Bicyclic polymerization

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

A potential drawback of all the routes discussed thus far is that there is little control over polydispersity and molecular weight of the resultant polymer. Ringopening metathesis polymerization (ROMP) is a living polymerization method and, in theory, affords materials with low polydispersities and predictable molecular weights. This methodology has been applied to the synthesis of polyacctylcne by Feast [23], and has recently been exploited in the synthesis of PPV. Bicyclic monomer 12 [24] and cyclophane 13 [25) afford well-defined precursor polymers which may be converted into PPV 1 by thermal elimination as described in Scheme 1-4. 

Okada, M. Ring-Opening Polymerization of Bicyclic and Spiro Compounds. Vol. 102, pp. 1-46. 

Sumitomo, H. and Okada, M. Ring-Opering Polymerization of Bicyclic Acetals, Oxalactone, and Oxalactam. Vol. 28, pp. 47—82. 

Yokoyama, Y. and Hall, H. K. Ring-Opening Polymerization of Atom-Bridged and Bond-Bridged Bicyclic Ethers, Acetals and Orthoesters. Vol. 42, pp. 107—138. 

When the cnolate of an enone is brought into reaction with an enone, usually a carbocyclic system is prepared by two consecutive Michael additions (M1MIRC reactions). Due to the lower temperatures employed and the absence of diene polymerization these reactions are useful alternatives for Diels-Alder reactions and proceed in general with high diastereoselectivities. When neither enolate nor enone is cyclic a monocyclic system is formed 338 which can be converted into a bicyclic system when the Michael addition is followed by an aldol reaction339. When, however, the enolate is cyclic a bicyclic or a tricyclic system is formed340 341. 

The first cyclization gives a mixture of cis- and from -isomers and only the cis-isomer goes on to give bicyclic products. The relatively slow rate of the second cyclization step, and the formation of rrou.s-product which does not cyclize, provides an explanation for the observation that radical polymerizations of triallyl monomers often give a crosslinked product. 

Ring-Opening Polymerization of Bicyclic Acetals, Oxalactone, and Oxalactam... 

In connection with studies on the ring-opening polymerization of cyclic acetals, we have undertaken investigations on the polymerization of bicyclic acetals, bicyclic oxalactone, and bicyclic oxalactam, which yield polysaccharide analogs, macrocyclic oligoesters, and a hydrophilic polyamide, respectively, some of which can be expected to be useful as novel speciality polymers. The monomers employed in the studies were prepared via synthetic routes presented in Scheme 1, starting from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) I. 

Polymerization of 6,8-dioxabicyclo[3.2.1]octane, 2, has been most extensively studied among bicyclic acetals. This monomer is readily prepared from 3,4-dihydro-2H-pyran-2-carbaldehyde 1 by reduction with sodium borohydride followed by add-... 

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

The difference in behavior of the cis- and trans-isomers 43 and 45 is ascribed mainly to the greater ring strain in the trans-isomer which results in a more favorable free energy change for its polymerization. This result is similar to that reported for the polymerization of cis- and frans-8-oxabicyclo[4.3. OJnonane 48 and 4936. The molecular weights of the polymers of the bicyclic ether 49 were, however, much higher than those of the polymers of the bicyclic acetal 45. 

The bicyclic acetals 43 and 45 can be regarded as 4,5-disubstituted-l,3-dioxolanes. In connection with their pdymerizabilities, it is interesting to note here that cis-4,5-dimethyl-l,3-dioxolane has a slightly greater tendency to polymerize than its trans-counterpart22, 37. The polymerization of 45 is an equilibrium reaction and the system is completely reversible. From the temperature dependence of the equilibrium... 

In contrast to the fact that cyclic acetals can be polymerized only by cationic initiators, lactones undergo polymerization both cationically and anionically, and therefore a wide variety of initiators including coordinated catalysts can be used. In this section, the polymerization of bicyclic lactones is described, although only a limited number of papers on this subject have been published. 

It has also been known that the polymerization of some bicyclic lactam ethers, for example, 3-aza-10-oxabicyclo[4.3.1]decan-4-one 60 was accompanied by the cleavage of the ether bond55,56. ... 

From the kinetic viewpoint the polymerizability of 61 is considered to be higher than that of e-caprolactam, which is polymerized usually at temperatures above 135 °C63,64 Thermodynamically, the polymerization of 61 appears to be more favored than that of a-pyrrolidone, for which no polymerization is observed in THF63-65 The higher polymerizability of 61 may be attributed not only to its highly strained bicyclic structure but also to the activation of the anion 66 by the... 

All of the reactions described above use anionic alkyl metal complexes as stoichiometric reductants. Cationic zirconium catalyst 58 was shown to re-ductively cyclize a variety of 1,5-dienes to give both mono- and bicyclic silane products when H3SiPh was employed as the stoichiometric reductant (Scheme 10) [32]. Poor yields due to competing polymerization processes were observed when less substituted dienes were employed. It is likely that... 

PS-fr-PBd) star-block copolymers were synthesized by the macromonomer technique in combination with anionic polymerization and ROMP [ 158], following the procedure outlined in Scheme 83. The macromonomers were prepared with two different methods. In the first the living diblock copolymer was reacted with ethylene oxide to reduce the nucleophihcity of the living end followed by termination with 5-carbonyl chloride bicycle (2.2.1) hept-2-ene, while in the second method the functional initiator 5-lithiomethyl bicycle... 

An alternative method for the preparation of polymer-chained pendant ligands has been recently developed.102 The polymerization of the (R)-hydroxylpiperidinyl fragment was realized by ROMP of a bicyclic olefin with the commercially available Grubbs catalyst. 

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