Mild and Efficient Modular Synthesis of Poly(acrylonitrile-co-butadiene) Block and Miktoarm Star Copolymer Architectures

Abstract

An efficient approach for the synthesis of block copolymers of poly(acrylonitrile-co-butadiene) (NBR) and poly(styrene-co-acrylonitrile) (SAN) is described. Conjugation of preformed polymer building blocks is achieved via a hetero-Diels-Alder (HDA) mechanism employing cyclopentadiene-capped NBRs with dienophile SAN copolymers, both synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. The protocol is further extended toward the synthesis of 4-miktoarm star polymers, consisting of two NBR and two SAN arms. Molar masses of the obtained complex macromolecular architectures range from below 10 000 g·mol–1 up to 110 000 g·mol–1 with dispersities below 1.5. Molecular verification of the coupling moieties is provided via NMR spectroscopy as well as ESI mass spectrometry. Size exclusion chromatography (SEC) traces of the obtained block copolymers and miktoarm star polymers were analyzed via deconvolution techniques, revealing the presence of 9.9–12.6 wt % (block copolymers) and 20 wt % (stars) of polymer chains not participating in the HDA conjugation, respectively. The residual polymers were analyzed toward their origin from either the loss of functionality during RAFT polymerization or incomplete conversion during the conjugation process. The comprehensive analysis of the macromolecular material was underpinned by kinetic simulations to estimate the fractions of nonfunctional polymer chains generated during the NBR and SAN polymerizations. The simulations evidenced that NBR-b-SAN samples cannot contain more than 94.4 wt % (M n 13 000 g·mol–1), 93.6 wt % (M n 57 000 g·mol–1), or 93.9 wt % (M n 110 000 g·mol–1) of polymer chains actually possessing the targeted block copolymer structures when assuming an ideal RAFT process. These results unambiguously reveal that nonfunctionalized polymer chains formed during RAFT polymerization cause the incomplete conjugation of polymer building blocks, evidencing the limitations of end-group control in controlled/living radical polymerizations.

Star Polymers

“…Barner-Kowollik and co-workers exploited this unique attribute to prepare unusual butadiene/acrylonitrile star polymers by conjugation of two block copolymers, i.e., of poly(acrylonitrile-co-butadiene) (NBR) and poly(styrene-coacrylonitrile) (SAN). 47 The conjugation is achieved via a hetero-Diels−Alder (HDA) click reaction employing cyclopentadiene-capped NBRs with dienophile SAN copolymers, leading to the formation of four-arm miktoarm star polymers. However, when preparing stars with larger arm numbers (>20), the grafting-onto approach may give ill-defined stars with less than the targeted numbers of arms, as the existing arm polymers may hinder further grafting by shielding the "free" arm polymers from accessing the reactive sites.…”

Poly(N‐isopropylacrylamide)‐Modified Styrene‐Butadiene Rubber as Thermoresponsive Material

“…Enhancement of elastomeric materials plays an important role in research today . It can be assumed that elastomers modified with LCST polymers can change their macroscopic polarity at the lower critical solution temperature.…”

Reversible addition-fragmentation chain transfer (co)polymerization of conjugated diene monomers: butadiene, isoprene and chloroprene

“…The earlier patents deal with the selection of RAFT agent and polymerization conditions for solution polymerization and some relate specifically to devolatilization and product isolation. Much of this work is also described in publications in the open literature (see also Enriquez‐Medrano et al ). The most recent paper in the series describes the synthesis of NBR by RAFT ab initio emulsion polymerization .…”

“…A series of papers has explored RAFT copolymerization of acrylonitrile and butadiene (NBR) in solution . Polymerizations with the trithiocarbonates DBTC ( 23 ) and DOPAT ( 26 ) and the 2‐phenylethanedithioate CUDTPA ( 17 ) were carried out at 110 °C with ACHN initiator in N , N ‐dimethylacetamide .…”

“…All three RAFT agents were effective in mediating the copolymerization. Other papers examined the effect of solvent and initiator on polymerization and the production of end‐functional polymers and their use in forming block and star polymers by copper‐catalysed azide–alkyne coupling or hetero‐Diels–Alder chemistry …”