Sub-5 μm balls possessing forest-like poly(methyloxazoline)/polyethyleneimine side chains and templated silica microballs with unusual internal structures

Abstract

Herein, sub-5 μm microballs, with unusual forest-like structures consisting of a polystyrene network and forest-like poly(2-methyl-2-oxazoline) (PMOZ) and/or linear polyethyleneimine (LPEI) side chains, were synthesized by combining two isolated processes. The first process is the dispersion radical polymerization of 4-chloromethylstyrene (CMS) and divinylbenzene (DVB) in the presence of a stabilizer, polyvinylpyrrolidone (PVP), using 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator, which produces a poly(4-chloromethylstyrene) type microgel (μ-PStCl). The second process is the cationic ring-opening polymerization of 2-methyl-2-oxazoline (MOZ) using μ-PStCl as a solid initiator, which produces poly(2-methyl-2-oxazoline)-grafted microballs (μ-PSt-g-PMOZ). The latter, possessing PMOZ side chains, was transformed into poly(ethyleneimine)-grafted microballs (μ-PSt-g-PEI) via treatment with HCl (aq). Moreover, the three types of microballs μ-PStCl, μ-PSt-g-PMOZ, and μ-PSt-g-PEI of 1–3 micrometer in diameter were characterized by FT-IR, ¹³C CP/MAS NMR, elemental analysis, XRD, and SEM. Both the microballs μ-PSt-g-PMOZ and μ-PSt-g-PEI, which resembled the assemblies of hydrophilic comb polymers in their chemical structure, exhibited good wettability in an aqueous phase. In particular, the μ-PSt-g-PEI microballs, which have forest-like basic polyamine chains throughout the microballs, act as catalytic templates in the hydrolytic polycondensation of tetramethoxysilane (TMOS) at room temperature to produce polymer/silica hybrid microballs of μ-PSt-g-PEI@SiO₂ with 2.5–3.5 μm diameter and 35–70 wt% silica content depending upon the mediation conditions. Calcination of the μ-PSt-g-PEI@SiO₂ hybrid microballs at 700, 800, and 900 °C resulted in silica microballs possessing 2–4 nm mesopores with reduced diameter from 3.5 to 1.2 μm and reduced BET surface area from 582 to 189 m² g⁻¹. It was confirmed that the sub-5 μm microballs of μ-PSt-g-PEI were very effective catalytic templates for the construction of silica microballs of different sizes (1–4 μm), different surface areas, and different (large hollow or co-continuous) internal structures.