Monomer, isomerization

The present study reports the synthesis, characterization and thermal reactions of phenyl and carbomethoxy substituted norbornenyl imides. These substrates were designed to model the reactive end-caps of the PMR-15 resin and allow an assessment of the effect that conjugating substituents would have on the high temperature cure of such systems. The effect of these substituents on both monomer isomerization and polymerization is reported and a possible use of the phenyl substituent as a probe of polymer structure is suggested. 

One of the complexities in the direct study of the PMR cure is the superposition of monomer isomerization on the polymerization chemistry of interest. To ensure our ability to dissect these two kinds of processes, we first studied the thermal behavior of each of our model compounds in the absence of any polymer forming process in dilute solution. 

Figure 1. Combined reaction profile diagram for monomer isomerization.

Having established the effect of substitution on the rates of both monomer isomerization and polymerization, we addressed the question of polymer structure. Specifically, are norbornenyl imide units incorporated into the fully cured polymer with their norbornyl rings intact If so, does the polymer also reflect the equilibrium ratio of exo and endo ring fused monomers For our parent monomers, PN and PX, this question has been unanswerable. We have not found any direct probe that allows an unambiguous assessment of specific substructures within the cured polymer. We do, however, have some evidence bearing on this question for the phenyl substituted monomer. This evidence is attributable in part to our discovery of an unexpected side-reaction in the cure of the phenyl substituted monomer, and in part to the presence of a unique NMR diagnostic for phenyl substituted, endo norbornyl N-phenyl imides. Both of these results are detailed below. 

Endo, K., Monomer-isomerization polymerization , in The Polymeric Materials Encyclopedia, CRC Press, Inc., Boca Raton, 1996, Vol. 6, pp. 4532 -536. 

These are manifested by the formation of a polymer with a structure which can only be explained by monomer isomerization in the course of addition. An active centre rich in energy is sometimes transformed to a more stable form by a transfer of atoms or atom groups, and further monomer is added only to this more stable form... 

NMR and IR spectra of the polyacetylenes formed support that their main chain consists of alternating double bonds. No evidence for monomer isomerization prior... 

Some monomers isomerize during cationic polymerization. The net result is that the structure of the polymer repeating unit is not the same as that of the monomer. This occurs if the carbocation which is formed by initial attack on the monomer can isomerize to a more stable form. Isomerization is more prevalent in cationic than in anionic or free radical polymerizations because carbenium or oxonium ions rearrange easily. 

The application of Ziegler-Natta catalysts to monomer isomerization polymerization, in which an olefin such as 2-butene is first isomerized to 1-butene and then polymerized, has been described in an extensive series of papers by Endo and Otsu.431 The non-participation of 2-butene in Ziegler-Natta polymerization (taking precautions to exclude the possibility of carbocationic polymerization)432 also allowed the selective polymerization of 1-butene from mixtures of butene isomers.433 It was proposed that isomerization and polymerization took place on Ti(ll) and Ti(lll) species, respectively.434 The relative rates of isomerization and polymerization were dependent on the catalyst system, polymerization being more rapid with TiCl3/AlEt3 than with Ti(OBu)4/AlEt3. 

Catalysts which lead to cis polymer show a significantly higher stereoselectivity in the bond forming reaction. When (ir-allyl nickel iodide)2 modified with TiCl is employed, the 1, 2 deuterium stereochemistry is 70% dl, and 30% meso as revealed by analysis of succinic anhydride. In addition, monomer isomerization is extensive, and could account for a large fraction of the meso structures which are formed, j ke use of (ir-allyl nickel trifluoroacetate)2 as catalyst led to a similar result (32% meso), accompanied by little if any monomer isomerization. Thus, it appears that in reactions to form cis polymer, some, but not always all, of the stereochemical information present in the starting diene is preserved in the polymer. In contrast, none of the initial diene stereochemistry can be detected in the trans polymer. 

The kinetic order dependence on the active chain-end concentration is approximately 0.25 for diene propagation, while the kinetic order dependence on the active chain end concentration is approximately 1.0 for cis-trans isomerization of the chains ends [3, 56]. Thus, while the unassociated chain ends add monomer, isomerization of the chain ends occurs in the aggregated state. Since aggregation is favored by increasing chain-end concentrations, high 1,2-microstructure is observed (47% for butadiene) for high chain-end concentrations ([PBDLi] = circa 0.1 M) and high CM-1,4 microstructure (86% for butadiene) is obtained at low chain-end concentrations (circa 10 M Table 7.4). 

The development of bifunctional catalysts for specific catalytic sequences of reactions in which the product of the first reaction can serve as substrate for the second is of great importance. There are many examples of such reactions. They are, for instance, the monomer-isomerizing polymerization of heptene-2, heptene-3 and 4-methyl-2-pentene and the combination of propene disproportionation with oligomerization, etc. Bifunctional catalysts are most widely used for ethylene copolymerization with a-butene in situ in the production of so-called low-density linear polyethylene (LDLPE). All general methods for LDLPE production are based on incorporation into a PE backbone of short-chain branches, which can be made by catalytic copolymerization of ethylene with a-olefins C3-C10. A macromolecnlar ligand offers wide possibilities of joining the different types of active site in the same matrix (see also Section 12.5.2). 

DRM does not homopolymerize under the same conditions as that for polymerization of DRF, but it can homopolymerize in the presence of a radical initiator and morpholine as an isomerization catalyst via a monomer-isomerization radical polymerization mechanism [3,29]. However, since morpholine also serves as a retarder, the polymerization reactivities do not exceed those for DRF in the absence of morpholine. 

