Topochemical polymerization
Various reactions have been adopted in the field of topochemical polymerisation, such as [2+2],[3] [4+2],[4] [4+4],[5] and [3+2][6] cycloaddition, linear addition between dienes, trienes, diacetylenes.
[7] The term "topochemistry" was first introduced by Kohlschütter in 1919, referring to the chemical reactions driven by the molecular alignments within the crystal.
In the 1960s, Schmidt's work on [2+2] photodimerization of cinnamic acids established the systematic approach to study the topochemical reactions.
They proposed that only double bonds adopting coplanar and parallel orientation within a distance of 3.5-4.2 Å could react with each other in the crystal lattice.
[11] Restricted by the experimental condition, early researchers of topochemical polymerization usually characterized the reaction process and product with traditional chemical methods.
The development of modern analysis technology such as single-crystal X-ray diffraction greatly facilitated the systematic study of topochemical polymerization and kept the popularity till these days.
If [2+2] cycloaddition is involved in the polymerization, then the alignment of double bonds within the crystal should fulfill the aforementioned Schmidt's criteria.
By comparing their polymerization reactivity and slightly different structure, the suitable range of lattice parameters can be derived.
[16] Although the movement of the mass center of the monomer is restricted by the crystal during the polymerization, the slight change of the bond length before and after the reaction give rise to the shifting of lattice parameters.
[17] Another possible routine is to introduce "soft" parts (C-C or C-O bond free to rotate instead of rigid conjugated system) in the monomer molecule.
[18] Light irradiation can initiate the reaction while avoiding exerting additional physical effects on the monomer crystal.
In some circumstances, however, the polymerization initiated by UV light is so slow that unbalanced pressure will accumulate more easily as previously stated.
Contrary to the light-initiated topochemical polymerization, the lower temperature and slower reaction rate would produce high quality polymer crystals.
[24] Interestingly, no polymerization was observed under light or heat due to the unfavorable distance between diacetylene units.
Few examples of polymer single crystals prepared in this way suffered from low quality and small size.
[26] Topochemical polymerization provides a potential solution to yield high-quality polymer single crystals.
