MOLECULAR WEIGHT AND KINETICS OF ACETYLENE POLYMERIZATION

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1 MOLECULAR WEIGHT AND KINETICS OF ACETYLENE POLYMERIZATION M. Schen, J. Chien, F. Karasz To cite this version: M. Schen, J. Chien, F. Karasz. MOLECULAR WEIGHT AND KINETICS OF ACETY LENE POLYMERIZATION. Journal de Physique Colloques, 198, 44 (C), pp.c16c166. < /jphyscol:1982>. <jpa > HAL Id: jpa Submitted on 1 Jan 198 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C, supplément au n 6, Tome 44, juin 198 page C16 MOLECULAR WEIGHT AND KINETICS OF ACETYLENE POLYMERIZATION M.A. Schen, J.C.W. Chien and F.E. Karasz Polymer Science and Engineering, Materials Research Laboratory, University of Massachusetts, Amherst, MA 0100Z, U.S.A. Résumé Nous avons mis au point une technique de marquage radioactif par CH 0 H, permettant de déterminer la masse moléculaire de polyacetylenes initiés par catalyseurs Ti(OBu) / 4Al(Et). Les effets du taux de Al/Ti, de son vieillissement, de sa concentration, ainsi que ceux de la température de polymérisation et la pression du monomère ont été étudiés. Simultanément des études utilisant CO et CH,0%[ comme marqueurs radioactifs ont permis de compter le nombre de sites actifs (C), et de déterminer le nombre total de liaisons métalpolymère (MPB), définissant ainsi la cinétique de réaction. Notre interprétation des résultats nous amène à la conclusion que l'espèce active C est initialement très réactive, et qu'en moins de 0 min. elle est totalement convertie par cinétique bimoléculaire en une espèce C beaucoup moins réactive. Abstract The molecular weight of polyacetylene initiated,by Ti(OBu),/ 4Al(Et) catalyst was studied by radioquenching using CH.,0 H. The effects of Al/Ti ratio, catalyst ageing, catalyst concentration, polymerization temperature, and monomer pressure on the polymerization were investigated. Concurrently, studies using CO as.,the radioquenching agent to count the number of active sites (C) and CH 0 H to determine the total metal polymer bonds (MPB) have defined the kinetics. Our interpretation of the results lead to the postulation that the active species C is initially extremely reactive and that within 0 min. are all converted, through bimolecular kinetics, to a much less active species C. Introduction Radiolabelling polyolefins during polymerization to study kinetics and molecular weight is a well established technique. In the ethylene polymerization by (T^0,H,)TiClVAK 4 CH,) 2 C1, a 4 C labelled titanium species is formed ana polymerization proceeds via insertion of ethylene into Ti CH, bond to give radiolabelled polymer(1). 14 * Similiarly, ^C labelled Al( C 2 H,) 2 C1 was used to produce active polymerization sites on the surface of TiCl, crystallites for propylene polymerization. Mn results from GPC and radioassay were found to be in excellent agreement(2). Also CO and C0 has been used as specific labels in ZieglerRatta polymerization of ethylene and propylene(5>4) In the present work, 14 ^CO was used to label actively growing polyacetylene chains according to Equation i. 0 (D To label all polymer chains, however, a nonspecific label is needed. For this work, it was deemed impratical to use radioactive catalyst such as Al Et, because of the large quantities of catalysts used in > # acetylene polymerization. Instead, CH,0 h was used. It is incor Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE porated into the polymer chain as follows: In this communication, a brief summary of our radioabtive labelling studies is presented but with an emphisis on molecular weights. A complete treatise of the subject can, however, be found elsewhere(5,6). Experimental A description of how all starting materials were prepared, purified, stored, and transferred can be found elsewhere(5,6). For each experiment, fresh catalysts were prepared at 195 K, allowed to age for 0 min. at ambient temperature and diluted to the desired concentration before use. Catlaysts at (Ti) = 1 mi4 and an Al/Ti ratio of 4 were employed unless otherwise stated. Polymerizations were performed in a crown capped glass pressure reactor equipped with a teflon stirbar for agitation and side arm through which they could be evacuated or filled with acetylene. A constant acetylene pressure was maintained, usually 1 atm throughout the polymerization unless otherwise stated. To label all polymer chains, an amount of C E ~ O ~ in H excess of that needed to react with all hydrolyzable bonds was injected and allowed to react for one hour or more before washing. To 14c label the active polymer chains, all remaining unreacted monomer was first removed by vacuum evacuation, then I4C0 in excess of that needed to react with all organotitanium bonds was injected and allowed to react for six hours before polymer workup. Products were washed with 10% RC1/ methanol to solublize catalyst debris, then methanol, and then dried under vacuum. Radioassay by combustion analysis and liquid scintillation counting was done by Kew England Nuclear. Results and Discussion In addition to Equation 2 occurring during CB 0 Y labelling, parallel reactions for the nonradioactive methanol that is also present occur. The rate for "cold'' methanol is expected to be faster than for "hotw methanol due to differences in bond dissociation energies. The ratio of these rate constants is the kinetic isotope effect, K. To evaluate K, parallel polymerizations were carried out and different nrnounts of' CE C ~ E added as illustrated on Figure 1. ;tith large exceses of PF 0 " ' H, the specific activity is low and constant. As the

