Bicyclic monomers

Polymerizability of bicyclic monomers is generally higher than the polymerizability of disubstituted cyclic monomers. This is because closure of an additional ring usually increases the angular strain moreover, conformational strain may also be increased due to the forced unfavourable conformation in the monomer molecule. In 1,4-epoxycyclohexane (7-oxabicyclo 2,2,l heptane) the 6-membered ring has to adopt the boatlike conformation while in the chain segment a more stable chair conformation may be attained (cf. Sect. 2.6.1)  

according to Plesch et al. 25,54) enthalpies of polymerization determined from the heats of combustion of 7-oxabicyclo 2,2,l heptane and its 2-methyl derivatives are considerably higher than those for unsubstituted and substituted 5-membered rings (Table 2.10). 

The enthalpy of polymerization is markedly different for the exo- and endo-isomers. The dependence of the polymerizability on configuration has been observed also by 

Exo-exo-, and endo-exo-2,3-dimethyl derivatives weue polymerized at 0 °C with 60% yield to polymer having Mn = 1000, whereas under the same conditions exo-exo- and endo-endo-2,5-dimethyl monomers gave only 15% polymer, and for exo-endo-derivatives the yield was 25 %55). Polymerization of these monomers with Lewis acid initiators is acompanied by termination. Thus, it is not clear whether the reported limited conversions reflect genuine monomer-polymer equilibria or are due to termination (cf. p. 12). 

Another example of the relationship between configuration and polymerizability concerns the polymerization of 8-oxabicyclo 4,3,0 nonane 57)  

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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. 

CHR—CO—), polyoxy acids (—O—CHR—CO—), poly-l-alkylbuta-dienes (—CH=CH—CHR—CH2—). To the same class iKlong the polymers with two asymmetric atoms for every monomer unit, such as polysoibates (—CH=CH—CHA—CHB—, 32 and 33) where both eiythio- and thieo-diisotactic forms are chiral, or polyhexadiene (—CH=CH—CHA—CHA—) and poly-2,3-epoxybutane (—O— CHA-CHA-), 39, where only the thieo-diisotactic structures are chiral, and the polymers of some bicyclic monomers such as those shown in 41 and 42. Other examples are the polymers obtained by hydrogen transfer fk m substituted benzalacetone (79, Scheme 17) (266, 267). 

This obstacle can be overcome by moving electron withdrawing substituents away from the double bond and increasing the reactivity of double bond by positioning it in a strained ring. This is achieved using bicyclic monomers. The monomers are readily obtained from the Diels-Alder reactions of substituted olefins with cyclopentadiene. This route is effective also for fluorinated monomers. These types of monomers undergo a ROMP with a variety of one component and two-component initiator systems. 

NMR studies14 have established that it is a symmetrical dimer (A2) in dry DMF solutions. Addition of H20 causes the dimer to dissociate to a hydrated, bicyclic monomer [A(OH2)] and its dihydrate [A(OH2)2] ... 

From the copolymer composition dependence of the molar ratio of the d- and L-enantiomeric units of 15e in the copolymer, the rate of reaction of the growing chain end of 21 with the D-enantiomer of 15e was estimated to be about four times faster than that with the L-enantiomer. Such asymmetric selection is mainly ascribable to the steric and electronic interactions between the asymmetric environment created by the bulky terminal unit of 21 and the rigid bicyclic monomer having three asymmetric centers and a polar bulky bromine substituent (Scheme 6, [23]). 

Both unsubstituted bicyclic monomers and related anhydrosugars polymerize by cationic mechanism [148]. In sugar derivatives, the remaining free hydroxyl groups have to be blocked before polymerization by typical methods known in carbohydrate chemistry. 

Also the polymerization of bicyclic monomer containing azetidine ring, l-azabicyclo[4.2.0]octane (conidine), proceeds without appreciable transfer and/or termination [178]. 

The review will be limited to atom- and bond-bridged bicyclic monomers. The important work of Bailey on the polymerization of spiro bicyclo orthoesters and spiro bicyclic orthocarbonates has been adequately described elsewhere 0 1 60 3. The outstanding and systematic body of work by Schu-erch2 3> and others on the ring opening polymerization of bicyclic acetals derived from carbohydrate precursors will also not be covered here. 

A bicyclic monomer containing a maleate ester unit (M-43) undergoes a very slow 2,6-addition polymerization with 1-12 (X = Cl)/CuCl/L-l in the bulk at 130 °C to afford narrow MWDs (Mw/Mn 1.2) (11% conversion in 2 weeks).223 Enchainment of this monomer to the chlorine-terminated polystyrene increases the decomposition temperature of the polymer. 

Acrylamide-based block copolymers B-2 7370 and B-28117 were prepared by the ruthenium- and copper-based systems, respectively. The vinylpyridine (B-29)214,371 segment can be introduced into the block copolymers with MMA. The polystyrene-based block copolymers B-30 with short segments of a bicyclic monomer had a higher decomposition temperature than the homopolystyrene with C—Br terminals.223... 

A review of the synthesis and polymerization of bicyclic acetals and orthoesters Is presented, and the relationship between ring structure and the ability to polymerize Is discussed. The highly labile bicyclic acetals and orthoesters were synthesized at high tenr-perature In dloctyl phthalate under vacuum In order to remove the monomers as soon as they form. The ability of the bicyclic monomers to polymerize falls In the same sequence as the ring strain [2.2.1] > [2.2.2] >... 

Recently, the thermodynamics of polymerization of bicyclic monomers has been extensively reviewed by Hall59) and for more details one should refer to this work this is the only comprehensive review of the polymerization of this class of compounds. 

During the last two decades, there has been a growing interest in the polymerization of bicyclic monomers containing one or more oxygen atoms, i.e. monomers belonging to one of the following groups ... 

Interest in the polymerization of bicyclic compounds stems from possibilities offered in synthetic polymer chemistry by this group of monomers. Thus, polymerization of anhydro sugars, i.e. substituted bicyclic acetals, leads to synthetic biopolymers-polysaccharides or their analogs. Polymerization of certain bicyclic monomers provides systems expanding on conversion from monomer into polymer. 

Various aspects of polymerization of bicyclic monomers have been recently reviewed. Sumitomo and Okada summarized research on the polymerization of bicyclic acetals, oxalactones, oxolactams and related heterobicyclic compounds n. Yokoyama and Hall gave an authoritative review on the ring-opening polymerization... 

More recently, Hall reported that in the polymerization of some bicyclic monomers, opening of one ring leads to contraction whereas opening of the second ring is accompanied by expansion47. In the following sections experimental details concerning some typical systems studied to date are are briefly summarized. 

The polymerization of a bicyclic monomer, in which only one or both rings are opened has recently been reported by Hall47. This monomer 2,6,7-trioxabicyclo 2.2.11-heptane (TBH) may be prepared in 73 % yield by the reaction of glycerol and triethyl-orthoformate ... 

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