5. Structure of polymers

0.025, and 0.05 l mole<sup>1</sup> s

2,3-norbornene dicarboxylic acid.

Position ke, l mole<sup>1</sup> s

34 Recent Research in Polymerization

<sup>1</sup> for 2, 3, and 4, respectively. At the same time, the constant of chain

<sup>1</sup> (30С) Ea, kJ mole<sup>1</sup> A, l mole<sup>1</sup> s

1 .

1

growth remains the same for all monomers and is within the range of 0.003–0.006 l mole<sup>1</sup> s

Exo,exo- 0.47 <sup>82</sup> <sup>9</sup> 1013 Exo,endo- 0.20 <sup>105</sup> <sup>2</sup> 1017 Endo,endo- 0.02 <sup>72</sup> <sup>7</sup> 1010

cymene)]2. In the first case, the steric factor would affect both constants k<sup>2</sup> and kg.

activation energy and pre-exponential multiplier.

energy and pre-exponential multiplier.

The initiation stage of monomer 2 catalyzed by 1 is much slower than the chain growth stage. If we compare the constant of initiation and growth of monomer 2 catalyzed by 1 and [RuCl2(pcymene)]2, then we would notice that initiation catalyzed by 1 is slower than chain growth in distinction from [RuCl2(p-cymene)]2, which affects initiation in a way that it is 10-fold faster than the growth of polymer chain. Comparison of constants defining polymerization initiated by these complexes is not adequate since these complexes have different structure and may have different activation mechanisms. However, in both the cases, ester groups in endo-position are located not far enough from the double bond of norbornene ring and sterically hinder the monomer attack by double bond of ruthenium. This would affect the polymerization rate of these esters both in the case of initiation by complex 1 and in the case of initiation by [RuCl2(p-

Table 4. Effective constants and activation parameters of polymerization of three-dimensional isomers of dimethyl ester

Each monomer is determined by its own set of activation parameters different from others. To explain the way the activation parameters change, we compiled a set of monomers in ascending order to form RudO bond and intramolecular complex. Exo,endo-isomer is more prone to form RudO bond since its ester substituents are located on different sides in relation to the norbornene ring and do not hinder each other during the formation of intramolecular complex. RudO bond is more strength, and intramolecular complex is more rigid in comparison with other isomers. That is why high activation energy and pre-exponential multiplier are typical for exo,endo-isomer. Exo,exo-isomer is the second on the capability to form RudO bond. This ester is inferior to exo,endo-isomer, since its ester substituents are located on one side in relation to norbornene ring. This sterically hinders their mutual distribution necessary for the formation of RudO bond. RudO bond has less strength, and intramolecular complex is more flexible. That is why if compared with exo,endo-isomer, exo,exo-isomer is defined by lower

Endo,endo-isomer is the third on the ability to form RudO bond. Because of the way ester substituents are located inside norbornene ring, this ester cannot form strong RudO bond. Ester group in endo-position cannot properly distribute in the coordination sphere of ruthenium to form intramolecular complex. That is why this molecule possesses low activation

In the paper [41], the authors estimated reactivity of these esters using the observed polymer-

ization constant k<sup>o</sup> as the criterion for comparing reaction capacity of monomers.

The products of chemical reaction are no less valuable as a source of information for kinetic parameters. Their structure helps us to learn how reaction components interact with each other. Using NMR method to study the kinetics of metathesis polymerization of norbornene acid, ester is more beneficial since it allows estimating the structure of the obtained polymers immediately [25, 39, 42–44].

To analyze structure of polymers 2, 5–8, we used data from the study [39], which demonstrated that cis-units have resonances of olefinic protons in a stronger field in relation to transunits (Figure 15(a)).

Overlap of resonances corresponding to cis- and trans-structures occurs in polymer obtained from monomer 4 (Figure 15(c)).

The spectrum of polymer 3 is more complex if compared with spectra of polymers 2 and 4, since molecule 3 possesses chiral properties. To correlate the shifts, we applied the approach suggested in the following study [43]; it was used to analyze the structure of polymers obtained from chiral products of norbornene using NMR-spectra COSY.

Implementation of this approach alongside with assumption that resonances of olefinic protons in cis-fragments are shifted to a higher field in relation to trans-fragments [44] allowed referring resonances of olefin region to four possible structures (Figures 15(b) and 16).

