Synthesis of Telechelic Polymers Initiated with Selected Functional Groups by Catalytic Chain Transfer

Synthesis of Telechelic Polymers Initiated With Selected Functional Groups by Catalytic Chain Transfer

ALEXEI A. GRIDNEV,1 WILLIAM J. SIMONSICK, JR.,1 STEVEN D. ITTEL2 1 DuPont Performance Coatings, Marshall Laboratory, 3401 Grays Ferry Avenue, Philadelphia, Pennsylvania 19146

2 DuPont Central Research and Development, Experimental Station, Contribution Number 8014, Wilmington, Delaware 19880-0328

Received 1 January 2000; accepted 1 March 2000

ABSTRACT: Catalytic chain transfer is found to be useful for making telechelic oligomers with a variety of initiating groups in a one-step reaction procedure. Two olefinic components are required, the first being a normal free-radical-polymerizable monomer such as a methacrylate. The second is a vicinal or other olefin generally considered to be unreactive in free radical polymerizations. Under conditions of radical polymerization in the presence of a CCT catalyst, the copolymer that results incorporates predom- inantly one molecule of the second component at the initiation of each polymer chain. The terminal end group is a geminal double bond. This geminal-disubstituted end group is radically polymerizable and would allow the preparation of functionalized arms on graft polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1911–1918, 2000 Keywords: polymers; catalytic chain transfer; oligomer; telechelics; functional groups; hydride; cobalt chelate; graft-polymers; radical polymerization

INTRODUCTION monomer.5 From a technological or commercial point of view, the high rate constants translate Catalytic chain transfer (CCT)1 remains one of into low concentrations of impurities in the final the most rapid and more selective catalytic pro- product. Generally, removal of the catalyst resi- cesses in radical polymerization. Its catalytic ef- dues from the resulting macromonomers or oli- ficiency (i.e., ratio of the rate constant of catalyzed gomers is a challenging problem made even more versus noncatalyzed reaction) is about 1010, difficult by the higher viscosities of high poly- which is comparable to the efficiency of enzymatic mers. In typical polymer applications, catalyst catalysis.2 In many cases, the rate constant of residue levels must be kept low to avoid color and Ϸ 8 ⅐ CCT, ka 10 L/mol s, is believed to be con- catalyst residues can cause problems in subse- 3 trolled by diffusion. Oligomers as low as dimers quent curing chemistry or polymer modification. and trimers can be obtained with CCT catalyst The removal of residual parts-per-million levels of concentration as low as 0.05 weight percent of the CCT catalyst residues from low viscosity products 4 monomer. For comparison, other catalytic reac- is relatively straight forward6 and requires rela- tions in radical polymerizations, like living radi- tively small quantities of sorbents and solvents. cal polymerization, require catalyst concentration The first reaction of the CCT process, hydrogen at the level of more than 2–3 weight percent of the abstraction from the propagating radical, Rn, by cobalt catalyst, LCo (where Pn is the corresponding polymer molecule with a terminal double Correspondence to: A. A. Gridnev bond, and LCo-H is the hydrido-Co(III) form of Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 1911–1918 (2000) 7 © 2000 John Wiley & Sons, Inc. the CCT catalyst) is well investigated. 1911 1912 GRIDNEV, SIMONSICK, AND ITTEL

ka the radical or oleﬁn. Thus, the more stable radi-

Rn ϩ LCo O¡ Pn ϩ LCoOH (1) cals of MMA or polyMMA may follow a reaction pathway more akin to the three-center intermediate while the concerted mechanism is more ap- The second reaction of the CCT catalytic cycle, propriate in the case of cyclic olefins that form transfer of the abstracted hydrogen atom to new less stable radical species. Recent discoveries in monomer, is the reaction of catalyst regeneration chemistry of vitamin B12 suggest that multiple and also chain initiation. mechanisms or more correctly, a continuum of reaction pathways are available to apparently similarly reactions whenever cobalt chelates are kr 11 M ϩ LCoOH ¡ R ϩ LCo (2) involved. In this article, we are reporting some 1 practical utilization of reaction 2 without regard to the exact mechanism, which may well vary for the different substrates employed. The hydrogen atom transfer from Co(III)-H to yield Co(II) and a new monomer radical is not yet well understood (M is the vinylic monomer, R1 is the monomeric radical). The difficulty arises from EXPERIMENTAL the very high reactivity of the cobalt hydride and its resulting low concentration in the reaction medium. Under normal condition of radical poly- Mass Spectrometry merization, its concentration is below detection ϩ 7(a),8 K IDS is an acronym for Potassium Ionization of limits. Despite the low steady-state concen- ϩ Desorbed Species.12 All K IDS experiments were tration, the addition of LCo-H to double bonds performed on a quadrupole mass spectrometer was thoroughly investigated.9 The radical formed (Finnigan model 4615B GC/MS, San Jose, CA). in reaction 2 yields a relatively stable adduct with An electron impact source configuration operat- Co(II) to give a Co(III)-R complex and the result- Ϫ ing at 200 °C and a source pressure of Ͻ 1 ⅐ 10 6 ing Co-alkyl chelate is often formed with domina- Torr was used in all experiments. tion of one isomer. This selectivity indicates that We present an abbreviated experimental pro- hydrogen transfer (reaction 2) goes through a con- ϩ cedure for the K IDS technique because a more certed mechanism followed by homolytic cleavage detailed procedure has been published else- of the CoOC bond (3): where.13 Commercially available (Finnigan, San Jose, CA) direct-exposure probe filaments were

