Controlled synthesis of acrylic homo- and copolymers in the presence of trithiocarbonates as reversible addition-fragmentation chain transfer agents

Formation of homo- and copolymers of various structures (random and block) based on tert-butyl acrylate and n-butyl acrylate via polymerization mediated by trithiocarbonates as reversible addition-fragmentation chain-transfer agents has been studied. The process is found to proceed according to a three-stage mechanism. As a result, it is possible to synthesize symmetric triblock copolymers with the use of polymer trithiocarbonates; the polymer reversible addition-fragmentation chain transfer agent predetermines the composition and molecular mass of end blocks, the composition of the monomer mixture determines the structure of the central block, and the concentration of the agent and the conversion of the monomers define its molecular-mass characteristics. The modification of polymerization products gives rise to amphiphilic copolymers.     

Emulsifier-free polymerization of <Emphasis Type="Italic">n</Emphasis>-butyl acrylate involving trithiocarbonates based on oligomer acrylic acid

The effect of the chain length of oligomer acrylic acid obtained in the presence of a low-molecularmass trithiocarbonate and the position of trithiocarbonate fragment (within the chain or at the chain end) on the process of emulsion polymerization of n-butyl acrylate and characteristics of the resulting dispersions has been studied for the first time. It has been found that, when using an oligomer with trithiocarbonate group located within the chain in the emulsion polymerization of n-butyl acrylate in a wide range of monomer–water phase compositions, triblock copolymers self-organizing in aqueous medium to give stable particles with the core–shell structure are formed. Oligomers with M _n ～ (5–10) × 10³ are optimal for synthesis of stable dispersions. In this case, block copolymers with the controlled length of hydrophobic block and a rather narrow MWD may be obtained. Thin films formed from these copolymers retain the structure of the initial dispersions on solvent removal. If the trithiocarbonate group in the oligomer is located at the chain end, the main polymerization product is a diblock copolymer. In this case, the formation of polymer–monomer particles occurs during a longer period of time, the control of MWD is weakened, and the dispersions of particles lose the aggregative stability after thin film formation.     

Ternary copolymers of acrylic acid, <Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide,and butyl acrylate: Synthesis and aggregative behavior in dilute solutions

Narrowly dispersed hydrophilic block and random copolymers of acrylic acid and N-isopropylacrylamide of various compositions and molecular masses are synthesized for the first time by controlled radical polymerization via the reversible addition-fragmentation chain-transfer mechanism. The copolymers are used for the synthesis of ternary copolymers with n-butyl acrylate. As shown by dynamic light scattering, the resulting amphiphilic copolymers can form stable dispersions in diluted aqueous solutions. It is found that the effective hydrodynamic radii of dispersion particles may be controlled via variation both in the primary structure of the ternary copolymer, i.e., the amount and sequence of hydrophilic and hydrophobic units, and in the external stimuli, i.e., the temperature and pH of a solution.     

Effect of the chemical natures of a monomer and a leaving group in symmetric trithiocarbonate as a reversible addition-fragmentation chain-transfer agent on the position of the trithiocarbonate group in macromolecules

The mechanism controlling the formation of polymer chains during the polymerization of vinyl monomers-namely, styrene, 4-vinylpyridine, n-butyl acrylates, and tert-butyl acrylate—mediated by symmetric trithiocarbonates (R-S-C(=S)-S-R) with different leaving groups R is studied. It is shown that the position of the trithiocarbonate fragment in a macromolecule depends on the nature of both the monomer and substituent R in trithiocarbonate. Variations in the structure of the leaving group in the initial reversible addition-fragmentation chain-transfer agent and the synthesis conditions makes it possible to direct polymerization to form a structure (symmetric, end, or asymmetric) relative to the trithiocarbonate group.     