Figure 15. The region of olefinic protons <sup>1</sup> H NMR-spectra of polymers obtained with polymerization of 2–4 catalyzed by 1.

Figure 16. COSY-spectrum of polymer exo,endo-2,3-dicarbomethoxy-5-norbornene.


Table 5. The number of cis- and trans-units in polymers obtained during polymerization of 2–8 over 1.

Table 5 demonstrates what of cis- and trans-units of polymers obtained from diesters of 5 norbornene-2,3-dicarboxylic acid contain.

Data given in Table 5 only offer estimative characteristic of polymers structure but allow comparing in series monomers under study. Given these data, we can highlight that polymers obtained from exo,exo-2,3-dicarbomethoxy-5-norbornenes have a similar structure. Neither elongation of radical of ester substituent nor its branching affects the ratio of cis- and transfragments. The change of substituents orientation in positions 2 and 3 in relation to norbornene ring causes the change in the number of cis- and trans-structures in the case of monomer 4. Transfer of one ester substituent from exo- into endo-position would not bring about the increase of trans-units. The situation observed can be explained if we take into consideration that there are two ways monomer molecules are attached to active ruthenium with the formation of trans- and cis-structures (Figure 17).

Figure 17. Two possible orientations 4 when attached to active form of ruthenium complex.

Table 5 demonstrates what of cis- and trans-units of polymers obtained from diesters of 5-

Monomer 2345678 Number of cis-units in polymer, % 57 54 43 56 55 56 55 Number of trans-units in polymer, % 43 44 57 44 45 44 45

Table 5. The number of cis- and trans-units in polymers obtained during polymerization of 2–8 over 1.

Data given in Table 5 only offer estimative characteristic of polymers structure but allow comparing in series monomers under study. Given these data, we can highlight that polymers obtained from exo,exo-2,3-dicarbomethoxy-5-norbornenes have a similar structure. Neither elongation of radical of ester substituent nor its branching affects the ratio of cis- and transfragments. The change of substituents orientation in positions 2 and 3 in relation to norbornene ring causes the change in the number of cis- and trans-structures in the case of monomer 4. Transfer of one ester substituent from exo- into endo-position would not bring about the increase of trans-units. The situation observed can be explained if we take into consideration that there are two ways monomer molecules are attached to active ruthenium with the forma-

norbornene-2,3-dicarboxylic acid contain.

36 Recent Research in Polymerization

Figure 16. COSY-spectrum of polymer exo,endo-2,3-dicarbomethoxy-5-norbornene.

tion of trans- and cis-structures (Figure 17).

In the case when it is attached with the formation of trans-structure, a methylene bridge of norbornene ring and bulky H2IMes-ligand hinder the monomer placement near the double bond of ruthenium. For exo,exo-derivatives, it is a more substantial hindrance if compared with ester groups which deter the attachment with the formation of cis-structures. On the contrary, for monomer 4, two atoms of oxygen in esters are more of an obstacle for the attachment to active ruthenium than a methylene bridge of norbornene ring.

Figure 17 demonstrates that both carbonyl oxygen hinder the distribution of monomer 4 near the double bond of active ruthenium in such a way that attachment with the formation of trans-unit is sterically more beneficial. This is also seen in an increased number of trans-units in polymer obtained with monomer 4. For monomer 3, only one carbonyl oxygen is a hindrance and that is why the part of trans-units in the obtained polymer remains practically the same if compared with monomer 2.

Thus, using monomers 2, 5–8, it is stated that the length and branching aliphatic radical of exo, exo-derivatives do not affect the ratio of cis- and trans-fragments in the obtained polymers. The orientation of ester substituents in relation to norbornene ring in 2,3-dicarbomethoxy-5 norbornenes affect the ratio of cis- and trans-fragments in polymers obtained from monomers 2–4. The transfer of two ester substituents to endo-position increases the share of trans-units, which is due to more substantial steric hindrances caused by carbonyl oxygen of ester monomer group and H2IMes-ligand of catalyst when forming cis-structure if compared with the obstacles caused by methylene bridge of norbornene ring and H2IMes-ligand of catalyst when forming trans-structure.
