Heterocyclic monomers, reactivity

Dreyfuss, P., Adaway, T., and Kennedy, J. P., Polymerization and grafting of heterocyclic monomers from reactive mono-and macrohaUdes, Appl. Polym. Symp.. 30, 183-192, 1977. 

The section on monomer reactivity would not be complete without mentioning heterocycles. 

The other limitation stems from very different structure of heterocyclic monomers and thus very different reactivity of resulting active species. As already discussed, oxonium ions may initiate the polymerization of cyclic amines, but ammonium ions would not initiate the polymerization of cyclic ethers. Thus, the sequential polymerization is possible only when the first monomer is not a stronger nucleophile than the second monomer. 

In the random copolymerization process, both types of active species should be able to participate in the cross-propagation reactions. This imposes certain limitations on the choice of comonomers in the cationic polymerization of heterocyclic monomers. Onium ions, being the active species of these polymerizations, differ considerably in reactivity thus, as already discussed, oxonium ions initiate the polymerization of cyclic amines, whereas ammonium ions do not initiate the polymerization of cyclic ethers and the corresponding cross-propagation reaction would not proceed ... 

Some heterocyclic monomers may undergo random copolymerization with vinyl monomers. This is a case of cyclic acetals (e.g., 1,3-dioxolane) which forms the random copolymers with styrene [308,309] or isoprene [310], Apparently, the oxycarbenium ions, being in equilibrium with tertiary oxonium ions (cf., Section II.B.6.b), are reactive enough to add styrene ... 

In the present communication some new results in the study of these highly living polymerizations are reported In the first part, the Influence of the presence of carbon-substituents on the "reactivity" of the azlridine monomers will be discussed In the second part, the possibility of sequential polymerization to form block copolymers is described Finally some preliminary results on the copolymerization of these azlrldines with other heterocyclic monomers will be presented ... 

General Considerations. The reactivity of heterocyclic monomers is governed by the size of the ring, nature of the heteroatom and its electronegativity and bond strength with the carbon atom, and steric factors. A detailed discussion of all these factors is presented by Penczek et al. (J ). Two basic principles will be outlined here. They will refer to the most simple heterocyclic monomers only. 

For less reactive anionic chain ends such as those involved in propagation of heterocyclic monomers, a wider range of solvents can be utilized. For example, dipolar... 

A-Heterocyclic carbenes (NHCs), used as organocatalysts, have received great interest due to their unique reactivity and selectivity observed in many different types of organic reactions (for selected recent reviews see [250-261]. More recently, NHC-mediated reactions have also been employed for polymer synthesis ([262-264] for selected reviews see [20, 265-267]), especially in the ROP of heterocyclic monomers, such as lactides [268-272], lactones [273-276], epoxides [277-279], cyclic carbonates [280], cyclic siloxanes [281, 282], and A-carboxyl-anhydrides [283, 284]. NHC-mediated step-growth polymerization has also been reported [285-287]. 

Heterocyclic Monomers.—Reviews of the polymerization of tetrahydrofuran (THF) were published. Rate constants of propagation of THF on macroesters and macroions were measured. In the polar solvent nitromethane, where macroesters are not important, it was shown that k and k t are identical within experimental error, and are not influenced by the nature of the counterion. It was postulated that the active centres are so highly solvated by monomer that free ions and ion-pairs are indistinguishable in terms of reactivity. 

High-Performance Polymers Aromatic Heterocyclic Monomers Thermally Stable Polymers High-Performance Composites Reactive Oligomers Aromatic Polyformals Star and Comb Shape Copolymers Polymers in Phase Transfer Catalysis Organometallic Polymers Poljmiers in Medical Applications Nitrogen-Containing Polymers Vinyl Organometallic Monomers Functionalized Polymers Anionic Copolymerizations Macromers... 

Pyrenemethanol, 19, is readily soluble in various epoxide monomers and has the advantage that it undergoes acid catalyzed and radical reactions that covalently bind it into the pol5mier matrix as photopolymerization proceeds. Similar considerations apply to compound 20 which, in addition to the reactive benzylic hydroxyl group, has a long alkyl chain that markedly improves solubility in a variety of vinyl and heterocyclic monomers. Like the above anthracene compounds, pyrene and its derivatives 19-21 are active as PSs for diaryliodonium, triarylsulfonium and also dialkylphenacylsulfonium salts. 

Investigations of CROP of cyclic ethers (mainly THE) provided the first thoroughly studied examples of polymerization with reversible deactivation of growing species involving equilibria between ionic and covalent (dormant) species. Studies of polymerization kinetics led to determination of rate constants of elementary reactions and in several cases equal reactivity of ions and ion-pairs in propagation was observed, which seems to be a general phenomenon in CROP of heterocyclic monomers. 

The above Lewis bases can also be utilized to initiate the polymerization of heterocyclic monomers. However, the high reactivity of the latter authorizes the use of weaker bases (for example, KOH) than those required for the polymerization of vinyl and related monomers. In this way it is possible to limit side reactions and thus to preserve the living character of the polymerization ... 

---