Crystal Structure of Atactic and Isotactic Poly(3-hydroxy-2,2-dimethylbutyrate): A Chemically Recyclable Poly(hydroxyalkanoate) with Tacticity-Independent Crystallinity

The crystal structures of isotactic and atactic poly(3-hydroxy-2,2-dimethylbutyrate) (P3H(Me)2B) are presented. Samples of atactic P3H(Me)2B (at-(R/S)-P3H(Me)2B) and of the R and S enantiomers of isotactic P3H(Me)2B (it-(R)-P3H(Me)2B and it-(S)-P3H(Me)2B, respectively) have been synthesized by ring opening polymerization of racemic and chiral dimethyl-butyrolactones employing a superbase catalyst. A 1:1 racemic mixture of the two R and S enantiomers (it-(R,S)-P3H(Me)2B) has been also prepared. Both the atactic and the pure enantiomer isotactic polymers crystallize showing identical diffraction patterns, indicating crystallization in the same crystalline form and identical crystal structure. The racemic mixture it-(R,S)-P3H(Me)2B also crystallizes giving identical diffraction pattern. This is one of the few examples of crystallization of an atactic polymer despite the configurational disorder and probably it is the first structurally confirmed example of crystallization of atactic and isotactic polymers in the identical crystal structure. This fascinating tacticity-independent crystallinity explains the remarkable thermal and mechanical behaviors of P3H(Me)2B, which is thermally stable and melt-processable and chemically recyclable to the monomer. The crystal structure has been resolved by analysis of X-ray powder diffraction and X-ray fiber diffraction of oriented fibers, combined with conformational analysis based on methods of density functional theory. The ordered crystal structure of the isotactic pure enantiomer it-(R)-P3H(Me)2B (or it-(S)-P3H(Me)2B) is described by chains in a nearly trans-planar conformation with chain axis of 4.7 Å packed in an orthorhombic unit cell with axes a = 13.30 Å, b = 9.99 Å, and c = 4.75 Å according to the chiral space groups P21212 or P212121. The atactic polymer at-(R/S)-P3H(Me)2B crystallizes in the same orthorhombic unit cell having only slightly larger a axis, a = 13.94 Å, b = 10.03 Å, and c = 4.75 Å, with chains characterized by a disordered succession of R and S monomers and a distorted trans-planar conformation that keeps a straight chain axis and the same periodicity of 4.7 Å of the ordered pure enantiomer. This proposed model of the conformation of at-(R/S)-P3H(Me)2B explains the crystallization of the atactic polymer. The crystal structure of at-(R/S)-P3H(Me)2B is, therefore, characterized by the packing of disordered chains in nearly trans-planar conformation according to both the chiral space groups P21212 or P212121 and the achiral space groups Pna21 or Pnn2.