Haloboration initiation

The convenient synthesis of a-hydroxyl-co-methoxycarbonyl asymmetric telechelic PIBs has been achieved by the combination of two recently discovered techniques, haloboration-initiation and end capping with 1,1-diphenylethylene followed by end quenching with silyl ketene acetals, 1 -methoxy-1 -trimethylsiloxy-2-methyl-propene (MTSMP), 1-methoxy-1-trimethylsiloxy-propene (MTSP), and 1-methoxy-l-trimethylsiloxy-ethene (MTSE). Nearly quantitative chain end functionalization has been proved by NMR, quantitative NMR, and FT-IR spectroscopy. The methoxycarhonyl end arising by quenching with MTSMP could not be hydrolyzed under either basic or acidic conditions. These methods also failed to yield the acid when the corresponding diisobutylene derivative was used. The sterically less hindered esters, however, readily underwent hydrolysis resulting in the formation of a-hydroxyl-co-carboxyl asymmetric telechelic PIBs. 

While BXa must be used for haloboration-initiation in order to obtain the precursor for the hydroxyl head group, capping was slow and incomplete using BCI3 [77]. Complete capping was only achieved in conjunction with TiCU. Thus after the complete polymerization of IB with BCI3 at -40 C in DCM, Hex and DCM were added to reach the DCM/Hex= 60/40 v/v ratio and the desired concentrations folldwed by the addition of TiCU and DPE at -80 C. 

The a,ft)-asymmetric polymers are available by the combination of the functional initiator and functional terminator methods. By the rational combination of haloboration-initiation and capping techniques, a series of a.w-asymmetrically functionalized PlBs have been prepared [139, 140]. Polymers prepared by haloboration-initiation invariably carry an alkylboron head group [42, 141, 142], which can easily be converted into a primary hydroxy [141] or a secondary amine group [139, 140]. To functionalize the (W-living ends, the functionalization strategy shown in Scheme 26.2 is applicable and has been used to incorporate methoxycarbonyl groups as w-functionahty [143]. 

Polymerization of IB from the PS macroinitiator was accomplished with BCl3 as coinitiator in CH2Cl2 at -78 °C. Due to the living nature of the polymerization of IB, high grafting efficiencies ( 85%) were reported. The resulting 15% homoPIB was most probably due to initiation from adventitious moisture or direct initiation (haloboration). 

Some Lewis acid-initiated polymerizations have been proposed to proceed by direct addition of the Lewis acid to the monomer s double bond. However, this is usually an exception, and has been clearly proven only for iodine [69,135] and boron halide [136,137] initiated systems. Iodi-nation and haloboration are reversible processes which produce deactivated alkyl halides due to the electron-withdrawing substituent at the neighboring carbon [Eq. (30)]. 

However, zwitterions have never been observed and the exact initiation mechanism is not known. It may occur by a one-step mechanism similar to haloboration [137]. 

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

Boron trichloride and tribromide successfully polymerize styrenes and isobutene. These Lewis acids are typically used in combination with water or alkyl chlorides, acetates, ethers, and alcohols [105,153]. In contrast to earlier reports, BC13 can self-initiate polymerization of styrene and isobutene [137] by haloboration, and subsequent activation of the resulting alkyl chlorides by excess Lewis acid. Direct initiation was confirmed by the formation of lower molecular weight polymers than pre-... 

Kinetic analysis of isobutene polymerizations initiated by BCh was recently used to distinguish between haloboration and self-ionization of the Lewis acid [Eq. (38)]. 

Trityl hexachloroantimonate-initiating systems behave similarly [145]. In contrast, only a small concentration of BCI3 is required to complete BCU-initiated polymerizations in the absence of water because haloboration (Section III.A.3.a.2) is usually slow. 

Auto-ionization of dimer Lewis acids was first suggested by Korshak and Lebedev (41). Friedel-Crafts metal halide Lewis acids show varions degrees of association in hydrocarbons. Aluminum halides can form dimers and trimers (42), but TiCh was shown to be monomeric in CH2CI2 (43). The low temperature polymerization of isobutylene in n-heptane initiated by AlBrs is an example of autoionization (44). Other possible mechanisms have been postulated for direct initiation, such as formation of the carbenium ion by the interaction of a halogenated solvent with the Lewis acid, electron transfer from the monomer to the Lewis acid, and allylic hydride transfer for a summary see Reference 45. Recently, direct initiation of isobutylene polymerization by haloboration of the monomer was demonstrated (46). 

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