Synthesis Of Helical Polyisocyanide Copolymer Using Pd Catalyst And The Study Of The Mechanism Of Helix-sense-selective Polymerization

Polyisocyanides have attracted great attention of scientists due to their unique rigid helical structures.They can be used in a range of applications including chiral recognition,enantiomer separation and liquid crystallization.However,the post modification of helix polyisocyanides is difficult,so the synthesis of block polymers containing helix polyisocyanides is rarely reported.Helix-sense-selective polymerization is one of the most common manners to synthesis the helical polymers.Achiral monomers can be polymerized to helical polymers by using this manner with chiral catalyst/initiator,additives,or solvents.The obtained chiral helical polymers can prepare more chiral materials through asymmetric catalysis,chiral separation,chiral recognition and other methods.In this way,we can use a few chiral materials to obtain a large number of chiral products,which can greatly reduce the price of chiral materials.However,the mechanism of helix-sense-selective polymerization is not very clear.Reversible addition-fragmentation chain transfer(RAFT)polymerization is one of the most versatile controlled radical polymerizations because of its universality of monomers,mild polymerization condition and the tolerance of solvents.Various functional polymers can be prepared using RAFT polymerization with different properties.Functional chiral materials can be obtained by introducing polyisocyanides to these polymers which will expand the applications of polyisocyanides.Star block copolymer and miktoarm star polymers containing two or more different blocks on the arms have attracted great attention for their structural diversity and variety applications.However,to the best of our knowledge,reports on such kinds of star polymers are very rare,probably owing to the limited synthetic approaches.On the other hand,the mechanism of helix-sense-selective polymerization was revealed through the polymerization of isocyanides by chiral catalyst.1.In this contribution,we reported a new method for facile synthesis of block and star copolymers composed of helical poly(phenyl isocyanide)and polyacrylate segments via combination of Pd(II)-initiated isocyanide polymerization and reversible addition-fragmentation chain transfer(RAFT)controlled radical polymerization.Firstly,a alkyne-Pd(II)complex containing benzyl trithiocarbonate substituent was designed and synthesized.The Pd(II)moiety of the complex can catalyzed the living polymerization of phenyl isocyanides,while the benzyl trithiocarbonate unit can be used as a chain transfer agent for RAFT polymerization.Combination of the two living polymerizations,a series of block copolymers containing poly(phenyl isocyanide)and polyacrylate,poly(aryl vinly)blocks with expected molecular weights(M_ns)and narrow molecular weight distributions(M_w/M_ns)can be facilely prepared under controlled manner.Some of these block copolymers exhibited interesting self-assembly and stimuli-responsive properties.2.Taking advantage of the Pd(II)catalyst,core cross-linked star block copolymer carrying polyacrylate-block-polyisocyanide arms can be readily prepared through the copolymerization with a cross linker,ethylene glycol dimethacrylate.Moreover,miktoarm star copolymers carrying helical polyisocyanides and polyacrylates arms can also be prepared under controlled manner.The star polymer has unique structure.Because of the chirality,the star polymer can be used in the chiral resolution;the enantiomeric excess(ee)value of the induced crystals of model a compound,racemic threonine,is up to 95%.3.Inspired by the exquisite helices in Nature,fabrication of helical materials with controlled handedness has attracted considerable attention.Herein,we report on precis synthesis of single left-and right-handed helical polyisocyanides through living polymerization of achiral monomers using chiral palladium catalysts under helix-sense-selective manner.Mechanism study revealed that the yielded helices with opposite handedness showed different activity of the living chain end.The helix with unfavoured handedness was self-terminated,while the other one with favoured handedness showed high activity and can undergoes chain propagation to form a high molecular weight polymer with maintained single-handed helicity.