Simple Photochemical Route to Block Copolymers via Two‐Step Sequential Type II Photoinitiation

A simple method for the preparation of block copolymers by a two‐step sequential Type II photoinitiation is described. In the first step, amine functionalized poly(methyl methacrylate) (PMMA‐N(Et)₂) is prepared by photopolymerization of methyl methacrylate at λ = 350 nm using benzophenone and triethyl amine as photosensitizer and hydrogen donor, respectively. Subsequent benzophenone‐sensitized photopolymerization of tert‐butyl acrylate using PMMA‐N(Et)₂ as hydrogen donor yielded poly(methyl methacrylate)‐block‐poly(tert‐butyl acrylate). The obtained hydrophobic block copolymer is readily converted to amphiphilic polymer by hydrolysis of the tert‐butyl ester moieties of the block copolymer as demonstrated by contact angle measurements. All polymers are characterized by NMR, Fourier transform infrared and UV–vis spectroscopies, and differential scanning calorimetry (DSC) thermal analyses.     

Morphology and rheology of poly(methyl methacrylate)‐block‐poly(isooctyl acrylate)‐block‐poly(methyl methacrylate) triblock copolymers,and potential as thermoplastic elastomers

The phase morphology and rheological properties of a series of poly(methyl methacrylate)‐block‐poly(isooctyl acrylate)‐block‐poly(methyl methacrylate) triblock copolymers (MIM) have been studied. These copolymers have well‐defined molecular structures, with a molecular weight (MW) of poly(methyl methacrylate) (PMMA) in the range of 3 500–50 000 and MW of poly(isooctyl acrylate) (PIOA) ranging from 100 000 to 140 000. Atomic force microscopy with phase detection imaging has shown a two‐phase morphology for all the MIM copolymers. The typical spherical, cylindrical, and lamellar phase morphologies have been observed depending on the copolymer composition. MIM consisting of very short PMMA end blocks (MW 3 500–5 000) behave as thermoplastic elastomers (TPEs), with however an upper‐service temperature higher than the traditional polystyrene‐block‐polyisoprene‐block‐polystyrene TPEs (Kraton D1107). A higher processing temperature is also noted, consistent with the higher viscosity of PMMA compared to PS.     

Nanostructure and Shape Control in Polymer‐Ceramic Hybrids from Poly(ethylene oxide)‐block‐Poly(hexyl methacrylate) and Aluminosilicates Derived from Them

Summary: The present study describes the use of poly(ethylene oxide)‐block‐poly(hexyl methacrylate) diblock copolymers (PEO‐b‐PHMA) as structure‐directing agents for the synthesis of nanostructured polymer‐inorganic hybrid materials from (3‐glycidylpropyl)trimethoxysilane and aluminium sec‐butoxide as precursors and organic, volatile solvents. Four different morphologies, i.e., inorganic spheres, cylinders, lamellae, and organic cylinders in an inorganic matrix, are obtained confirmed by a combination of small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM). The composites are further characterized by differential scanning calorimetry (DSC) and solid‐state ¹³C, ²⁹Si, and ²⁷Al NMR. It is demonstrated that the change in the hydrophobic block from polyisoprene (PI) to poly(hexyl methacrylate) (PHMA) has no significant effect on the local structure of the inorganic rich phase. By the dissolution of the composites rich in poly(hexyl methacrylate), nano‐particles of different shapes, i.e., spheres, cylinders, and lamellae, are obtained as demonstrated by atomic force microscopy (AFM) and TEM. Finally, calcination of composites with the inverse hexagonal structure at elevated temperatures up to 600 °C results in nanostructured aluminosilicates that retain their structure as evidenced through a combination of SAXS and TEM. The study opens pathways towards tailoring filler‐matrix interactions in model nanocomposites and builds the bases for the preparation of composites from multiblock copolymers with polyisoprene (PI), poly(ethylene oxide) (PEO), and poly(hexyl methacrylate) (PHMA) as building blocks.

Bright field TEM micrograph of composite T55/1 with inverse hexagonal morphology after calcination.     

