Chirality, entropy and crystallization in polymers: isotactic poly(3-methyl-1-pentene) as an example of influence of chirality and entropy on the crystal structure

We report the synthesis and the crystal structure of isotactic poly((R,S)-3-methyl-1-pentene) (iP(R,S)3MP). A purely random achiral copolymer of two enantiomeric (R) and (S) 3-methyl-1-pentene monomers cannot be obtained by polymerization of the racemic mixture of the (R) and (S) monomers but has been obtained by stereospecific polymerization of 3-methyl-1,3-pentadiene to isotactic 1,2-poly(3-methyl-1,3-pentadiene) (iP3MPD12) and successive hydrogenation. The crystal structures of achiral iP(R,S)3MP and chiral poly((S)-3-methyl-1-pentene) (iP(S)3MP) are different, indicating that crystal structures of polymers may be driven by different entropic effects related to chirality and type of crystal disorder. The conformational disorder of the chiral lateral groups prevails in chiral iP(S)3MP, inducing crystallization of chains of one helical chirality, whereas the entropic effect due to the statistical substitution of chains of different helical chirality in the sites of the lattice prevails in achiral iP(R,S)3MP.