coated with a 10% w/w slurry of Al2O3, 2KNO3, and 2SiO2 in acetone. The coated filaments were (3) dried and conditioned by heating the filament to approximately 500 °C under vacuum. Neat sam- where EWG is an electron-withdrawing group. ples (0.1 mg) or 10% w/w solutions (0.2 ␮L) were On the other hand, hydrogen transfer from deposited onto a stainless steel ribbon adjacent to radical to catalyst by reaction 1 is believed to the conditioned filament. The filament and sam- proceed by a three-center intermediate as indi- ple were inserted into the ion source of the mass cated by kinetic data for reaction 17(h) and the spectrometer and a 1.3-A current applied to the substantial isotope effect, Ϸ 3.5.10 Thus it would filament. The application of a high current to the seem that CCT contradicts the principle of micro- filament causes resistive heating of the filament scopic reversibility that requires both forward with subsequent Kϩ emission from the alumino- and reverse reaction to have the same intermedi- silicate matrix. The sample, which is in close ate state. proximity to the filament, is heated and intact This apparent dichotomy regarding the princi- large organic molecules are desorbed. The organic ple of microscopic reversibility indicates that the molecules collide with Kϩ and are cationized. Ions reversible hydrogen exchange process between are seen as [M] Kϩ, the mass of the analyte, plus radicals and the cobalt catalyst can follow differ- 39 Da, the mass of potassium. For these studies, ent reaction pathways depending on the elec- the mass spectrometer was scanned from 100– tronic and steric nature of both the catalyst and 1000 Da/s. Under these conditions, we obtained TELECHELIC POLYMERS BY CATALYTIC CHAIN TRANSFER 1913

ϩ Figure 1. K IDS mass spectrum of copolymer of MMA2 copolymerization with BMA in the presence of CCT catalyst.

between three and ﬁve scans per sample that Copolymerization of MMA2 and BMA were averaged to yield the reported spectra. A solution of 8 mg of (tetra-p-anisylporphin)cobal-

t(II), 24 mg of AIBN, 2 mL of MMA2 and1mL Materials BMA in 10 mL of chloroform was degassed by three freeze-pump-thaw cycles and sealed in a VAZO-88௡ [azo-bis(cyclohexanecarbonitrile)] was Ϫ flask at 10 2 Torr residual pressure. The flask recrystallized from methanol and chloroform at was immersed into an isothermal circulating bath temperatures below 23 °C. MMA was vacuum for5hat75°C.Thecontent was analyzed by distilled immediately prior to the experiments. ϩ K IDS and the results are presented in Figure 1. Other olefins (Aldrich) were used as received. Cat- As is readily seen, the product is a mixture of alyst (triphenylphosphine)-bis(diphenylglyoximato)- MMA -initiated BMA macromonomers and homo- Co(III)chloride was made as reported earlier.7(e) 2 BMA macromonomers. Conversion of BMA was Another CCT catalyst, tetrakis(n-heptyloxyphe- approximately 90% and conversion of the MMA nyl)porphin cobalt(II), was obtained from Dr. 2 was approximately 10–15%. G. V. Ponomarev (Institute of Biophysics RAN). MMA oligomers were prepared by a standard Copolymerization of Olefins and MMA CTC polymerization of MMA utilizing bis(meth- ylglyoximato)Co(III)isopropyl.4 The resulting In a typical experiment,a5mLreaction flask product was vacuum distilled to yield pure dimer equipped with a Teflon௡ stopcock and magnetic