A Novel Strategy to Synthesize Double Comb‐Shaped Water Soluble Copolymer by RAFT Polymerization

Summary: A double comb‐shaped water soluble copolymer, poly[poly(ethylene oxide) methyl ether methacrylate]‐block‐poly(N‐isopropylacrylamide)‐block‐poly[poly (ethylene oxide) methyl ether methacrylate], abbreviated as [P(MA‐MPEO)‐block‐PNIPAM‐block‐P(MA‐MPEO)], with a controlled molecular weight and narrow polydispersity was successively synthesized using a macromonomer technique. The ⁶⁰Co γ irradiation polymerization of MA‐MPEO in the presence of dibenzyl trithiocarbonate (DBTTC) at room temperature afforded a polymer, P(MA‐MPEO)‐SC(S)S‐P(MA‐MPEO), which was subsequently used as a macro RAFT agent in the RAFT polymerization of N‐isopropylacrylamide, and water soluble double comb‐shaped copolymers, P(MA‐MPEO)‐block‐PNIPAM‐block‐P(MA‐MPEO), were successfully obtained.

Structure of the double comb‐shaped copolymer.     

Stabilizing the Microphase Separation of Block Copolymers by Controlled Photo‐crosslinking

Photo‐crosslinkable material is produced via self‐assembly of poly(methyl methacrylate)‐block‐poly(n‐butyl acrylate) (PMMA‐b‐PBA) block copolymers, in which the MMA block is decorated with coumarin moieties. The block copolymer is synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization. Atomic force microscopy (AFM) reveals microphase separation into nanodomains of controlled and regular morphology. The local elastic modulus of different nanodomains is determined using the PeakForce Quantitative Nano‐Mechanics (PFQNM) mode of AFM. The global modulus, as determined by mechanical spectroscopy, agrees well with the local values. The crosslinking of the block copolymer, via photodimerization of the incorporated coumarin moieties using UV light, is traced by UV–vis absorption measurements and resulted in an increase of the global elastic modulus. The crosslinking reaction proceeding in the solid state is surprisingly effective and stabilizes the morphology to such an extent that equilibrium phase separation, as targeted by annealing, is effectively suppressed.

     

Self‐Assembled Structures in Blends of Block Copolymer A‐block‐B with Homopolymer A: SAXS and SANS Study

Summary: The study describes the investigation of the microphase‐separated morphology of a block copolymer A‐block‐B with the parent homopolymer A, where A is polystyrene and A‐block‐B is poly(perdeuterated styrene)‐block‐poly(methyl methacrylate) (dPS‐block‐PMMA). The microdomain morphology and phase behavior in blends of dPS‐block‐PMMA with polystyrene of = 8 000 and 35 000 were investigated. Binary blends of the diblock copolymer and homopolymers were prepared with various amounts of homopolymers. The criterion for “wet and dry brush” taking into account different solubilization of homopolymer has been applied to explain the changes in microdomain morphology during the self‐assembling process.

     

Synthesis and Characterization of Poly(trimethylene oxide)‐block‐Polystyrene and Poly(trimethylene oxide)‐block‐Polystyrene‐block‐Poly(methyl methacrylate) by Combination of Atom Transfer Radical Polymerization (ATRP) and Cationic Ring‐Opening Polymerization (CROP)

Summary: Diblock copolymers, poly(trimethylene oxide)‐block‐poly(styrene)s abbreviated as poly(TMO)‐block‐poly(St), and triblock copolymers, poly(TMO)‐block‐poly(St)‐block‐poly(MMA)s (MMA = methyl methacrylate), with controlled molecular weight and narrow polydispersity have been successively synthesized by a combination of atom transfer radical polymerization (ATRP) and cationic ring‐opening polymerization using the bifunctional initiator, 2‐hydroxylethyl α‐bromoisobutyrate, without intermediate function transformation. The gel permeation chromatography (GPC) and NMR analyses confirmed the structures of di‐ and triblock copolymers obtained.