Controlled synthesis of copolymers of vinyl acetate and n-butyl acrylate mediated by trithiocarbonates as reversible addition-fragmentation chain-transfer agents

The formation of copolymers of vinyl acetate and n-butyl acrylate via polymerization mediated by di-tert-butyl trithiocarbonate and dibenzyl trithiocarbonate as reversible addition-fragmentation chain-transfer agents is studied. Copolymerization mediated by low-molecular-mass reversible addition-fragmentation chain-transfer agents and by the copolymers formed in their presence proceeds via the pseudoliving mechanism. As a result, the controlled synthesis of narrowly dispersed copolymers of various compositions and desired molecular masses may be implemented. Variation in the compositions of the copolymers with conversion is investigated, and the reactivity ratios of the comonomers are found to differ significantly (r _VA = 0.01 and r _BA = 5.38). Our experimental data make it possible to infer that gradient copolymers are formed in the systems of interest in a wide range of comonomer mixture compositions.     

Controlled copolymerization of acrylonitrile in bulk via the reversible addition-fragmentation chain-transfer mechanism

The pseudoliving radical binary copolymerization of acrylonitrile with methyl acrylate, styrene, n-butyl acrylate, and tert-butyl acrylate in bulk in the presence of the reversible addition-fragmentation chain-transfer agent dibenzyl trithiocarbonate is performed for the first time. The addition of trithiocarbonate makes it possible to prepare a narrowly dispersed visually optically transparent copolymer in a wide range of monomer-feed compositions even at limiting conversions. Conditions for the synthesis of acrylonitrile copolymers with controlled molecular masses and narrow molecular-mass distributions are ascertained. In the above copolymers, the trithiocarbonate group is shown to be located within the chain.     

Controlled free-radical azeotropic copolymerization of styrene and n-butyl acrylate in the presence of tert-butyl dithiobenzoate

The free-radical azeotropic bulk copolymerization of styrene and n-butyl acrylate at 90°C mediated by tert-butyl dithiobenzoate and copoly(strene—n-butyl acrylate) dithiobenzoate as reversible chain-transfer agents has been studied. It has been shown that low-and high-molecular mass chain-transfer agents allow one to efficiently control the molecular-mass characteristics of the copolymers. For all studied systems, the molecular mass linearly increases with conversion, and the copolymers are characterized by low polydispersity indexes. When polystyryl dithiobenzoate and poly(butyl acrylate) dithiobenzoate are used as polymer reversible chain-transfer agents in the azeotropic copolymerization of styrene and n-butyl acrylate, the diblock copolymers with the controlled block lengths are prepared. As evidenced by ESR studies, radical intermediates are formed in the course of the azeotropic copolymerization of styrene and n-butyl acrylate mediated by tert-butyl dithiobenzoate and the copolymer reversible chain-transfer agent; the kinetics of formation of these intermediates has been investigated. It has been demonstrated that the rate of the azeotropic copolymerization mediated by low-and high-molecular-mass reversible chain-transfer agents decreases with an increase in their concentration. The possible causes of this phenomenon are discussed.     

Synthesis of urethane—acrylic multi-block copolymers via electrochemically mediated ATRP

The electrochemically mediated atom transfer radical polymerisation (eATRP) of n-butyl acrylate was investigated under a variety of catalyst concentrations. Poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate) copolymers were prepared via electrochemically mediated atom transfer radical polymerisation (eATRP) using only 7 × 10^?6 mole % of Cu^II complex. The successful chain extension and formation of penta-block copolymers confirmed the living nature of the poly(alkyl acrylates) prepared by eATRP. In this work, the tri-block and penta-block urethane-acrylate copolymers were synthesised for the first time by using tertiary bromine-terminated polyurethane macro-initiators as transitional products reacting with n-butyl acrylate, and subsequently with tert-butyl acrylate in the presence of the Cu^IIBr₂/TPMA catalyst complex. The results of ¹H NMR spectral studies support the formation of tri-block poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate) copolymers, and penta-block poly(tert-butyl acrylate)-block-poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate)-block-poly(tert-butyl acrylate) copolymers.     

