Multi-walled carbon nanotubes were functionalized by sequential HNO3 oxidation and amidation with high-molecular-weight poly(oxyalkylene)-amines (400–2000 g mol−1) as the modifying agents. These nanotube-tethering organic portions include two types of polyether backbone, hydrophobic poly(oxypropylene) (POP) and hydrophilic poly(oxyethylene) (POE). By varying these hydrophilic or hydrophobic structures and their lengths, the organically modified nanotubes can be tailored to have different dispersing properties, either in water or in organic solvents. Homogeneous dispersion further allowed the observation of the detailed morphology of individual nanotubes and their aggregated forms by using atomic force microscopy and field emission-scanning electron microscopy analyses.

The ionic exchange reaction of the smectite clays was affected by using a series of polymeric amine salts, synthesized by grafting poly(oxyalkylene)-diamines (POA-amine) onto poly(styrene-co-maleic anhydrides) (SMA). The comb-branched polyamines, comprised of a hydrophobic SMA backbone and multiple amine pendants, are amphiphilic in nature and capable of reducing the toluene/H 2 O interfacial tension (from 36 to 4 mN/m at 10 -3 wt % concentration). After acidification to cationic amines (-NH 3 + Cl -), the polyamines increased solubility in water and effectiveness for the clay intercalation. Depending on the structural variation and their amphiphilic property, the SMA-derived polyamines may intercalate with sodium montmorillonite to yield the organoclays with a broad range of XRD basal spacing from 12.9 to 78.0 Å. The intercalation profile is suspected to proceed with a critical d spacing expansion, due to hydrophobic phase separation from the embedded organics in the gallery confinement. The variations of hybrid basal spacing, TGA degradation pattern, and DSC analyses indicated the presence of at least two different types of intercalated hybrids. The finding on clay intercalating mechanism could lead to the synthesis of new organoclays for suiting versatile applications.

Introduction. Carbodiimides (CDI), easily prepared from isocyanates, 1 have been known to react with carboxylic acids to form a mixture of acid anhydrides (anhydrides), N, N′-substituted ureas (ureas), and N-acyl-N,N′-disubstituted ureas (or N-acylureas). Earlier mechanistic studies by Khorana 2,3 and Silverstein 4,5 showed that two parallel reaction pathways might account for the diversity of products observed. The initial formation of an O-acylisourea intermediate that can either rearrange into N-acylurea or undergo a further substitution reaction with another acid molecule to produce the corresponding ureas and anhydrides as final products (Scheme 1). The earlier literature indicated that aromatic CDIs seem to favor the formation of N-acylureas upon treatment of carboxylic acids, whereas aliphatic CDIs often lead to the formation a mixture of anhydrides and N,N′-disubstituted ureas. 6 Recent evidence has shown that ferrocenecarboxylic acid was able to selectively add onto an aromatic CDI to yield an N-acylurea as the main product. 7 In another report by Lau, 8 the synthesis of di-and trisubstituted N-acylureas on a solid support was also achieved in excellent yields.Although the thermal conversion of an N-acylurea into an isocyanate and an amide has been well documented, 9 the overall transformation from isocyanate to CDI and then from CDI to amide through thermolysis of the N-acylurea has not been fully exploited either as synthetic intermediates or as latent isocyanate sources. The contamination by urea and anhydride as byproducts may prevent a clean isolation of N-acylurea. To the best of our knowledge, high-yield formation of an N-acylurea as an isocyanate precursor or as an isolable intermediate for polyamide synthesis has not been developed previously. Herein, we report the preparation of aryl N-acylureas in high yield, their consequential thermal reactions, and applications in a stepwise synthesis of aryl amides and polyamides.Results and Discussion. In our recent selectivity study of CDI reactions, carboxylic acids were allowed to react separately with two CDI model compounds: dicyclohexylcarbodiimide (DCC) as an aliphatic CDI whereas diphenylcarbodiimide (DPCDI) which was prepared from phenyl isocyanate and a catalytic amount of 1,3-dimethyl-3-phospholene oxide (DMPO) as the aromatic CDI (Scheme 1). In doing so, we found two distinctive product types indicating the existence of different pathways in the CDI reaction (Table 1). Using DCC as starting material to react with benzoic acid 2a or acetic acid 2g, for example, the reaction yielded anhydrides and ureas as the major product. Low yields (38 and 25%; entry 1 and 2) of N-acylurea were observed in the product mixture. Other carboxylic acids also afforded poor yields of N-acylureas when treated with DCC.The selectivity for N-acylurea was enhanced dramatically when DPCDI was used instead of DCC. The migration of the acyl group from the O to N atom in the initial isourea seems to be dominant. For example, treatment of DPCDI with acetic acid, 2a, at ro...

We have uncovered a novel polycondensation strategy for the synthesis of well-defined polyamides of narrow molecular weight distributions based on modifications of our sequential self-repetitive reaction (SSRR) previously developed for diisocyanatedicarboxylic acid polymerization. In our newly discovered SSRR polyamide formation mechanism, a small amount of hindered carbodiimide, N,N-bis(2,6-diisopropylphenyl)carbodiimide (iPr-CDI) or a hindered isocyanate such as 2,6-diisopropylphenyl isocyanate (iPr-NCO), was introduced to the polymerization as an initiator, followed by simultaneous addition of diisocyanates and diacids monomers. By using this new reaction mode, the SSRR mechanism produces polyamide products of narrow molecular weight distributions with their dispersities reduced to 1.21.4, which is far lower than a range of >2.5 found in regular SSRR reactions. Significantly different from a conventional step-growth or standard SSRR reaction, the formation of a polymer backbone is preferential when the diacid is added to the requisite iPr-CDI in the first step, followed by a rearrangement to form amide and fragmented components for SSRR. The control of molecular weight is mainly attributed to the acid addition favoring the unhindered poly-CDI intermediates in the middle of the growing chains over the hindered-CDI at the chain terminals. It appears that the formation of a hindered isocyanate and the subsequent formation of a new hindered-CDI at the terminal end of growing amide-chains in each SSRR cycle force the acid again toward the preferred unhindered CDI sites dictating the observed outcome. This simple polyamide synthesis methodology is unique and unconventional, and it could significantly facilitate the development of tailored-made polyamides from a variety of diisocyanates and diacids