GPC curves of (a) poly(St); (b) diblock copolymer, poly(St)‐block‐poly(MMA) before precipitation; (c) poly(St)‐block‐poly(MMA) after precipitation in cyclohexane/ethanol (2:1); (d) triblock copolymer, poly(TMO)‐block‐poly(St)‐block‐poly(MMA).     

Anionic Grafting to Strategies for Functional Polymethacrylates: Convenient Preparation of Stimuli‐Responsive Block Copolymer Architectures

Functional block copolymers (BCP) are promising candidates for many important applications in fields of separations technologies, drug delivery, or (nano)lithography. Here, a universal strategy is described for the preparation of functional poly(methacrylate)s grafted (PMA) to polystyrene‐block‐polyisoprene (PS‐b‐PI) BCPs via a convenient postmodification strategy. PS‐b‐PIs are functionalized by means of hydrosilylation protocols for the introduction of chlorosilane moieties. Subsequent anionic grafting‐to polymerization of different functional PMA macro anions leads to grafted BCP. Various functional and non‐functional homopolymers, that is, poly(di(ethylene glycol) methyl ether methacrylate), poly(methacrylic acid), and poly(3‐methacryloxypropyl)heptaisobutyl‐T8‐silsesquioxane as well as poly(methyl methacrylate), poly(n‐butyl methacrylate), poly(iso‐propyl methacrylate), poly(tert‐butyl methacrylate), and poly(2‐hydroxy ethyl methacrylate) are block‐selectively incorporated into the PI segment. Applied grafting strategies for the non‐functional PMA derivatives, which feature different sizes of the alkyl substituent, reveal a strong influence on the grafting‐to efficiency. The grafted BCP architectures are capable of undergoing microphase separation in the bulk state, while the resulting morphology is significantly influenced by the introduction of PMA segments as shown by transmission electron microscopy measurements. Additionally, the structure formation and pH‐switching capability of the poly(methacrylic acid)‐grafted BCP is studied by dynamic light scattering, proving the feasibility for the herein investigated synthesis strategy.     

Block copolymers of narrow molecular-weight distribution with a side-chain liquid-crystalline polymeric block as one component

Syntheses of block copolymers have been undertaken by photo-polymerization of 6-{4-[4-(butoxyphenoxy)carbonyl]phenoxy}hexyl methacrylate ( 1 ) using the living propagating chain end of poly(methyl methacrylate) as an initiating species. The latter was derived from photo-polymerization of methyl methacrylate (MMA) in the presence of methyl(5,10,15,20-tetraphenylporphinato)aluminium. The polymerization of 1 proceeded from the propagating end of poly(MMA) with high blocking efficiency to produce poly(MMA)-block-poly( 1 ) with narrow molecular-weight distribution. The length of the block chain could be controlled by varying the amounts of monomers. An endothermic peak of the block copolymer with higher mass of liquid-crystalline chain was observed at almost the same temperature region as that of poly( 1 ) on differential scanning calorimetric analyses, which suggests that the former has a structure with microphase separation.     

Selective Grafting of Block Copolymers, 3a. Multigraft Copolymers

Double‐grafted copolymers, poly(hydrogenated isoprene)‐block‐{polystyrene‐graft‐[poly(4‐methylstyrene)‐graft‐polystyrene]} and poly(hydrogenated isoprene)‐block‐{polystyrene‐graft‐[poly(4‐methylstyrene)‐graft‐poly(tert‐butyl methacrylate)]}, were prepared using a procedure consisting of two steps. The starting block copolymer, poly(hydrogenated isoprene)‐block‐polystyrene, was metallated using a sec‐butyllithium/N,N,N ´,N ´‐tetramethylethylenediamine complex. The multi‐metallated intermediate produced served as a multifunctional initiator for the anionic polymerization of 4‐methylstyrene, giving rise to the corresponding graft copolymer. The similar grafting‐from procedure was again applied to the synthesized graft copolymer. Using phenylpotassium superbase as the metallating agent and styrene or tert‐butyl methacrylate as the monomers to be grafted, enabled the corresponding double‐grafted copolymers to be prepared.     