Synthesis of bis(2,2′:6′,2″-terpyridine)-terminated telechelic polymers by RAFT polymerization and ruthenium-polymer complexation thereof

Well-defined polystyrenes and poly(n-butyl acrylate)s of the two ends being functionalized with terpyridine groups were synthesized via addition-fragmentation chain transfer (RAFT) polymerization using a symmetric bisterpyridine-functionalized trithiocarbonate as a chain transfer agent (CTA). Kinetic studies on RAFT mediated thermal polymerization of styrene indicated the controlled polymerization. Corresponding triblock copolymers of styrene and n-butyl acrylate were obtained by utilizing the bisterpyridine-functionalized homopolymers as the macro-CTAs. Supramolecular metallo-polystyrenes with different repeat blocks were prepared by the chelating interaction between the terpyridine ends and Ru(II) ions. The formation of the metallo-polymers was proven by UV-vis spectra and dynamic light scattering (DLS).     

Specific features of radical copolymerization of methyl methacrylate and <Emphasis Type="Italic">n</Emphasis>-butyl acrylate in the presence of the tributylborane–<Emphasis Type="Italic">p</Emphasis>-quinone system

Copolymerization of methyl methacrylate (MMA) and n-butyl acrylate (n-BA) in the presence of the tributylborane–p-quinone system has been investigated. Reactivity ratios of the monomers differ from the values known for conventional radical polymerization. The copolymerization proceeds in a controlled manner according to the reversible inhibition mechanism. The nature of p-quinone and the composition of the monomer mixture affect the realization of this mechanism. The microstructure of the copolymers obtained in the presence of borane and p-quinones depends on the nature of the latter.     

Pseudoliving polymerization of vinyl acetate mediated by reversible addition-fragmentation chain-transfer agents

The homopolymerization of vinyl acetate mediated by dithiobenzoates and trithiocarbonates as reversible addition-fragmentation chain-transfer agents is studied. The polymerization of vinyl acetate is characterized by some distinct features: (i) a substantial role of chain-termination reactions involving radical intermediates in the kinetics of the process that increases as the concentrations of the reversible additionfragmentation chain-transfer agent and the initiator increase and as temperature decreases and (ii) the occurrence of side reactions of chain transfer to monomers and polymers. The role of these reactions significantly increases with conversion of the monomer. Thus, in order to prepare a narrowly dispersed PVA via the reversible addition-fragmentation chain-transfer mechanism, the process should be conducted to small conversions (15–20%) at moderately high temperatures (80°C) and at a small molar excess of the reversible addition-fragmentation chain-transfer agent with respect to the initiator. A technique for the synthesis of block copolymers based on PVA and poly(n-butyl acrylate) via the reversible addition-fragmentation chain-transfer mechanism is developed.     

Controlled synthesis of multiblock copolymers by pseudoliving radical polymerization via the reversible addition-fragmentation chain-transfer mechanism

With the use of two classes of reversible addition-fragmentation chain-transfer agents??dithiobenzoates and trithiocarbonates??multiblock copolymers based on styrene and n-butyl acrylate, which are the best-studied monomers in these processes, are synthesized. It is shown that the polymers containing dithiobenzoate and trithiocarbonate groups are highly efficient for the synthesis of block copolymers, which is independent of the number of stages at which the polymeric RAFT agents are used in polymerization: In all cases, the polymeric RAFT agent is fully consumed in the polymerization of the ??alien?? monomer. The mechanism governing chain formation during the synthesis of multiblock copolymers, that is, the character of monomer insertion into the polymer chain, via one or both ends, is studied. It is found that the order of monomer loading determines the ratio of chains growing through one or two ends. The thermal stability of amphiphilic multiblock copolymers, their solubility in various solvents, and self-organizing ability are investigated.     

Controlled radical copolymerization of styrene and <Emphasis Type="Italic">tert</Emphasis>-butyl acrylate in the presence of tri-<Emphasis Type="Italic">n</Emphasis>-butylborane–<Emphasis Type="Italic">p</Emphasis>-quinone catalytic system

Radical copolymerization of styrene with tert-butyl acrylate is studied under different conditions. It is found that the addition of tri-n-butylborane or tri-n-butylborane along with p-quinones (2,3-dimethylbenzoquinone, 2,5-di-tert-butylbenzoquinone) results in changes in the relative activities of monomers. Copolymerization in the presence of tri-n-butylborane and p-quinones proceeds via the mechanism of reversible inhibition and is characterized by the linear increase in number-average molecular weight with conversion and by the capacity of copolymers of reinitiation. The hydrolyzed copolymer samples form more stable films compared to copolymers prepared via conventional radical copolymerization.     

