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Advances in Polycarbodiimide Chemistry

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Advances in Polycarbodiimide Chemistry Polymer 52 (2011) 1693e1710 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Feature Article Advances in polycarbodiimide chemistry Justin G. Kennemur, Bruce M. Novak* Department of Chemistry, North Carolina State University, USA article info abstract Article history: Over the last 15 years, large strides have been taken with regards to synthesis, catalysis, structural Received 21 December 2010 control, and functional application of polycarbodiimides (polyguanidines); a unique class of helical Received in revised form macromolecules that are derived from the polymerization of carbodiimide monomers using transition 13 February 2011 metal catalysis. This manuscript will provide a summary of the synthesis and characterization studies in Accepted 18 February 2011 addition to the large variety of properties discovered for these systems and potential applications Available online 3 March 2011 associated with these properties. In large part, it is the chiral helical backbone coupled with synthetically selective pendant groups which has allowed creation of polycarbodiimides spanning a large range of Keywords: Helical polymer potential applications. Such applications include cooperative chiral materials, liquid crystalline materials, Chiral polymer optical sensors and display materials, tunable polarizers, sites for asymmetric catalysis, and recyclable Polycarbodiimide polymers. Manipulation of the two pendant groups that span away from the amidine backbone repeat Polyguanidine unit has allowed solubility of these polymers in solvents ranging from cyclohexane to water, maximizing Asymmetric Catalysis the potential application of these polymers in various media. Liquid Crystals Ó 2011 Elsevier Ltd. All rights reserved. Chiro-optical Materials 1. Introduction further specificity will require their full description as non-linear or helical polycarbodiimides. Before discussing the chemistry of these polymer systems, it is In 1964, Robinson first reported anionic low molecular weight first necessary to clarify some of the confusions associated with polymerization of a variety of dialkyl-, diaryl-, and diallyl-carbo- their identification. In literature, the name polycarbodiimide is diimides using n-butyllithium catalyst [12]. Radical and cationic shared between the polymers of which this manuscript will review polymerization attempts were deemed unsuccessful. Thermal and the linear polycarbodiimides derived from diisocyanates which polymerization of diethylcarbodiimide at temperatures between have garnered much attention for their dopant stabilization, cross- 115 C and 125 C was also reported but only modest yields (w50%) linking, and dehydration applications [1e11]. The reader should were obtained after a period of 30 h and product was also deemed know that the latter linear polycarbodiimides are not discussed low in molecular weight [12]. Due to the lack of control over the within this text. We refer to our systems as polycarbodiimides in polymerizations and the relatively low molecular weight obtained, a traditional sense since they are derived from carbodiimide no further work was reported. It wasn’t until 1994 when Goodwin monomers. The backbone of these polymers contains a repeating et al. reported the ability to polymerize carbodiimides in a living amidine or fully substituted guanidine-like structure and previous fashion through the use of organotitanium (IV) catalysts [13]. literature has attempted to rectify confusion by naming these Patten et al. had previously proven these catalysts to be successful systems polyguanidines. However, we feel this name is also for controlled living polymerizations of isocyanates which are misleading since the backbone nitrogens are not planar and are isoelectronic with carbodiimides [14]. The discovery of synthe- fully substituted, thus removing typical properties that are often sizing polycarbodiimides in a living fashion was monumental due associated with the guanidine or guanidinium moiety. For the to the potential properties that these polymers may obtain. purpose of consistency and simplicity we will continue to refer to One of the most prolific properties of polycarbodiimides is their our polymer systems as polycarbodiimides throughout this text but asymmetric backbone which allows the formation of excess helical sense and/or chiral polycarbodiimides. Over the last thirty years, the field of synthetic helical polymer chemistry has taken tremendous strides after large potential was realized in such * Corresponding author. Postal address: Dabney Hall, 2620 Yarbrough Dr., Box research during the early 1950’s. It was during this time when the 8204, Raleigh, NC 27695, USA. Tel.: