Facile one-spot synthesis of highly branched polycaprolactone

Reported is the first solvent-free (bulk) synthesis of degradable/bioresorbable, highly branched polymers via tin octanoate Sn(Oct2) catalysed controlled ring opening co-polymerisation (ROP) of mono and di-functional lactone monomers that proceed to near quantitative conversion. The successful isolation of solvent soluble, highly branched structures was shown to be dependent on both the concentration of the di-functional monomer and the overall reaction time. Comparison with analogous systems utilising controlled radical polymerisation (CRP) to form the highly/hyper branched polymers suggested significant experimental differences between the two chain growth methods. The maximum proportion of di-functional monomer without gelation ensuing was found to be 0.6 equivalents w.r.t. mono-functional monomer (c.f. 1 with CRP) and the onset of significant levels of branching occurred at approximately 90% conversion (c.f. ~70% with CRP). These differences and significant disparity in reaction times were attributed to (a) the coordination and insertion (C+I) propagation mechanism adopted by the Sn catalyst and (b) the presence of additional trans-esterification reactions at high conversion. Evidence is presented to support the conclusion that there are two mechanisms contributing to the overall branching process in the ROP system at high conversion. First, the C+I mechanism promotes growth of linear polymer until approximately 90% conversion, after which both the C+I and trans-esterification processes contribute to the interchain branching process. The branched nature of the molecular structures was supported by confirmation plots generated from static light scattering. This data demonstrated that the polymers synthesised exhibit varying degrees of branching, consistent with the di-functional monomer (4,4’-bioxepanyl-7,7’-dione - BOD) concentration in the feed. The degree of branching was calculated using 3 different methods and the results were shown to be independent of method. Finally, DSC analysis of the polymers demonstrated correlation between the degree of branching achieved and the observed Tm for the material where increased branching leads to a drop in the recorded Tm

Polymerization-Induced Self-Assembly of Block Copolymer Nano-objects via RAFT Aqueous Dispersion Polymerization

In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells

RAFT aqueous dispersion polymerization yields poly(ethylene glycol)-based diblock copolymer nano-objects with predictable single phase morphologies

A poly(ethylene glycol) (PEG) macromolecular chain transfer agent (macro-CTA) is prepared in high yield (>95%) with 97% dithiobenzoate chain-end functionality in a three-step synthesis starting from a monohydroxy PEG113 precursor. This PEG113-dithiobenzoate is then used for the reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). Polymerizations conducted under optimized conditions at 50 °C led to high conversions as judged by 1H NMR spectroscopy and relatively low diblock copolymer polydispersities (Mw/Mn < 1.25) as judged by GPC. The latter technique also indicated good blocking efficiencies, since there was minimal PEG113 macro-CTA contamination. Systematic variation of the mean degree of polymerization of the core-forming PHPMA block allowed PEG113-PHPMAx diblock copolymer spheres, worms, or vesicles to be prepared at up to 17.5% w/w solids, as judged by dynamic light scattering and transmission electron microscopy studies. Small-angle X-ray scattering (SAXS) analysis revealed that more exotic oligolamellar vesicles were observed at 20% w/w solids when targeting highly asymmetric diblock compositions. Detailed analysis of SAXS curves indicated that the mean number of membranes per oligolamellar vesicle is approximately three. A PEG 113-PHPMAx phase diagram was constructed to enable the reproducible targeting of pure phases, as opposed to mixed morphologies (e.g., spheres plus worms or worms plus vesicles). This new RAFT PISA formulation is expected to be important for the rational and efficient synthesis of a wide range of biocompatible, thermo-responsive PEGylated diblock copolymer nano-objects for various biomedical applications

Extent of intramolecular cyclization in RAFT-synthesized methacrylic branched copolymers using C-13 NMR spectroscopy

Recently, we reported using 1H NMR spectroscopy to assess the degree of intramolecular cyclization in a series of soluble methacrylic branched copolymers (see J. Rosselgong and S. P. Armes, Macromolecules, 2012, 45, 2731–2737). The key to success in addressing this long-standing problem in polymer science was the selection of a suitable disulfide-based dimethacrylate as the branching comonomer, which was statistically copolymerized with methyl methacrylate using reversible addition-fragmentation chain transfer (RAFT) polymerization. In this earlier work, estimation of the degree of intramolecular cyclization required peak deconvolution of the relevant thiamethylene proton signals. In the present work, we show that quantitative 13C NMR spectroscopy can also be utilized to determine the intramolecular cyclization for the same series of methacrylic copolymers via peak deconvolution of the oxymethylene carbon signals. Although this technique requires long spectral accumulation times, it offers superior resolution compared to 1H NMR spectroscopy and hence may ultimately enable this analytical approach to be applied to less esoteric divinyl comonomers

