What are Polyisobutylene and Butyl Rubber? (Executive Summary)

What are Polyisobutylee ad Butyl Rubber? Polyisobutylee (PIB) ad butyl rubber (IIR) belog to a class of materials called polymers (i.e. poly = may mers = uits) or macromolecules (e.g. macro = large molecules). 1 Polymers are large molecules that are built from the chemical likig of smaller buildig blocks typically referred to as moomers. PIB (1) is a homopolymer of isobutylee (i.e. a polymer built completely of isobutylee uits {Figure 1}) whereas butyl () is a copolymer of isobutylee ad isopree (i.e. a polymer built of isobutylee ad isopree uits {Figure 1}). Butyl rubber is CH C CH C CH C CH CH 0.985 0.015 1 Figure 1. Repeat Uits for PIB (1) ad Butyl (). cosists primarily of isobutylee (ca 98.5 mol %) ad mior amouts of isopree (ca 1.5 mol %). Polymerizatio of isopree results i the icorporatio of a alkee (i.e. double bod or usaturatio) ito the polymer chai. These double bods serve as crosslikig sites (i.e. sites where oe polymer chai ca be chemically liked to aother). Vulcaizatio of the butyl copolymers results i the formatio of a etwork structure i the form of a crossliked rubber (Figure ). I this case every butyl polymer chai is liked to each other to form oe macroscopic molecule (e.g. the ier tube is actually oe molecule). As such butyl is a thermoset polymer ad oce vulcaized it caot be reformed ito a ew shape. PIB is a thermoplastic polymer ad ca be reshaped by applicatio of heat/pressure sice oe of the idividual PIB chais are chemically liked together. CH C CH C CH CH S S CH C CH C CH CH Figure. Example Crosslik of Sulfur Vulcaized Butyl. The Basics of Carbocatioic Polymerizatio The chemical structure of a moomer determies the exact way or mechaism by which the likig reactio operates durig the formatio of a particular polymer. I the case of PIB ad butyl the mechaism ivolves carbocatioic itermediates (i.e. carbo species that have a deficiecy of electros givig them a positive charge {Figure 3}). This deficiecy of electros R R C R R = uspecified substituet Figure 3. Geeric Structure for a Carbocatio. makes carbocatios extremely reactive ad highly acidic. They will literally react with the weakest of bases (e.g. isobutylee moomer) uder reactio times of less tha oe aosecod (i.e. oe billioth of a secod). Polymerizatio begis with a iitiatig evet that ca occur i a

variety of ways; all of which ultimately geerate a carbocatio. This process is termed iitiatio ad each carbocatio formed durig this process has the ability to geerate oe or more polymer chai(s) (Figure 4). Uder ideal coditios each carbocatio formed durig iitiatio gives rise H + G + CH C Figure 4. Iitiatio by Protoatio. to oly oe polymer chai; however, due to side reactios (e.g. chai trasfer, termiatio) this is ot always the case. Moomer uits are the coected by successive additio of moomer to the carbocatio resultig i growth ad simultaeous regeeratio of the carbocatioic chai-ed (Figure 5). This process is termed propagatio ad ca occur repetitively thousads of times i CH C + CH C C CH C CH C CH Figure 5. Propagatio. a extremely short period of time (ca < 1 secod). Growig polymer chais ca udergo complex reactios collectively kow as chai trasfer (Figure 6). The et result of chai PIB CH H 3 C δ G δ PIB C C + C PIB CH H C CH C + H δ CH δ CH δ G C PIB CH + CH C H C H PIB CH + CH C CH PIB CH + Figure 6. Couteraio Assisted Chai Trasfer ad Spotaeous Chai Trasfer. trasfer is that a growig polymer chai is trasformed ito a dead chai with cocomitat productio of a ew active carbocatio that ca iitiate growth of a ew polymer chai. The