þ1 919 515 2996; fax: þ1 919 513 2828. E-mail addresses: [email protected] (J.G. Kennemur), [email protected] combined efforts of Pauling, Watson, and Crick revealed the helical (B.M. Novak). macrostructures of proteins and DNA [15,16] and this revelation 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.02.040 1694 J.G. Kennemur, B.M. Novak / Polymer 52 (2011) 1693e1710 was soon followed by the discovery that helical polymers could be many of the bulkier BINOL based catalysts (Cat-7e10)were synthesized in a laboratory when, in 1955, Natta revealed that synthesized to reduce aggregation [42,43]. This is also the stereo-regular isotactic polypropylene exhibits a helical structure in reasoning behind using bulkier alkoxide initiator ligands such as the crystalline solid state [17]. Since that time, further under- isopropoxy (Cat-4,5,8) or tert-butoxy (Cat-6,7,9). standing of the advanced functions governed by biological helices Of the BINOL catalysts made, only Cat-7, 9, and 10 were found to has propelled research into a variety of synthetic helical polymers be monomeric as a solid crystal, the most notable being Cat-7 due designed for frontier applications. These polymers presently to its moderate activity and good yields of resulting polymer encompass an assortment of carbon and heteroatom-based back- [42,43]. Even though crystals of these catalysts are shown to be bones synthesized from an even larger variety of catalysts. An in- monomeric, aggregation behavior may be different when dissolved depth review of the current advances in chiral macromolecular in various solvents. The tridentate BINOL based titanium(IV) cata- design lies outside the scope of this manuscript, however, the lysts (Cat-11e14) were made in an effort to maximize helix-sense- reader is encouraged to refer thorough and well written reviews of selective polymerization however reaction times were very slow this subject and the references therein that have been previously and resulting polymers were of low yield (36%) and likely low in published [18e39]. Relevant advances as they pertain to helical molecular weight [43]. The general trend is that bulky BINOL ligand polycarbodiimide chemistry will be discussed vide infra. based titanium catalysts are less aggregated but, as a consequence, polymerize carbodiimides much slower. Additionally, if the carbo- 1.1. Catalysis of polycarbodiimides diimides themselves have bulky pendant groups, polymerization may not occur at all. The achiral trichlorotitanium(IV) tri- Since the discovery of Cat-1e3 (Fig. 1) and their ability to fluoroethoxide catalyst (Cat-15) is a highly active catalyst. The three polymerize carbodiimides in a living fashion [13], a variety of chloride ligands are less bulky than BINOL ligands and they also titanium, copper [40], nickel, and zirconium catalysts [41] have increase the activity of the titanium center by inductively with- been synthesized (Fig. 2). The titanium (IV) catalysts are air and drawing electron density. As a result, Cat-15 allows fast, controlled moisture sensitive requiring the use of a Schlenk apparatus under polymerizations and provides high polymer yields [46]. Conse- an inert nitrogen blanket or an inert dry box system for synthesis quently, since there is no chiral influence from this catalyst, it is not and polymerizations. The amine and alkoxide ligands that originate a candidate for helix-sense-selective polymerization of achiral on the titanium center are known as the initiation or transfer monomers. ligands. Upon initial coordination of the first carbodiimide nitrogen Cationic dicyclopentadienyl-methyl zirconium(IV) catalyst with onto the titanium metal center, this ligand transfers onto the a large chiral tris(tetrachlorobenzenediolato)phosphate(V) counter electrophilic center carbon of the carbodiimide (Scheme 1). As the anion (Cat-24) has shown some initial success toward zirconium polymerization progresses and sequential carbodiimides insert, based polymerizations [41]. The larger metal center belongs to the the initiator ligand remains on the polymer as an end group while same periodic group as titanium and was shown to also success- the rest of the catalyst carries along the propagating chain end. The fully polymerize achiral symmetric and asymmetric carbodiimides retained ligands on the metal center can be chiral or achiral and such as N-(methyl)-N0-(phenyl) carbodiimide (92% yield of polymer their size and inductive effects can greatly alter the kinetics of reported) [51]. The large chiral anion was designed to govern the polymerization. The 1,10-binaphth-2,20-ol (BINOL) ligand was direction of insertion and propagation of the polymer chain and employed as a chiral ligand for titanium catalysts to induce helix- achieve helix-sense-selective polymerizations but
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