(MMA2) and trimer (MMA3), leaving higher oli- stirrer was charged with 3 mg tetrakis(n-hep- gomers behind in the residue. tyloxyphenyl)porphyrinCo, 16 mg VAZO-88௡, 1.8 mL of the nonhomopolymerizable olefin and 0.08 mL of MMA. The sample was degassed by three freeze-pump-thaw cycles and the reaction flask was sealed at 10Ϫ2 Torr pressure. The flask was placed into an oil bath and kept at 100 °C for 1 h. Then the flask was chilled, another 0.08 mL of MMA was added and the flask was degassed the same manner as described above. After1hanother0.08 mL of MMA was added 1914 GRIDNEV, SIMONSICK, AND ITTEL followed by the degassing and temperature logical question was whether this fast H-ex- treatment. change takes place between the polymeric radical In the specific example of MMA polymerization and the cobalt catalyst or Co-H and oligomeric with 2-pentenenitrile, shown in Figure 2, it is product molecules. This might be expected be- apparent that the product is a mixture of PN- cause through the CTC mechanism, these oli- initiated MMA macromonomers and homo-MMA gomers have a terminal double bond, which ap- macromonomers. pears to be very similar to that in the monomer. In most work, it had been presumed that this product molecule is “dead” in that the radical polymerization has been terminated. The question is whether Co-H is able to pass a hydrogen-

atom back to Pn thus converting it back to an active radical Rn. To the best of our knowledge, this possibility has not previously been consid- 14 Conversions of the PN were in the range of 5–15% ered in CCT-related research; hydrogen trans- while conversions of the MMA were in the range fer from a radical to a monomer has always been of 80–95%. Semibatch, starved feed copolymer- drawn with one forward arrow. Most mechanistic izations would be expected to produce higher per- investigations of CCT were carried out at low centages of the PN-initiated product with almost conversions to simplify the kinetic interpretation, total consumption of the MMA. Unreacted prod- and under these low-conversion conditions, a de- ucts are easily removed by vacuum stripping of tectable level of reinitiation of the “dead” oligo- the product mixture. meric molecules is not expected. However, under In the case of cyclopente-1-one, 1 mg of (triph- commercial production where high conversion is enylphosphine)bis(diphenylglyoximato)-Co(III)- required for economic viability, the concentration chloride was used as the CCT catalyst instead of of macromonomers can begin to approach the con- the porphyrin complex. PPVE was added to the centration of monomer in solution. reaction flask as 50% solution in 50/50 mixture of At conversions of 50% or more, the molar con- 1,2-dichloroethane and Freon 113 since PPVE it- centration of polymeric product becomes compa- self does not dissolve the cobalt porphyrin cata- rable with the concentration of monomer and Co- lyst. H may react by pathway 4 rather than 5. When the reaction was conducted in the presence of CCT catalyst, the final solution was evap- orated in high vacuum and residual oligomers were analyzed by proton NMR and KϩIDS mass (4) spectrometry. In control experiments (no CCT catalyst added), the reaction mixture was treated with hexane to cause precipitation of polymer. The collected polymer was dried under high vacuum and analyzed by proton NMR because the (5) molecular weights were too high for KϩIDS. NMR spectra were taken in CDCl3 using 300 MHz GE instruments. Of course, the relative reactivity of the two double bonds is another important factor and the relative RESULTS AND DISCUSSION stability of the two resulting products will be an issue. Since the double bonds in both monomer and oligomer have similar substituents, it is ex- Macromonomers pected that reactivity of the two species should be In our previous work7 we reported that the Co-H comparable. bond in Co(III)-H complexes undergoes reversible The effect can be further exaggerated by hydrogen atom exchange with external monomer switching from high conversion batch polymeriza- multiple times before the hydrogen atom finally tions to starved-feed polymerizations in which leaves the radical cage attached to R1. The next monomer is fed with catalyst and radical initiator TELECHELIC POLYMERS BY CATALYTIC CHAIN TRANSFER 1915 to preformed oligomer. In this case, gradual BMA was approximately one. During the poly- growth of the supposedly “dead” oligomers is pos- merization, the ratio was increasing due to higher sible: molar consumption of BMA. The average ratio during the process was thought to be3:1or4:1, though this is difficult to ascertain. Figure 1 is the KϩIDS mass spectrum of the reaction of MMA-dimer and butyl methacrylate monomer. KϩIDS is soft ionization technique in which molecular ions are produced without frag- mentation. The abundancies (y-axis) are approximately proportional to the number of molecules present in the mixture. MMA has a molecular