K. Endo and T. Otsu, "Monomer-Isomerization Polymerization", in Handbook of Mass and Heat Transfer, Ed., N. P. Cheremisinoff, Vol.3., pp.553-581, Gulf Publishing, 1988... 

Weak links can be formed through a variation in the linkages between monomeric units as well as through monomer isomerization. For example, in the polymerization of vinyl compounds, CH2=CHR, a certain proportion of head-to-head and tail-to-tail linkages will occur along with the... 

B.C. Gandara, M. Cafle, H.J. Jo, A. Hernandez, J.G. Campa, J. Abajo, A.E. Lozano, Y.M. Lee, Thermally rearranged polybenzoxazoles membranes with biphenyl moieties monomer isomeric effect, J. Membr. Sci. 450 (2014) 369-379. 

An unusual way to produce butene-1 copolymers with other linear olefins has been described 165,166) , these copolymers were obtained starting from p-olefins as a result of a monomer isomerization process. The isomerization of butene-2 and pentene-2 to butene-1 and pentene-1 was confirmed by the presence of the 766 cm (ethyl group) and 740 cm" (propyl group) bands, and the copolymer compositions were measured as functions of the 766Mi3so ratios 166). 

The early observations made in 1964 which opened up the field of pol3mierizations with monomer-isomerization have been followed up and extended particularly by Japanese and French researchers so that at the present time sufficient experimental and theoretical information is available for an orderly review of this interesting area of pol3mier science. 

Otsu etal. 16) found that pentene-1 pdymerizes rapidly wit ut monconer-isomerization in the presence of AlEts/VClg catalyst but pen-tene-2 does not polymerize under similar conditions. However, by the addition of Fe (acac)3, an isomerization catalyst, to the AlEts/VCI3 system, the pol3mierization of pentene-2 to po q>entene-l is effected. This observation also indicates that monomer-isomerization polymerization of -olefins involves two steps and requires the presence of two indq>endent catalytic sites one for isomerization and another for pc43unerization. 

In the coordinated anicHiic pol3unerization invoilving monomer-isomerization of butene-2 above 60° C only the homopolymer of butene-1 is obtained in spite of the presence of a large excess od butene-2 in the system. Similar results were obtained in the polymerizations of pentene-2 and hexene-2 at 80° C winch yielded by a monomer-isomerization mechanism polypentene-1 and polybexene-1, reflectively 17). It was also found that in the copolymerizations of butene-2 with propylene and 3-phenylpropene (allylbenzene) with AlEtj/TiCIa or AlEtj/VQ , catalyst with an added isomerization catalyst (seeTable 3) the homopolymers... 

The copolymers described in Table 4 were exclusively of butene-1 and pentene-1 units. This was concluded from the IR spectra of the products which were identical to those obtained by the direct copol5unerization of butene-1 and pentene-1 (Fig. 1). As indicated by the data in Table 4, both / -olefins underwent geometric and positional isomerizations during the copolymerization, and the observed isomer distributions of the unreacted olefins approached those of an equilibrium mixture (see footnote in Table 4). These results are similar to those observed in the monomer-isomerization homopolsnnerization of butene-2 to polybutene-1 8) and indicate... 

The field of polymerizations with monomer-isomerization-preceding-prppagation is quite youi. Conceptually these polymerizations are two step reactions in which the initial monomer in the charge is unable to polymerize, however, it may isomerize to give an equilibrium-mixture of potential monomers only one of which being able to pdymerize. 

The chemistry of polymerizations with monomer-isomerization-pre-ceeding-propagation is not confined to the catalysts of the Ziegler-Matta type, i.e., anionic coordinated mechanisms. For examjde, allylbenzene gives (among other products) poly-/3-methylst3 rene by a conventional cationic process, however, this s tem is not well defined because of disturbing side reactions (alkylations, etc.) also occur. 

Finally, a glance in the future. Polymerizations with dual-site (for isomerization and polymerization) catalysts as discussed in this paper rmse interesting possiblities. At least theoretically, it appears possible to couple disproportionation with polymerization. In this process, for example, the first catalyst site would disproportionate, say. butene-2 to 2 moles of ethylene which would then polymerize at another active site to polyethylene or to a copolymer of ethylene and butene-2. Or, in the presence of propylene in this system ethylene-propylene copolymers might be produced. The p< biUties and ramifications are limitless and only further research could provide the dues how such a process could be reduced to practice. Polymerizations with monomer-isomerizations on dual site catalysts might be the first crack in the door to new vistas in pol3uner science. 

This type of polymerization, the so-called monomer-isomerization polymerization, often proceeds with Ziegler-Natta catalysts (e.g., polymerization of 2-butene)... 

In the field of cationic polymerization, on the other hand, a number of examples are known in which a propagating carbocation isomerizes (rearranges) into an energitically more stable carbocation that in turn propagates This isomerization polymerization should be clearly distinguished from the monomer-isomerization polymerization that involves isomerization of the starting monomer, not the propagating species. 

Monomer-isomerization poly- or oligomerization by a cationic mechanism appears to be specific for oxo-acid initiators, because facile monomer isomerization requires a rapid proton elimination from a protonated olefin. No examples of monomer-isomerization polymerization have been reported for MX initiators. The following examples of cationic monomer-isomerization oligomerization may be helpful to demonstrate another characteristic of oxo-acid initiators. 

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