Chain-Folding of Alternating Copolymers to Achieve Main-Chain Alkyl Chain Crystallization with Minimum Six Carbon Atoms

It is generally accepted that the critical alkyl chain length for crystallization in polymers is 9–12 carbon atoms. How to achieve crystallization of a shorter alkyl chain in polymers is a fundamental challenge and has attracted great interest in polymer physics. Herein, we synthesize five main-chain alternating copolymers (ACPs) with different alkyl chain lengths (the length ≤10 carbon atoms) to investigate the critical alkyl chain length for crystallization in main-chain polymer crystallization. It is found that when the alkyl chain length is greater than or equal to six carbon atoms, the alkyl chain in the polymer can crystallize in a chain-folding manner according to the DSC measurements. Some small crystalline clusters might also exist in the polymers with two or four carbon atoms in the alkyl chain according to the small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) measurements. Moreover, the layer spacing is linearly related to the number of carbon atoms at a far smaller scale of sub-2 nm. To our knowledge, it is the smallest critical alkyl chain length for crystallization in main-chain polymers, which is attributed to the chain-folding self-assembly mechanism of ACPs to promote an ordered arrangement of alkyl chains.