formula of C5H8O2 corresponding to an average molecular weight of 100.12 Daltons (Da). Vinyl- terminated MMA-dimer has a molecular formula

of C10H16O4; this species has a molecular weight of 200.24. Potassium adds 39 Da, hence the MMA- dimer is seen at 239 Da. Addition of butyl methac-

rylate (C8H14O2; 142.20 Da) to the MMA-dimer while preserving the vinyl-termination yields a

species having a formula of C18H30O6; 342.43 Da. Addition of potassium affords the ion seen at 381 Da. The cluster of peaks observed around the 381 Da ion are due primarily to the 13C-isotopes. These isotope peaks can be used in support of our molecular formula assignments and indeed, the isotope clusters do support our assignments. In a similar fashion, we can assign the 523 Da, 665 Da, and 808 Da ions to the potassiated vinyl- terminated [MMA] [BMA] , MMA[BMA] and (6) 2 2 3 [MMA]2[BMA]4 oligomers, respectively. Unfortu- nately, single-stage soft ionization mass spec- For example, reaction of previously isolated trometry does not provide any sequence informa- MMA-dimer was conducted with the addition of tion. The 323 Da, 465 Da, 607 Da, and 808 Da ions small quantities of other methacrylates in the are due to potassiated vinyl-terminated BMA ho- presence of CCT catalyst. In the case of MMA- mopolymer. All ions seen in Figure 1 are due to dimer with butyl methacrylate, most of the result- vinyl-terminated species. If any saturated poly- ing oligomeric product contained two units of mer chains were present, we expect to see signif- MMA and varying amounts of butyl methacrylate icant ions currents at 2 Da above the vinyl-termi- (see Fig.1). There are two possible mechanisms by nated ions due to the addition of two hydrogens. which the MMA-dimer could be incorporated into Other than the expected 13C isotope peaks no the polymer chain. One of them is reaction 6 and signiﬁcant ion currents are seen 2 Da above the the other is of course, the trivial copolymerization vinyl-terminated signals. of the MMA-dimer with polyBMA radical. The We may conclude that rate constants for Co-H latter process was found not to occur under the addition to the double bond of BMA might not be particular conditions employed. Control experi- substantially different from that of MMA2. It sup- ments involving polymerizations conducted with- ports our previous conclusion that CCT rate con- out the CCT catalyst gave no copolymer. Instead, stants, in contrast to propagation rate constants, MMA-dimer and poly(BMA) were isolated from are not highly dependent on radical length.15 This the reaction mixture. Hence, dimer incorporation process relies upon the known fact that methac- most likely occurs through MMA-dimer initiation rylate macromonomers do not homopolymerize or of the polymerization by pathway 6. In the exper- copolymerize with methacrylates, though they iment, the initial molar ratio of MMA-dimer to will copolymerize with acrylates. The ceiling tem- 1916 GRIDNEV, SIMONSICK, AND ITTEL

Figure 2. KϩIDS mass spectrum of copolymer of MMA and 2-pentenenitrile synthe- sized in the presence of CCT catalyst.

perature of the resulting polymers would be below the polymerization temperatures. (7)

Unreactive Olefins

This hydrogen atom exchange process with methacrylate oligomers and the resulting oligomer-initiated product can be extended to other classes of organic molecules. Consider now 1,2- subtituted olefins that are selected for their in- ability to effectively homopolymerize. This fea- The resulting polymer shown in eq 7 will have ture of nonhomopolymerization is common to both functional groups X and Y that originated with 1,2-disubstituted olefins and oligomethacrylates.1 the vicinal olefin. Previously, it was shown that the Co-H bond can Figure 2 is the KϩIDS mass spectrum of the transfer a hydrogen atom to a variety of such reaction product of MMA (100.12 Da) and 2-pen- olefins. Since the resulting radical cannot add tenenitrile (81.12 Da). All ions are seen as their another 1,2-olefin molecule, it will eventually ter- potassium adducts, [M]Kϩ. The base peak at 439 minate another radical or transfer a hydrogen Da is due to the potassium attached vinyl-termi- atom back to the CCT catalyst. The latter reaction nated MMA tetramer. Substitution of a 2-pente- is a reverse reaction and thus is unproductive. If nenitrile (81.12 Da) for an MMA (100.12 Da) af- on the other hand, we feed such a polymerization fords an oligomer that is approximately 19 Da reaction with some homo-polymerizable monomer less in molecular weight, hence the ion at 420 Da. that is highly reactive in the CCT process, then Substitution of another MMA by a 2-penteneni- the radical would add to the polymerizable mono- trile yields the ion seen at 401 Da. Similar oli- mer, thus initiating a polymerization until the Co gomer distributions are seen for the trimer, pen- catalyst terminates the propagating radical by tamers, and heptamers. Hence, the KϩIDS data CCT. We would then have a reaction scheme very indicate that in an MMA copolymerization with similar to reaction 6 but the initiating end group 2-pentenenitrile, a substantial fraction of the re- would be different. sulting oligomers have incorporated at least one TELECHELIC POLYMERS BY CATALYTIC CHAIN TRANSFER 1917