4 ,&~ ; CH,O=H added mol amount of CHgO K decreases, the 10 lo2 A gel k,/k,= 2 7 specific activity increases until 6 point a to maximum the corresponds stoichiometric value is approximately reached. amounts This of 4 h methanol and reactive organo metallic groups(indicated by the arrow). The average value of K o for subsequent determinations lo of E2 and (MPB) is taken to be a The influence _of catalyst con + centration on Mn can be seen in to2 10 " 10' Figure 2. Depending on experi 10 mental conditions, three types CH,O~~ added m~~ of polyacetylene macromorpholo Figure 1: Variation of il specific gies were obtained. kt high activity in CH, with the amount of catalyst concentrations, a poly C H ~ O ~ added. H mer film was formed on the reactor wall and a gel layer on 7 the catalyst surface. A t intermediate concentrations, both low density gel on the catalyst t; surface and powder suspended in the solution result. In the most dilute regime, only pow A ders are formed. Interestingly, L within a single preparation, samples from different zones also give different molecular weights. This behavior can be m : : " \ d,di traced to two factors. First,.COW.". monomer accessibility to the I catalyst solution below the polymer gel is retarded. Second, 1 chain transfer and termination I$, I occurs much more frequently in 10' ' the reaction medium than on the :.I reactor wall because of the Figure 2: Variation of Wn at vary abundance of catalyst components. ing (Ti) concentrations. The Al/Ti ratio was found to affect acetylene Rn. A t high Al/Ti ratios, chain transfer 6 5 ::: 0. increases the frequency of transfer and decreases the Bn. AS seen in Table 1, for polymerizations carried out at 298 K and 2 hours, %n decreased with higher c 2 I= Al/Ti ratios. Table T ' K ' AL/TI!N Figure : Variation of zn with polymerization temperature. 4/l 40,000 7/1 6,200 10/1,100

5 C166 JOURNAL DE PHYSIQUE The effect of temperature is shown in Figure. From 26 K to 4 K, Mn decreases three fold. This is due to different temperature dependencies for the indivisual kinetic processes and inherent catalyst stability. Below 26 K, 14n decreases with decreasing temperature. We believe this to be due to slow propagation rates at low temperatures. The variation of zn with polymerizatio_n time is shown in Figure 4b at 195 X. The polymer reaches a maximum Mn value within 50 min. There was no futh~r increase in Mn even though more polymer was formed. Thus, we conclude that chain gr~wth is limited by transfer processes. Many of the preceeding experiments were performed with dilute catalyst { 4 ). As a consequence, polyacetylene gel and powder macromorphologies resulted. Figure 5 shows however the results of labelling a polyacetylene film. Polymerization conditions included: (~i)= 0.2 M, Al/Ti= 4, temp.= 195 K, time= 60 min., and acetylene pressure= atm. As is seen, materials of two distinctly different molecular weights were formed; a gel layer sitting on the catalyst solution and a free standing film above it. Xhen analyzed seperately, the Mn+of different portions along the film height was found to be 11, and invarient; as compared to 5500 for the gel. Figure 4: Variation of (a) (C) and (b) Mn with polymerization time at 195 K. Fi~ure 5: Mn of CHx from a standard film preparation. References (1) Chien J.C.W., J. Am. Chem. Soc. 81, 86(1959). (2) Chien J.C.Y., J. Poly. Sci, A1, 2, 425(196). () Zakharov V,1?., Eulatov G.D., Chumaerskii N.E., Yermakov Yu I., React. Kin. Cat. Lett. 2, 247(1974). (4) Zakharov V.A., Zhedan P. A., Vernal E.E., Artamonora S.G., Kin. Katal. l6, 1184(1975). (5) Chien J.C.W., Karasz F.E., Schen N.A., Acad., in press. Hirsch J.A., Proc. Nat. (6) Schen?lea,, Karasz F.E., press. Chien J.C.W., J. Am. Phys. Soc., in