fial mai reactio that ca occur is trasformatio of a growig polymer chai ito a dead polymer chai, a process referred to as termiatio (Figure 7). Termiatio ivolves the reactio Figure 7. Termiatio by Collapse of a Io Pair. of a carbocatio with aother species i such a maer as to geerate products icapable of iitiatig the growth of ew polymer chais. Uses ad Market Data for PIB ad Butyl Rubber Depedig o the umber average molecular weight ( M ) physical properties for PIBs ca rage from liquids to semiliquids ad solids. PIBs have excellet stability to a wide rage of chemicals ad are soluble i a umber of hydrocarbos ad chlorohydrocarbos. They have a low glass trasitio temperature, low specific thermal coductivity, low water permeability, ad high dielectric stregth. Because they do ot corrode metals or leave carbo residues whe volatilized or depolymerized low M PIBs are useful as lubricats, additives, ad viscosity modifiers. Uses for higher M PIBs iclude sealats, ad adhesives. Traditioal methods a for the preparatio of high molecular weight PIBs ( M > 10 5 g mol -1 ) require polymerizatio temperatures -100 C i order to reduce chai trasfer to a acceptable level. I some cases b-f (processes usig AlCl 3 ) toxic solvets (e.g. methyl chloride) are required for polymerizatio; strict regulatios prohibit the buildig of ew or expasio of existig plats that require such materials. BASF is the largest maufacturer of PIBs (10 6 metric tos year -1 with costs ragig $0.8 kg -1 for low M grades ad $1.7- kg -1 for medium to high M grades). 3 Low M PIBs bearig a high (> 80 %) percetage of termial usaturatio (i.e. reactive PIBs) are very desirable as lubricatio additives but a cost effective, o-pollutig system for their productio has ot bee disclosed. f,4 Vulcaized butyl has the lowest gas ad moisture permeability of all elastomers, excellet heat/ozoe resistace, ad high dampig properties. f-i The mai use of butyl is i tirecurig bladders ad ier tubes. Commercial processes for the maufacture of butyl are costly ad detrimetal to the eviromet as they operate at temperatures -100 C ad require the use of toxic methyl chloride. f-i Legislatio desiged to protect the eviromet prevets the expaded use of methyl chloride i such processes. Curretly > 760,000 metric tos of butyl is produced each year at a price of $.8-3 kg -1. f,5 Exxo ad Laxess are the major maufacturers of butyl rubber. Uses for butyl rubber o a percetage basis iclude: 1. Tires, tubes ad other peumatic products = 80 %. Automotive mechaical goods = 9 % 3. Adhesives, caulks ad sealats = 6 % 4. Pharmaceutical = 4 % 5. Miscellaeous = 1 %

Refereces 1. Two good geeral refereces for polymer sciece are: (a) Odia, G. Priciples of Polymerizatio; 3 rd Ed.; Joh Wiley & Sos, Ic.: New York, 1991. (b) Billmeyer, F. W., Jr. Textbook of Polymer Sciece; 3 rd Ed.; Joh Wiley & Sos, Ic.: New York, 1984.. (a) Otto, M.; Müller-Curadi, M. Ger. Patet 641,84 (1931). (b) Güterbock, H. Polyisobutee, Spriger Verlag, Berli, 1959. (c) Immel, W. Polyisobutee i Ullma s Ecyclopedia of Idustrial Chemistry, 5 th ed.; Elvers, B.; Haskis, S.; Schulz, G. Eds.; VCH Publishers: Weiheim; 199; Vol. A1, pp. 555-561. (d) Keedy, J.P.; Marechal Carbocatioic Polymerizatio, Wiley-Itersciece, New York; 198; Chap. 10. (e) Kresge, E. N.; Schatz, R. H.; Wag, H-C. Isobutee polymers i Ecyclopedia of Polymer Sciece ad Egieerig, d ed.; Elvers, B.; Haskis, S.; Schulz, G. Eds.; Wiley-Itersciece: New York; 1987; Vol. 8, pp. 43-448. (f) Vairo, J. P.; Spassky, N. i Catioic Polymerizatio; Matyjaszewski, K. Ed.; Marcel Dekker: New York, 1996; p. 683-704. (g) Thomas, R. M.; Sparks, W. J. U.S. Patet,356,18 (1944). (h) Duffy, J.; Wilso, G.J. Sythesis of butyl rubber by catioic polymerizatio i Ullma s Ecyclopedia of Idustrial Chemistry, 5 th ed.; Elvers, B.; Haskis, S.; Russey, W.; Schulz, G. Eds.; VCH Publishers: Weiheim; 1993; Vol. A3, pp. 88-94. (i) Duffy, J.; Wilso, G.J. Halobutyl rubber i Ullma s Ecyclopedia of Idustrial Chemistry, 5 th ed.; Elvers, B.; Haskis, S.; Russey, W.; Schulz, G. Eds.; VCH Publishers: Weiheim; 1993; Vol. A3, pp. 314-318. 3. Marsalko, T. (BASF) persoal commuicatio, 005. 4. (a) Rath, H. P.; Hah, D.; Sadrock, G.; Deyck, F.; Straete, B. V.; Vree, E. D. US Patet 6,753,389 (004). (b) Nola, J. T., Jr.; Chafetz, H. US Patet 3,04,6 (196). (c) Nola, J. T., Jr.; Chafetz, H. US Patet 3,166,546 (1965). (d) Vierle, M.; Schö, D.; Bohepoll, M.; Küh, F. E.; Nuyke, O. CA Patet,41,688 (003). (e) Guerrero, A. Kulbaba, K. Bochma, M. Macromolecules 007, 40(1), 414-416. 5. Kirscher, M. Chem. Market. Rep., 005, Jue 6-15, 34.