“Compatibilizing effect” in interpenetrating polymer networks

α,ω-Acrylate terminated poly(1,3-dioxolane) (polyDXL) was copolymerized with methyl acrylate and methyl methacrylate for the synthesis of block copolymer networks. It was shown by dynamic mechanical thermal analysis (DMTA) that the presence of covalent bonds between poly DXL and the polyacrylate leads to a high degree of compatibility. In contrast, the combination of the same two components in network form, leading to sequential interpenetrating polymer networks (IPN's), resulted in phase-separated materials. When the above-mentioned block copolymer structures, in which one of the blocks is chemically identical to the polyacrylate used in the second network, were applied in the first network of the IPN, an increase of miscibility between the two types of polymers was observed. DMTA analysis revealed that the morphology of these IPN's could be varied from phase-separated to homogeneous materials by increasing the polyacrylate fraction in the first block copolymer network. The possibility to degrade the poly DXL segments in an acid environment allows the complete degradation and extraction of one network in the IPN's and hence provided an interesting approach to the study of domain characteristics by optical microscopy.     

Triblock copolymer based thermoplastic elastomeric gels of a large service temperature range: preparation and characterization

Poly(methyl methacrylate)-block-polybutadiene-block-poly(methyl methacrylate) (MBM) triblock copolymers and their hydrogenated counterparts with poly(ethylene-co-1,2-butylene) midblock (MEBM) were swollen by an aliphatic oil of high boiling point which is a selective solvent for the central block. Thermoreversible gels are accordingly formed by both MBM and MEBM copolymers above a critical polymer content (Cr), which depends on the nature of the midblock and not on the copolymer molecular weight, at least in the investigated range. Cr has been found to be 5 wt.-% for an MBM block copolymer and 2 wt.-% for MEBM copolymers of various molecular weights. Gels of MEBM triblock copolymers exhibit interesting mechanical properties, such as high elongation at break (up to 870%) and high tensile strength (32 kPa). The most interesting feature of the MEBM gels is an upper service temperature as high as 170°C, thus more than 100°C higher than the value (47°C) reported for gels of an SEBS copolymer (S = polystyrene) of comparable molecular weight (100000) and composition (ca. 30 wt.-% hard block). The morphology of MEBM gels was studied by scanning electron microscopy (SEM) and found to be cocontinuous in case of a gel containing 20 wt.-% copolymer.     

Clay‐PMMA Nanocomposites by Photoinitiated Radical Polymerization Using Intercalated Phenacyl Pyridinium Salt Initiators

Summary: In situ synthesis of poly(methyl methacrylate) (PMMA) nanocomposites by photopolymerization using organophilic montmorillonite (MMT) as the layered clay is reported. MMT clay was ion exchanged with N‐phenacyl, N,N‐dimethylanilinium hexafluoro phosphate (PDA) which acts as both suitable intercalant‐ and photo‐initiator. These modified clays were then dispersed in methyl methacrylate (MMA) monomer in different loading degrees to carry out the in situ photopolymerization. Intercalation ability of the photoinitiator and exfoliated nanocomposite structure were evidenced by both X‐ray diffraction (XRD) spectroscopy and transmission electron microscopy (TEM). Thermal properties and morphologies of the resultant nanocomposites were also studied.

Schematic representation of clay‐PMMA nanocomposites by photoinitiated radical polymerization.     

A New Approach for the Synthesis of pH‐Responsive Cross‐Linked Micelles from a Poly(glycidyl methacrylate)‐Based Functional Copolymer