Microstructure of Polymers Prepared in the Presence of Tri-<Emphasis Type="Italic">n</Emphasis>-butylboron–<Emphasis Type="Italic">p</Emphasis>-Quinone System

The microstructure of polystyrene, poly(methyl acrylate), and poly(tert-butyl acrylate) obtained in the presence of tri-n-butylboron in combination with naphthoquinone-1,4 and 2,3-dimethylbenzoquinone is studied. The stereoregularity of polymers formed under these conditions may be described in the framework of the first-order Markov chain model. The use of p-quinones in combination with tri-n-butylboron favors an increase in the content of isotactic triads in the polymers. The causes of the observed deviations are discussed.     

Controlled radical polymerization of styrene mediated by dithiobenzoates as reversible addition-fragmentation chain-transfer agents

The radical polymerization of styrene at 60 and 80°C mediated by benzyl dithiobenzoate and poly(styrene dithiobenzoate) as reversible addition-fragmentation chain-transfer agents has been studied. It has been shown that both agents are characterized by high chain-transfer constants and provide control over molecular-mass characteristics of polymerization products. The number-average molecular mass of polystyrene linearly grows with conversion, and the polymers are characterized by low values of polydispersity indexes. It has been demonstrated that the rate of polymerization significantly decreases with an increase in the concentration of reversible addition-fragmentation chain-transfer agents. This effect is typical of styrene polymerization mediated by dithiobenzoates. The possible reasons for this phenomenon are discussed.     

Synthesis of Block Copolymers of Styrene with <Emphasis Type="Italic">D</Emphasis>,<Emphasis Type="Italic">L</Emphasis>-Lactide by the Sequential Controlled Cationic Polymerization and Ring-Opening Anionic Polymerization

The cationic polymerization of styrene initiated by the system 2-chloro-2-phenylpropane–TiCl₄–pyridine is studied in a mixture CH₂Cl₂–n-hexane at a temperature of –80°С. It is shown that under these conditions polymerization occurs via the living mechanism at [monomer]: [initiator] ≤ 100. The method of preparing polystyrenes with terminal primary hydroxyl groups (M_n = 4000–10000 g/mol) by the sequential controlled cationic polymerization of styrene and the in situ alkylation of 4-phenoxy-1-butanol by polystyrene macrocations is proposed. The resulting functionalized polystyrenes are used as macroinitiators of anionic-coordination ring-opening polymerization of D,L-lactide in the presence of tin bis(2-ethyl hexanoate) [Sn(Oct)₂] in toluene at 80°С. Copolymers polystyrene-block-poly(D,L-lactide) with the controlled length of the poly(D,L-lactide) block (M_n = 10000–17000 g/mol) and a relatively low molecular-weight distribution (M_w/M_n = 1.6–1.8) are synthesized. Formation of the block copolymers is confirmed by ¹Н NMR spectroscopy, gel-permeation chromatography, and atomic force microscopy.     

Aggregative behavior of AB and ABC block copolymers in the solid phase and in a nonselective solvent

Effects of the amount of chemically dissimilar blocks (two or three) and their polarity on the aggregative behavior of АВ and АВС linear block copolymers of various compositions that are based on polystyrene, poly(n-butyl acrylate), and either poly(acrylic acid) or poly(tert-butyl acrylate) in bulk and in the nonselective solvent DMF are studied via differential scanning calorimetry and dynamic light scattering. АВ block copolymers composed of two chemically dissimilar blocks in the diluted solution in DMF are fully dispersed into macromolecular coils. However, the simultaneous incorporation of three incompatible blocks of different polarities (polystyrene, poly(acrylic acid), and poly(n-butyl acrylate)) into the copolymer is accompanied by a well-defined segregation of blocks in the nonselective solvent, regardless of the composition of the block copolymer and the length and sequence of blocks. This phenomenon makes itself evident as the formation of intermacromolecular aggregates in diluted solutions with a mean hydrodynamic radius of 60–120 nm that are stable in the range 10–60°C. A decrease in the level of the thermodynamic incompatibility of blocks (replacement of a poly(acrylic acid) polar block with a less polar poly(tert-butyl acrylate) block) or the selective improvement of solvent quality with respect to the polar block (the addition of LiBr to DMF) suppresses the segregation of blocks and may lead to the formation of a molecularly dispersed solution of the block copolymer.     

