Synthesis and Polymerizability of Atom-Bridged Bicyclic Monomers

The synthesis and polymerizability of atom-bridged bicyclic monomers was surveyed. The monomers included lactams, ureas, urethanes, lactones, carbonates, ethers, acetals, orthoesters, and amines. Despite widely-varying structures, they almost all polymerized to give polymers with monocyclic rings in the chain. The polymerizations are grouped by mechanism: uncoordinated anionic, coordinated anionic, and cationic.


Introduction
Ring-opening polymerizations, which convert cyclic monomers into linear polymers, are a major type of polymerization.A recent authoritative treatise [1] covered ring-opening polymerization of monocyclic monomers; this Review covers ring-opening polymerization of atom-bridged bicyclic monomers."Atom-bridged" means all three chains connecting the bridgehead atoms contain at least one atom.Bicyclic compounds with bridgehead atoms directly attached to one another will not be considered here because they usually polymerize like monocyclics.Alkene metathesis of bicyclic monomers has been well-reviewed elsewhere and will not be included here.
The reactions are arranged below according to mechanism: UNCOORDINATED ANIONIC POLYMERIZATIONS of lactams, ureas, and urethanes COORDINATED ANIONIC POLYMERIZATIONS of lactones and carbonates CATIONIC POLYMERIZATIONS of ethers, acetals, orthoesters and amines

OPEN ACCESS
Although these monomers may appear exotic, they are often synthesized rather easily.Hydrogenation of suitable benzene derivatives followed by cyclization has been used most often.This is easy for industrial research laboratories equipped with high pressure equipment but less so for academic laboratories, perhaps accounting for the limited data in this field.
All of the syntheses involve the possibility of forming linear polymer directly instead of bicyclic monomer.The synthesis conditions may have to be adjusted to favor the formation of bicyclic monomer.These include dilute solution, high temperature, and other factors.
Since the polymer structure is usually obvious from the monomer structure, the polymer structure is only given below for the first example and in some later cases where more than one polymer structure is possible.
Lactam synthesis usually involves removal of a small molecule from an amino acid derivative, often by simply heating the amino acid under vacuum to remove water.Cyclization of an aminoacyl chloride hydrochloride with triethylamine has also been employed.The polymerizations are catalyzed by the lactam anion, assisted by cocatalyst N-acyllactam.Bicyclic amides 1-16 (except the one in brackets) have been converted to polymers; the polymer is shown for monomer 1 only.
When the carbonyl group is attached to the ring in the initial polymer, it may be epimerized by the strongly basic lactam anion (Scheme I).
Scheme I. Ring-opening of bicyclic lactam 3 to give either cis or trans polymer [3].Oxabicyclic lactam 17 (Scheme II) and derivatives were synthesized from starting material 17 sm by internal addition of the carboxamide group to the dihydropyran group.Although monomer 17 is both a bicyclic lactam and a bicyclic ether, it has only been polymerized as the former by an anionic mechanism; cationic ether polymerization is excluded because of the basic amide group.This topic has been reviewed [15][16][17].
An especially interesting class of lactam and related N-containing monomers contains a bridgehead N next to a carbonyl group.These are greatly destabilized relative to normal amides because the usual N-CO resonance is absent according to Bredt's Rule, which forbids the occurrence of double bonds at bridgeheads as pointed out by Lukes [5].Yakhontov also discussed this problem [6].Pracejus [7] synthesized 1-aza-6,6-dimethylbicyclo[2.2.2]octan-2-one, while recent workers synthesized the parent structure as a salt by a nitrene insertion reaction; the free lactam could not be generated from the salt [8].Although polymerization studies were not reported, these anti-Bredt lactams undoubtedly polymerize, being destabilized by their boat form as well as Bredt's Rule violation.The effect can be offset with an adjacent heteroatom, or in ureas and urethanes; the results of these studies are discussed below.

Amines (58-59)
Finally, bicyclic amines 58 and 59 (Table 8) have also been polymerized cationically via S N 2 displacements on ammonium ion intermediates [67].1,4-Diazabicyclo[2.2.2]octane (58), widely used as a catalyst, is now seen to be a monomer itself.N H 59 [69] 2.9.Attempted Correlations with Monomer Properties 2.9.1.Infrared Carbonyl groups in small rings absorb infrared radiation at much higher frequencies than carbonyls in open chain analogs.However, we determined that the carbonyl absorption did not correlate with polymerizability of monocyclic monomers [70].This also proved true for bicyclic monomers [71].The higher carbonyl absorption frequency in small rings is attributed to orbital hybridization effects.

Saponification Rates
We tried and failed to correlate saponification rates with polymerizability for monocyclic monomers [72].Our thought was that strain relief in polymerization might parallel strain relief in saponification reactions.However, no parallel was found [71].The same was true for bicyclic monomers [73].The reason is that formation of tetrahedral intermediates, not ring opening, is the rate-determining step.

Dipole Moments
We noticed that lactones possess much higher dipole moments than their open-chain analogs and wondered whether this factor might be related to polymerization.However, this proved not to be the case [74].For example, γ-butyrolactone and δ-valerolactone both have high dipole moments, yet only the latter polymerizes.

Polymer Properties and Uses
The physical properties of the polymers described herein have not been investigated to any great extent.Ring-opening polymerization of an atom-bridged-bicyclic monomer gives a linear polymer containing a ring in the chain.This often imparts desirable physical properties to the polymer, including increased melting point and glass transition temperature [75,76].Polymerization of bicyclic ether 42 gives a crystalline polymer melting at 250 °C.while the polymer from tetrahydrofuran melts at 50 °C [32].Given that the polymerization mechanisms for the bicyclic and monocyclic monomers are the same, the synthesis of new copolymers can be anticipated.

Conclusions
The polymerizabilities of the atom-bridged bicyclic compounds in this review are listed in Table 9.
They almost all polymerize!The sole exceptions are bicyclo[3.3.1]nonanes; even here, N-bridgehead anti-Bredt lactams do polymerize.This surprising generalization requires explanation.For the small ring sizes (three and four members), angle strain and eclipsing interactions are important.For the common ring sizes (five to seven members), eclipsing interactions dominate.Often the six-membered rings in bicyclic monomers are locked into either boat or chair forms (the boat forms of cyclohexane are destabilized by about 8 kcal/mole relative to the chair forms).For larger rings, transannular hydrogen crowding as well as entropic considerations come into play.The monomers described possess combined 5-and 6-membered rings.Although no significant angle strain occurs, ring fusion causes eclipsing H-H repulsions.The cyclohexane rings in many of the monomers are locked into boat forms with strain energies of ∼8 kcal/mole.The non-polymerizability of bicyclo [3.3.1]nonanespossessing two stable chair forms supports this line of thought.Second, amide and ester bridges are forced into energetically unfavorable cis conformations.Third, entropic considerations favor polymerization; chain polymers have many more available conformations than the rigid bicyclic monomers.Fourth, the bridgehead N-C=O structure present in the anti-Bredt monomers described above is a powerful driving force for polymerization.Fifth, isomerization of an initially formed cis-disubstituted ring to a thermodynamically favored trans-disubstituted ring is another driving force in cases where the carbonyl group is attached to the ring and epimerization is permitted.Bicyclic monomers possess these attributes to varying degrees, but the sum is almost always sufficient to cause polymerization.
Unsymmetrical acetals like 46 can cleave in either or both of two ways (Scheme III).Scheme III.Ring-opening of an unsymmetrical acetal.

Table 9 .
Polymerizability of bicyclic monomers in this review.