Polymerization haloboration

Recently, we explored haloboration polymerization between boron tribromide and terminal diynes to give poly(organoboron halide)s as a polymeric Lewis acid, which has scarcely ever been known [13]. These poly(organoboron halide)s were also prepared by means of hydro-boration polymerization between dienes and monobromoborane as shown in Scheme 4 [14]. As a typical example, 1,7-octadiene was added to a 1.0 M dichloromethane solution of monobromoborane-dimethyl sulfide complex with stirring at 0°C under nitrogen. The molecular weight of the polymer obtained was increased when the feed ratio of monobromoborane to diene approached or slightly exceeded unity. 

Substituted boron halides are known to be useful reagents for ether cleavage (52) and selective haloboration reactions (33-34) under mild conditions. Polymeric homologs of these materials, therefore, may have a potential to show unique properties as reactive polymers. Recently, we reported a polyaddition between boron tribromide and terminal diynes as a novel methodology for the preparation of poly(organoboron halide)s (55). We termed this haloboration polymerization. 

Table V Haloboration Polymerization between BBrs and various Diynes...

Figure 9 The -77-conjugated organoboron polymer (9) produced by haloboration-phenylboration polymerization between 2,7-diethynylflourene and Ph2BBr. (Adapted from ref. 29.)...

Conjugated organoboron polymers were prepared by haloboration-phenylboration polymerization between diyne monomers and bromodiphenylborane (scheme 29).54 The polymerization was carried out by adding a slight excess of bromodiphenylborane to a tetrachloroethane solution of diynes at room temperature... 

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)]. 

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. 

It seems that the rate constants for the deactivation of the carbocations ( r) are higher than the reported for propagation rate constants, kp [257], This indicates that polymerization of IBVE in the presence of a small amount of BC13 could have led to well-defined polymers. Unfortunately, BC13 adds to alkenes via a haloboration and this concept could not be verified [256,258]. 

As depicted in Scheme 4, the treatment of 1,4-diethynylbenzene with 2eq. of diphenylbromoborane (selective haloboration) and the subsequent reaction with leq. of tolylene-2,4-diisocyanate (phenylboration polymerization) gives the corresponding alternating copolymers. 

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. 

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). 

A wide range of novel polymers with boron in the backbone have been prepared by means of boration polymerizations (65,70-82). Diynes can be polymerized by hydroboration (70,71), phenylboration (65), and haloboration (72) to yield polymers (14),(15), and (16) (eq. 17). When an appropriate aromatic or heteroaromatic diyne is used, the resulting polymers have been shown to have extended 7T-conjugation through the vacant p-orbital of the boron atom (73,74). In fact, several have been shown to exhibit blue fluorescence emission. 

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