Disulfide-Functionalized Diblock Copolymer Worm Gels

Two strategies for introducing disulfide groups at the outer surface of RAFT-synthesized poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (PGMA–PHPMA, or Gx-Hy for brevity) diblock copolymer worms are investigated. The first approach involved statistical copolymerization of GMA with a small amount of disulfide dimethacrylate (DSDMA, or D) comonomer to afford a G54-D0.50 macromolecular chain transfer agent (macro-CTA); this synthesis was conducted in relatively dilute solution in order to ensure mainly intramolecular cyclization and hence the formation of linear chains. Alternatively, a new disulfide-based bifunctional RAFT agent (DSDB) was used to prepare a G45-S-S-G45 (or (G45-S)2) macro-CTA. A binary mixture of a non-functionalized G55 macro-CTA was utilized with each of these two disulfide-based macro-CTAs in turn for the RAFT aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). By targeting a PHPMA DP of 130 and systematically varying the molar ratio of the two macro-CTAs, a series of disulfide-functionalized diblock copolymer worm gels were obtained. For both formulations, oscillatory rheology studies confirmed that higher disulfide contents led to stronger gels, presumably as a result of inter-worm covalent bond formation via disulfide/thiol exchange. Using the DSDB-based macro-CTA led to the strongest worm gels, and this formulation also proved to be more effective in suppressing the thermosensitive behavior that is observed for the nondisulfide-functionalized control worm gel. However, macroscopic precipitation occurred when the proportion of DSDB-based macro-CTA was increased to 50 mol %, whereas the DSDMA-based macro-CTA could be utilized at up to 80 mol %. Finally, the worm gel modulus could be reduced to that of a nondisulfide-containing worm gel by reductive cleavage of the inter-worm disulfide bonds using excess tris(2-carboxyethyl)phosphine (TCEP) to yield thiol groups. These new biomimetic worm gels are expected to exhibit enhanced muco-adhesion

Accelerated combinatorial high throughput star polymer synthesis via a rapid one-pot sequential aqueous RAFT (rosa-RAFT) polymerization scheme

Advanced polymerization methodologies, such as reversible addition-fragmentation transfer (RAFT), allow unprecedented control over star polymer composition, topology, and functionality. However, using RAFT to produce high throughput (HTP) combinatorial star polymer libraries remains, to date, impracticable due to several technical limitations. Herein, the methodology “rapid one-pot sequential aqueous RAFT” or “rosa-RAFT,” in which well-defined homo-, copolymer, and mikto-arm star polymers can be prepared in very low to medium reaction volumes (50 µL to 2 mL) via an “arm-first” approach in air within minutes, is reported. Due to the high conversion of a variety of acrylamide/acrylate monomers achieved during each successive short reaction step (each taking 3 min), the requirement for intermediary purification is avoided, drastically facilitating and accelerating the star synthesis process. The presented methodology enables RAFT to be applied to HTP polymeric bio/nanomaterials discovery pipelines, in which hundreds of complex polymeric formulations can be rapidly produced, screened, and scaled up for assessment in a wide range of applications

RAFT synthesis of branched acrylic copolymers

We report the synthesis of branched acrylic copolymers based on 2-hydroxypropyl acrylate using reversible addition fragmentation chain transfer (RAFT) polymerization in tert-butanol at 80 °C. Three branching comonomers were investigated in this study:  ethylene glycol diacrylate, bisphenol A ethoxylated diacrylate and a disulfide-based diacrylate. The latter comonomer allows chemical degradation of the branched acrylic copolymers to produce thiol-functionalized primary chains. Gel permeation chromatography analysis of these degraded copolymer chains indicated low polydispersities (Mw/Mn < 1.22), which confirmed that the living character of the RAFT chemistry was retained under branching conditions. RAFT allows significantly more than one branching agent per primary chain to be used in the copolymerization without causing gelation. This result was obtained with all three branching comonomers and differs from the near-ideal copolymerizations previously reported for the ATRP synthesis of branched methacrylic copolymers (Macromolecules 2006, 39, 7483−7492). Detailed HPLC analysis of the RAFT copolymerization of 2-hydroxypropyl acrylate with bisphenol A ethoxylated diacrylate indicates near-statistical incorporation of the latter comonomer. We suggest that intramolecular cyclization is the primary reason for the apparent violation of classical Flory−Stockmayer gelation theory. This hypothesis is supported by the observation that substantially more ethylene glycol diacrylate than bisphenol A ethoxylated diacrylate can be tolerated in such branching copolymerizations without causing gelation

Thiol-Functionalized Block Copolymer Vesicles

Thiol-functionalized block copolymer vesicles are readily prepared via RAFT-mediated polymerization-induced self-assembly (PISA). More specifically, a disulfide-functionalized poly­(glycerol monomethacrylate) macro-CTA is chain-extended using 2-hydroxypropyl methacrylate): the growing water-insoluble poly­(2-hydroxypropyl methacrylate) chains drive in situ self-assembly to produce diblock copolymer vesicles in concentrated aqueous solution. The disulfide bonds in the poly­(glycerol monomethacrylate) stabilizer chains are reductively cleaved in situ using either tributyl phosphine or tris­(2-carboxyethyl)­phosphine to generate thiol groups, which react immediately with either a quaternary acrylate to introduce cationic character or with rhodamine B acrylate or rhodamine B isothiocyanate to confer a convenient fluorescent tag. In addition to such facile derivatization, such thiol-functionalized vesicles may offer some potential for drug delivery applications, because enhanced muco-adhesion is anticipated for these nano-objects