How Do Spherical Diblock Copolymer Nanoparticles Grow during RAFT Alcoholic Dispersion Polymerization?

Abstract

A poly(2-(dimethylamino)ethyl methacrylate) (PDMA) chain transfer agent (CTA) is used for the reversible addition–fragmentation chain transfer (RAFT) alcoholic dispersion polymerization of benzyl methacrylate (BzMA) in ethanol at 70 °C. THF GPC analysis indicated a well-controlled polymerization with molecular weight increasing linearly with conversion. GPC traces also showed high blocking efficiency with no homopolymer contamination apparent and Mw/Mn values below 1.35 in all cases. 1H NMR studies confirmed greater than 98% BzMA conversion for a target PBzMA degree of polymerization (DP) of up to 600. The PBzMA block becomes insoluble as it grows, leading to the in situ formation of sterically stabilized diblock copolymer nanoparticles via polymerization-induced self-assembly (PISA). Fixing the mean DP of the PDMA stabilizer block at 94 units and systematically varying the DP of the PBzMA block enabled a series of spherical nanoparticles of tunable diameter to be obtained. These nanoparticles were characterized by TEM, DLS, MALLS, and SAXS, with mean diameters ranging from 35 to 100 nm. The latter technique was particularly informative: data fits to a spherical micelle model enabled calculation of the core diameter, surface area occupied per copolymer chain, and the mean aggregation number (Nagg). The scaling exponent derived from a double-logarithmic plot of core diameter vs PBzMA DP suggests that the conformation of the PBzMA chains is intermediate between the collapsed and fully extended state. This is in good agreement with 1H NMR studies, which suggest that only 5−13% of the BzMA residues of the core-forming chains are solvated. The Nagg values calculated from SAXS and MALLS are in good agreement and scale approximately linearly with PBzMA DP. This suggests that spherical micelles grow in size not only as a result of the increase in copolymer molecular weight during the PISA synthesis but also by exchange of individual copolymer chains between micelles and/or by sphere–sphere fusion events.

Addition of water to an alcoholic RAFT PISA formulation leads to faster kinetics but limits the evolution of copolymer morphology

“…PISA formulations are robust, efficient and highly versatile: aqueous emulsion, aqueous dispersion and non-aqueous polymerization protocols have been developed using various LRP chemistries. 29 As expected, using a longer PDMA macro-CTA (DP = 94) limited the morphology of the resulting particles to kineticallytrapped spheres. [19][20][21][22][23][24][25][26][27] Of particular relevance to the present study, we reported an all-methacrylic RAFT dispersion polymerization protocol based on chain extension of a poly(2-(dimethylamino)ethyl methacrylate) (PDMA) macro-CTA using benzyl methacrylate (BzMA) in ethanol.…”

“…14,29,30 However, using the PDMA 43 macro-CTA led to phase separation when targeting a DP of 1200 or higher for BzMA polymerizations conducted in 80/20 ethanol/water mixtures. 14,29,30 However, using the PDMA 43 macro-CTA led to phase separation when targeting a DP of 1200 or higher for BzMA polymerizations conducted in 80/20 ethanol/water mixtures.…”

Controlling Nanomaterial Size and Shape for Biomedical Applications via Polymerization‐Induced Self‐Assembly

“…Although spherical particles are commonly produced by PISA systems, controlling the diameter of PISA micelles was not demonstrated in early work . In 2010, Armes and coworkers reported the use of RAFT dispersion polymerization for the production of sterically stabilized micelles with a range of controlled diameters .…”

Overcoming Kinetic Trapping for Morphology Evolution during Polymerization‐Induced Self‐Assembly

“…No morphology transition is observed as the DP of PBzMA increased, when using long stabilizers. Armes and coworkers have studied and addressed it as kinetic trapping, which is caused by the strong repulsions between long PDMA 51 stabilizers preventing the fusion and reorganization of nanoparticles . Contrarily, using short stabilizer of PDMA 33 ‐CTA, D 33 ‐ b ‐B 106 copolymers are vesicles …”