise as low‐surface‐energy additives or surfactants for supercritical CO₂ applications. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 853–861, 2004     

Synthesis and characterization of poly(phenylene oxide) graft copolymers by atom transfer radical polymerizations

A series of comb-like poly(phenylene oxide)s (PPO) graft copolymers with controlled grafting density and length of grafts were synthesized by atom transfer radical polymerization (ATRP). The α-bromo-poly(2,6-dimethyl-1,4-phenylene oxide)s (BPPO) were used as macroinitiators to polymerize vinyl monomers and the graft copolymers carrying polystyrene (PS), poly(p-acetoxystyrene) (PAS), and poly(methyl methacrylate) (PMMA) as side chains were synthesized and characterized by NMR, FTIR, GPC, DSC and TGA. The composition-dependent glass-transition temperatures (T_g) of PPO-g-PS exhibited good correlation with theoretical curve in Couchman equations except for the cases of low PS content (<40 mol%) copolymers in which a positive deviation was observed due to enhanced molecular interactions. The increase in monomer/initiator ratio led to the increase of degree of polymerization and the decrease of polydispersity. Despite the immiscibility nature between PPO and PMMA, the PPO-g-PMMA exhibited enhanced compatibilization as apparent single T_g in a wide temperature window throughout various compositions revealing an efficient segmental mixing on a molecular scale due to grafting structure.     

Kinetic studies of free-radical polymerizations of I-vinylimidazole initiated by benzoyl peroxide and azoisobutyronitrile

Kinetic features of radical polymerizations of 1-vinylimidazole initiated by azoisobutyronitrile and by benzoyl peroxide have been investigated in several solvents and at several temperatures. Polymerizations initiated both by the azo compound and by the peroxide have abnormally low monomer exponents and abnormally high initiator exponents, features first noted by Bamford and Schofield and attributed by them to degradative addition of monomer. Polymerizations initiated by benzoyl peroxide appear to be further complicated by induced decomposition of the initiator; conversion vs time curves, especially for polymerizations in dimethylformamide and particularly at 80°C, show pronounced curvature. These curves can be approximately interpreted in terms of the Tobolsky theory of “dead-end” radical polymerization.     

Synthesis and characterization of sulfonated block copolymers by atom transfer radical polymerization

Well‐defined sulfonated polystyrene and block copolymers with n‐butyl acrylate (nBA) were synthesized by CuBr catalyzed living radical polymerization. Neopentyl p‐styrene sulfonate (NSS) was polymerized with ethyl‐2‐bromopropionate initiator and CuBr catalyst with N,N,N′,N′‐pentamethylethyleneamine to give poly(NSS) (PNSS) with a narrow molecular weight distribution (MWD < 1.12). PNSS was then acidified by thermolysis resulting in a polystyrene backbone with 100% sulfonic acid groups. Random copolymers of NSS and styrene with various composition ratios were also synthesized by copolymerization of NSS and styrene with different feed ratios (MWD < 1.11). Well defined block copolymers with nBA were synthesized by sequential polymerization of NSS from a poly(n‐butyl acrylate) (PnBA) precursor using CuBr catalyzed living radical polymerization (MWD < 1.29). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5991–5998, 2008     

Synthesis of liquid crystalline–amorphous block copolymers by the combination of atom transfer and photoinduced radical polymerization mechanisms

The synthesis of ABA‐type block copolymers, involving liquid‐crystalline 6‐(4‐cyanobiphenyl‐4′‐oxy)hexyl acrylate (LC6) and styrene (St) monomer with copper‐based atom transfer radical polymerization (ATRP) and photoinduced radical polymerization (PIRP), was studied. First, photoactive α‐methylol benzoin methyl ether was esterified with 2‐bromopropionyl bromide, and it was subsequently used for ATRP of LC6 in diphenylether in conjunction with CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as a catalyst. The obtained photoactive functional liquid‐crystalline polymer, poly[6‐(4‐cyanobiphenyl‐4′‐oxy)hexyl acrylate] (PLC6), was used as an initiator in PIRP of St. Similarly, photoactive polystyrenes were also synthesized and employed for the block copolymerization of LC6 in the second stage. The spectral, thermal, and optical measurements confirmed a full combination of ATRP and PIRP, which resulted in the formation of ABA‐type block copolymers with very narrow polydispersities. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1892–1903, 2003     

Characterization of polyoxyalkylene block copolymers by combination of different chromatographic techniques and MALDI-TOF-MS

Polyoxyalkylene diblock copolymers (consisting of PEO as hydrophilic block and PBO or PHO as hydrophobic block) are characterized by combination of two dimensional liquid chromatography and MALDI-TOF-MS. Liquid chromatography under critical conditions (LCCC) is used as first dimension and fractions are collected, mobile phase evaporated and diluted in the mobile phase used in the second dimension (SEC, LCCC or LAC). This two-dimensional chromatography in combination of MALDI-TOF-MS gives information about purity of reaction products, presence of the byproducts, chemical composition and molar mass distribution of all the products.     

Synthesis of polystyrene-poly(methacrylic acid) amphiphilic block copolymers by free-radical polymerization through chain-transfer and hydrosilylation reactions

Amphiphilic polystyrene-poly(methacrylic acid) block copolymers of various compositions have been synthesized by free-radical polymerization via chain-transfer and hydrosilylation reactions, as established by viscometry, IR spectroscopy, and fractionation measurements. The compositional homogeneity of the block copolymers worsens with an increase in the content of a low-molecular-mass monomer in the starting mixture and is independent of the nature of the terminal unit of a macromonomer.     

Formation of superlattices via blending of block copolymers

Block copolymers are well‐known for their large number of microphase morphologies on mesoscopic length scales. After a short review of the different morphologies observed in binary block copolymers and ternary triblock copolymers, the self‐assembling in blends of different block copolymers into common superlattices is discussed in detail. Besides similar morphologies known for pure triblock and diblock copolymers, the blends can also show new morphologies. Examples of such new morphologies are periodic non‐centrosymmetric lamellae and multiple gyroid interface structures. The discussion of the superlattices is primarily based on investigations by transmission electron microscopy (TEM), which are supplemented in a few cases by small angle X‐ray scattering (SAXS) or results from computer simulations.     

Surface active semifluorinated diblock copolymers prepared by group transfer polymerization

Diblock copolymers consisting of poly(methyl methacrylate) and poly(2-perfluorobutylethyl methacrylate) with narrow molecular weight distribution have been synthesized by group transfer polymerization. Different ratios of the PMMA block to the fluorinated block have been prepared. It was found that all polymers are surface active. Critical micelle concentrations are not dependent on the fluorinated block length. Critical surface tensions, extrapolated from Zisman plots and the dispersion force component of the surface energies extrapolated from Girifalco-Good-Fowkes-Young plots were decreasing with increasing length of the fluorinated block.