Table I. Inclusion of Nonpolymerizable Oleﬁn (NPO) into PolyMMA Chains: Ratio of MMA to Oleﬁn Refers to the Beginning of the Polymerization

NPO Nonpolymerizable Ratio Temp Incorporation1 Oleﬁn (NPO) NPO : MMA (°C) (%) Remarks

2-pentenenitrile 10 : 1 100 30 2-cyano-2-butene 5 : 1 70 10 crotonoaldehyde 20 : 1 100 60 dimethyl maleate 20 : 1 100 85 50% of this number includes 2 maleate units ethyl crotonate 20 : 1 100 80 40% of this amount includes 2 crotonate units cyclopentene-1-one 20 : 1 100 60

1 Molar percent of polymer chains containing the nonpolymerizable olefin versus all polymer chains. molecule of 2-pentenenitrile into polymer back- end groups has been demonstrated. For effective bone. Proton NMR of the telechelic oligomers con- incorporation of the vicinal olefins they should be firms the presence of geminal vinylidene protons employed as the solvent for the reaction. While with chemical shifts appropriate for methacrylate batch polymerization was utilized in this work for oligomers obtained via CCT. Hence, the co-oli- simplicity, starved-feed polymerization is ex- gomer of 2-pentenenitrile with MMA has the pected to provide better selectivity and a wider structure: choice of functional groups. The presence of a polymerizable double bond as the terminal end group makes these telechelic oligomers attractive (8) starting materials for synthesis of graft copoly- mers. The telechelic oligomers reported here could In reference experiments in which no cobalt was find applications not only in polymer science but added, molecular weights were much higher and more generally in organic chemistry. Applying the level of pentenenitrile incorporation were less three or more olefinic components and variable than one percent. feeding speed will provide a large range of prod- The chemistry is easily extended to a variety of ucts. Additional chemical modification will other olefins; these are shown in Table I. In each broaden the scope of the method. For example, case, reference experiments with no cobalt cata- the resulting double bonds can be converted into lyst added were conducted. In most cases, very corresponding tert-alkyl bromide to be utilized low levels of incorporation of the second olefin into further in ATRP. the poly(methyl methacrylate) backbone were de- tected in the absence of cobalt catalyst. Thus without cobalt, ethyl crotonate was incorporated REFERENCES AND NOTES at 5%, 2-pentenenitrile at Ͻ1%, crotonaldehyde at 4%, diethyl maleate 6%, and cyclopentene-1- 1. (a) Parshall, G. W.; Ittel, S. D. Homogeneous Ca- one at Ͻ2% as determined by the integrals of the talysis; Wiley: New York, 1992; p 85; (b) Davis, NMR signals in the final product after evapora- T. P.; Haddleton, D. M.; Richards, S. N. J. Macro- tion in high vacuum. In other control experi- mol Sci Rev Chem Phys 1994, C34, 243; (c) Davis, T. P.; Haddleton, D. M.; Maloney, D. R. Trends ments, the Mn of the resulting polyMMA were in the range of 2000–5500. In the presence of CCT Polym Sci 1995, 3, 365. catalyst, all of the number-average molecular 2. Karmilova, L. V.; Ponomarev, G. V.; Smirnov, B. R.; weights were in the range of 500 to 800 g/mol. Bel’govskii, I. M. Russ Chem Rev 1984, 53, 132. 3. Kukulj, D.; Davis, T. P. Macromol Chem Phys 1998, 199, 1697. CONCLUSIONS 4. Gridnev, A. A.; Ittel, S. D. Macromolecules 1996, 29, 5864. The feasibility of utilizing CCT catalysts in the 5. (a) Uegaki, H.; Kotani, Y.; Kamigaito, M.; Sawamoto, synthesis of telechelic oligomers with different M. Macromolecules 1998, 31, 6756; (b) Kotani, Y.; 1918 GRIDNEV, SIMONSICK, AND ITTEL

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