An amphiphilic poly(ethylene glycol)methyl ether‐block‐poly(glycidyl methacrylate) (MPEG‐b‐PGMA) diblock copolymer is first synthesized via atom transfer radical polymerization. Epoxy groups of this precursor block copolymer are converted to hydroxyl and tertiary amine residues by reacting with diethyl amine over a ring‐opening reaction. The resulting diblock copolymer is poly(ethylene glycol)methyl ether‐block‐poly(3‐diethylamino‐2‐hydroxypropyl methacrylate) (MPEG‐b‐PDEAHPMA). Micellar solution of this diblock copolymer is prepared at pH 12.0 in aqueous media. At high pH (12.0), core – shell micelles are formed with PDEAHPMA‐core and MPEG‐shell. PDEAHPMA blocks have both hydroxyl and tertiary amine groups that provide reactivity against divinyl sulfone (DVS) and a response to solution pH, respectively. Cross‐linking of hydroxyl groups of PDEAHPMA chains in the micelle core is successfully achieved by adding DVS. At pH 2.0, the DEAHPMA‐core of core cross‐linked (CCL) micelles becomes protonated and hence swelling is observed. The effect of varying the DVS concentrations is also studied. The micelle formation of diblock copolymer and its response to solution pH are investigated by using surface tensiometer, ¹H NMR spectroscopy, and dynamic light scattering (DLS) techniques. CCL micellar structure and its response to solution conditions are investigated with transmission electron microscopy and DLS studies, respectively.

     

Phase separation in poly(methyl methacrylate)-block-poly(2,3-dibromopropyl methacrylate) copolymers

AB block copolymers poly(methyl methacrylate)-block-poly(2,3-dibromopropyl methacrylate) were prepared by a two-stage process involving the sequential anionic polymerization of methyl methacrylate and allyl methacrylate followed by bromination of the pendant allylic groups. Transmission electron microscopy of thin sections of cast films revealed phase separation, the morphology depending upon composition of the block. In particulate form these copolymers gelled with methyl methacrylate to form doughs that could be moulded and heat cured to produce hard X-ray opaque copolymers.     

Block copolymer micelles as seed in emulsion polymerization

The size of aggregates formed by poly(acrylic acid)-block-poly(methyl methacrylate) block copolymers was determined and the applicability of these block copolymers as stabilizers in emulsion polymerization was investigated. The analytical methods included transmission electron microscopy, light scattering, and analytical ultracentrifugation. Polymers with a hydrophilic poly(acrylic acid) block of equal or larger size than the hydrophobic poly(methyl methacrylate) block are efficient as stabilizers down to block copolymer-to-monomer ratios of less than 1 wt.-%. From the influence of the block copolymer-to-monomer ratio on the latex particle size, from the relation between the number of block copolymer molecules per latex particle and the aggregation number of the block copolymer micelles, and from fluorescence studies we conclude that micelles consisting of block copolymers with 35 or more hydrophobic MMA units act as a seed in the emulsion polymerization of acrylic and methacrylic monomers.     

Double Hydrophilic Poly(ethylene oxide)‐block‐Poly(dehydroalanine) Block Copolymers: Comparison of Two Different Synthetic Routes

Two different synthetic pathways give access to the amphiphilic block copolymer poly(ethylene oxide)‐block‐poly(tert‐butoxycarbonylaminomethylacrylate). In the first approach, two end‐functionalized segments are linked via click chemistry; and in the second approach, a poly(ethylene oxide) (PEO) based macroinitiator is chain extended via atom transfer radical polymerization (ATRP). In both cases the linking unit consists of an amide group, which is necessary to effectively deprotect the corresponding polymer precursor without cleavage of both segments. For this, amide‐containing ATRP initiators are employed and successful synthesis by nuclear magnetic resonance (NMR) and size exclusion chromatography (SEC) analyses before comparing both pathways is demonstrated. After deprotection, a novel double hydrophilic block copolymer, poly(ethylene oxide)‐block‐poly(dehydroalanine), is obtained, which is investigated using SEC (aqueous and DMSO) and ¹H‐NMR spectroscopy. Containing a potentially zwitterionic PDha segment and a high density of both amino and carboxylic groups, pH‐dependent aggregation of the block copolymer is expected and is studied using dynamic light scattering, revealing interesting solution properties. The corresponding polymers are applied in various areas including drug delivery systems or in biomineralization.     