Tracing drug release process with dual-modal hyperbranched polymer-gold nanoparticle complexes

Dual-modal surface enhanced Raman spectrum(SERS)-fluorescence polymer/metal hybrid complexes have been prepared for tracing drug release process in tumor cells. Firstly, the hyperbranched poly((S-(4-vinyl) benzyl S′-propyltrithiocarbonate)-co-(poly(ethylene glycol) methacrylate))(HPVBEG) was synthesized via the combination of reversible addition-fragmentation chain-transfer(RAFT) polymerization and self-condensing vinyl polymerization(SCVP). Subsequently, the anticancer drug doxorubicin(DOX) was linked to HPVBEG via pH sensitive Schiff base bonds to form HPVBEG-g-DOX conjugates.Through aminolysis reaction, HPVBEG-g-DOX was coordinated with gold nanoparticles(GNP), resulting in the formation of HPVBEG-g-DOX/GNP complexes. In neutral condition, the HPVBEG-g-DOX/GNP complexes were stable, and DOX was bound to the surface of GNPs. Therefore, the SERS of DOX could be observed, while the fluorescence of DOX was quenched by GNPs. Under an acidic environment, DOX was released from the surface of GNPs with breakage of Schiff base bonds.Thus, the SERS signal of DOX was gradually reduced. Correspondingly, the fluorescence signal of DOX was enhanced.Through dual-modal SERS-fluorescence technique, the DOX delivery and release process was traced in tumor cells. Moreover,the viability of MCF-7 cells incubated with HPVBEG-g-DOX/GNP complexes was investigated by Cell Counting Kit-8(CCK-8) assay. The experimental results showed that HPVBEG-g-DOX/GNP complexes had similar proliferation inhibition effect compared with free DOX. Definitely, the dual-modal SERS-fluorescence complexes for tracing drug delivery and release will have promising prospects on tumor diagnosis and therapy.     

RAFT-Mediated Emulsion Polymerization-Induced Self-Assembly for the Synthesis of Core-Degradable Waterborne Particles

Poly(N-acryloylmorpholine) (PNAM)-decorated waterborne nanoparticles comprising a core of either degradable polystyrene (PS) or poly(n-butyl acrylate) (PBA) were synthesized by polymerization-induced self-assembly (PISA) in water. A PNAM bearing a trithiocarbonate chain end (PNAM-TTC) was extended via reversible addition-fragmentation chain transfer (RAFT)-mediated emulsion copolymerization of either styrene (S) or n-butyl acrylate (BA) with dibenzo[c,e]oxepane-5-thione (DOT). Well-defined amphiphilic block copolymers were obtained. The in situ self-assembly of these polymers resulted in the formation of stable nanoparticles. The insertion of thioester units in the vinylic blocks enabled their degradation under basic conditions. The same strategy was then applied to the emulsion copolymerization of BA with DOT using a poly(ethylene glycol) (PEG) equipped with a trithiocarbonate end group, resulting in PEG-decorated nanoparticles with degradable PBA-based cores.     

Aqueous RAFT polymerization of acrylamide: A convenient method for polyacrylamide with narrow molecular weight distribution

Controlled and homogeneous free-radical polymerization of acrylamide(AM) in aqueous phase was realized by using S,S'-bis(α,α'-dimethyl-α'-acetic acid)-trithiocarbonate as a reversible addition-fragmentation transfer(RAFT) agent. Linear increases in molecular weight with conversion and narrow molecular weight distribution were observed for polyacrylamide(PAM) throughout the polymerization. By this method, PAMs with controlled molecular weight(up to 1.0 × 10~6) and narrow molecular weight distribution(M_w/M_n 1.2) were prepared. This study provides an effective method for synthesis of PAMs with narrow molecular weight distribution under environmentally friendly conditions.