Stereochemical control by inclusion polymerization

Abstract

Inclusion polymerization consists in the polymerization of unsaturated monomers inserted, as guest molecules, into the crystal lattice of a different host molecule. Polymers thus (ibt;iined often possess a high constitutional and configurational regularity. I n some cases they also present optical activity and an extended-chain macroconformation in the nascent state. Structural, kinetic,and thersodynamic studies provide evidence that the reaction takes place in the solid phase. Various types of reaction control are examined and data are presented, showing that asymmetric induction is transmitted through space and not through bonds.It is significant that a large part of the Florence Symposium o n Macromolecules i s dedicated to solid-state polymerization. Crystals are characterized by a very high degree of order and it seems obvious t o utilize crystalline compounds in some way to induce order into macromolecules. This has artually been done in various ways, the most important of whirh concerns the use of heterogeneous ciitalysts. Crystalline l'iC1, in its a,y,&-forms and MgC1;-supported titanium halides are the constituents of stereospecific catalytic systems f o r a-olefin polymerization. Other lines of research concern direct polymerization of crystallinemonomcrs or polymerization of monomers trapped in a crystal matrix. My subject is a specific treatment of this last problem, the polymerization of monomers inserted as guests into the crystal lattice of other substances. This kind of polymerization is indifferently called inclusion polymerization, polymerization in channel-like complexes, canal polymerization, and so on. My own interest in this field dates back t o about 1 5 years ago, when I was working at the Polytechnic Institute of Milan with Professor Natta. At that time I discovered a new very versatile host, the trann~-ant~-t.r_ans-anti-traosisomer of perhydrotriphenylene (PHTP)(I,2). In the last years of his scientific activity, Professor Natta took an enthusiastic part in this research, which combined two of his main interPsts: crystallography and macromolecular chemistry. My cooperation with Professor Allegra, to whom we owe the determination of the crystal structure of the new class of inclusion compounds, also dates back to that time. The results of inclusion polymerization in perhydrotriphenylene u p to 1974 were reviewed by myself at a previous IUPAC Symposium o n Macromolecules ( 3 ) . Pclyrnerization in urea and thiourea was reviewed by Chatani in the same year ( 4 ) . I n this paper I wish to illustrate the development of the research carried o u t during recent years. I shall particularly deal with thermodynamic aspects of inclusion polymerizacion, with the search for new hosts and with the control of consticut i o n , configuration and macroconformation observed in the different cases, with attrnrion to polymerizations carried out i n perhydrotriphenylene.

Metal Free Reversible-Deactivation Radical Polymerizations: Advances, Challenges, and Opportunities

“…This not only minimizes the risk of side reactions but also allows for partial stereo-control which is not common in radical polymerization because of the planarity of the generated sp 2 hybridized radicals. For this reason, polymerizations in the presence of additives and mediators or by using monomers with predetermined stereo specificity are commonly employed techniques to still gain control over stereospecifity [ 132 , 133 , 134 ]. Partial stereo-control, however, could also be achieved lately by photo NMRP where approximately 60% syndiotactic content was obtained [ 135 ].…”

“Absolute” Asymmetric Polymerization within Crystalline Architectures: Relevance to the Origin of Homochirality

“…Farina and Natta [9][10][11] provided the first successful demonstration of such an induction process in the polymerization of achiral pentadienes as inclusion complexes of enantiopure perhydrotriphenylene (Scheme 8.1) -that is, a hydrocarbon molecule that crystallizes into a hexagonal form. In the crystal, the enantiopure perhydrotriphenylene molecules form enantiomorphous channels that can be filled with achiral guest molecules of suitable shape.…”

“…In the crystal, the enantiopure perhydrotriphenylene molecules form enantiomorphous channels that can be filled with achiral guest molecules of suitable shape. Farina and Natta showed that the guest trans-l,3-pentadiene would undergo, as induced by γ -rays, a 1-4 isotactic trans-addition via ''through-space'' and not ''through-bond'' within the confined environment of the channel, to yield an asymmetric polymer which contains chiral C-atoms [10,11]. The same method has been successfully applied to the synthesis of asymmetric block copolymers composed from achiral blocks of isoprene and the homochiral blocks of trans-1,3-pentadiene [6].…”

“…10 Cartoon representing a racemic template, composed of isotactic oligopeptide chains [shown as sequences of left-and right-hands], exerting a regioenantioselective incorporation of new monomers (shown as single isolated left-and right-hands) during the chain-elongation step of the polymerization. 11 Proposed chain-elongation mechanism via the formation of racemic antiparallel (ap) β-sheets comprising alternating oligo-R and oligo-S chains, both with the enantiopure initiator S-Phe-OMe, I s , at their C terminus, as modeled on the basis of the (RS)-PheNCA monomer crystal structure viewed along the c-axis.…”

Stereocontrol of Methyl Methacrylate during Photoinduced Nitroxide-Mediated Polymerization in the Presence of Photosensitive Alkoxyamine

“…However, radical polymerization is much less successful in controlling tacticity because of the radical nature of the propagation step such that the planarity of the generated radicals (sp 2 hybridization) offers similar probabilities of meso and racemic addition, resulting in atactic polymers. Several approaches have been established to control tacticity by polymerization under confined media such as the addition of solvents or additive-mediated system, even by polymerizing monomer with inherent stereospecificity [6][7][8].…”