Bioinspired All‐Polyester Diblock Copolymers Made from Poly(pentadecalactone) and Poly(2‐(2‐hydroxyethoxy)benzoate): Synthesis and Polymer Film Properties

The bioinspired diblock copolymers poly(pentadecalactone)‐block‐poly(2‐(2‐hydroxyethoxy)benzoate) (PPDL‐block‐P2HEB) are synthesized from pentadecalactone and dihydro‐5H‐1,4‐benzodioxepin‐5‐one (2,3‐DHB). No transesterification between the blocks is observed. In a sequential approach, PPDL obtained by ring‐opening polymerization (ROP) is used to initiate 2,3‐DHB. Here, the molar mass M_n of the P2HEB block is limited. In a modular approach, end‐functionalized PPDL and P2HEB are obtained separately by ROP with functional initiators, and connected by 1,3‐dipolar Huisgen reaction (“click‐chemistry”). Block copolymer compositions from 85:15 mass percent to 28:72 mass percent (PPDL:P2HEB) are synthesized, with M_n of from about 30 000–50 000 g mol^?1. The structure of the block copolymer is confirmed by proton NMR, Fourier‐transform infrared spectroscopy, and gel permeation chromatography. Morphological studies by atomic force microscopy (AFM) further confirms the block copolymer structure, while quantitative nanomechanical AFM measurements reveal that the Derjaguin–Muller–Toporov moduli of the block copolymers range between 17.2 ± 1.8 and 62.3 ± 5.7 MPa, i.e., between the values of the parent P2HEB and PPDL homopolymers (7.6 ± 1.4 and 801 ± 42 MPa, respectively). Differential scanning calorimetry shows that the thermal properties of the homopolymers are retained by each of the copolymer blocks (melting temperature 90 °C, glass transition temperature 36 °C).     

Direct Observation of Poly(propylene)‐block‐Poly(ethylene‐co‐propylene) Molecules by Atomic Force Microscopy

Summary: Atomic force microscopy (AFM) has been applied to get molecular images of diblock poly(propylene)‐block‐poly(ethylene‐co‐propylene) copolymers deposited on mica from diluted solutions at elevated temperatures. Both isolated molecules and their small aggregates have been visualized as compact particles of various sizes with outspreading poly(ethylene‐co‐propylene) chains. On the base of the height, volume and morphological analysis AFM images were divided into three groups. In the first group the compact particles are suggested to be small regular‐shaped crystallites formed by a few poly(propylene) blocks. Some isolated particles of this group were connected with single copolymer chains. In the second group the compact particles have larger dimensions and irregular or round shapes implying unordered packing of constituents. The third group were represented by isolated poly(ethylene‐co‐propylene) coils. The two‐dimensional expansion of coils on mica both isolated and included in aggregates exceeds several times their dimensions in a solution. The probable mechanism of such an expansion is proposed relying on the existence of van‐der‐Waals surface force field of a sufficient strength in the vicinity of the crystal surface.

Enlarged AFM height images of block‐copolymer aggregates of group A with small compact particles of regular shapes (frames a, b, c) and group B (frame d) with a large globular compact particle.     

Planet‐Like Nanostructures Formed by an ABC Triblock Terpolymer

A novel planet‐like nanostructure formed by an asymmetric polystyrene‐block‐polyisoprene‐block‐poly(methyl methacrylate) (SIM) triblock terpolymer is presented, where polystyrene (PS) forms spherical microdomains, covered by polyisoprene (PI) rings/helices/polar caps and poly(methyl methacrylate) (PMMA) forms the matrix. The new nanostructure is the result of a thermally induced morphological transition of a kinetically trapped helical nanostructure by thermal annealing above its glass transition temperature. This new nanostructure is observed when the triblock terpolymer solution is cast from chloroform (CHCl₃). When cast from tetrahydrofuran (THF) or toluene, mainly cylindrical and to a lesser extent spherical microdomains are observed. This is caused by a selectivity of the CHCl₃ for the PS and PMMA microdomains. The SIM triblock terpolymer is synthesized by anionic polymerization, and the morphology is characterized by transmission electron microscopy, atomic force microscopy, and small angle X‐ray scattering.