Polymerization inifer method

For PIB, a method resulting in end-reactive polymers, however, based on chain transfer reactions during polymerization, was addressed as the cationic inifer method (54). 

Nevertheless, a few years ago, Kennedy 66 69) developed a method yielding co-functional polymers by cationic polymerization of vinyl monomers. The principle of the socalled inifer method is to kinetically favor transfer to the initiating species with respect to all other kinds of transfer reactions (especially the transfer to monomer). A typical initiating system is composed of an allyl or benzyl halide and boron trichloride BCl3. This mixture behaves like an alkenium tetrachloro-borate and readily initiates the polymerization of monomers such as isobutene or a-methylstyrene. The efficiency of the halide as a transfer agent depends on the lability of the C—Cl bond and on the molar ratio [RC1]/[BC13],... 

Techniques derived from anionic or cationic living polymerization methods have widely been used. They are efficient because of the long lifetime of the active sites. Once polymerization is completed these sites are used for functionalization purposes. Alternately, unsaturated ionic initiators have been used but to a lesser extent because of the requirement involved that the polymerizable groups remain unscathed during the macromonomer formation. The versatile inifer method has also been applied to the synthesis of macromonomers. 

In addition to the sequential living polymerizations, block copolymers may be synthesized by various reactions of end-functionalized polymers. The method may be subdivided into two classes, as discussed in Section III. A. Most of the examples via living cationic polymerizations are based on vinyl ethers, and Fig. 8 lists the second monomers used in these vinyl ether-based block copolymers. Similar syntheses based on the inifer methods are discussed elsewhere [1,3]. 

As pointed out already, rather few end-functionalized poly isobutenes have been obtained from isobutene via living cationic polymerization, whereas abundantly via the inifer method followed by various chemical reactions to convert the resulting tertiary chloride terminal (Section IV.A.3) [3],... 

Apart from the syntheses by living cationic polymerizations, a variety of polyisobutene macromonomers have been prepared by converting the chloride terminal derived from the inifer method (Section IV.A.3) [166-170] and more recently from living cationic polymerization... 

Kennedy and Carter used the inifer method to prepare polyisobutylene (PIB) macromonomers with phenol end-groups [186]. The method involves the polymerization of IB with BCl3 andp-hydroxycumylchloride as the initiation system. This system gives rise to transfer reactions but both these reactions and the ion splitting of the BCI4 anion produce the same species, as shown in the following Scheme 54. 

Macromers can be prepared not only by anionic polymerization, but also by cationic and radical polymerization. Kennedy and his coworkers utilized the inifer method of cationic polymerization in synthesizing polyisobutylene macromers, as follows ... 

Polystyrene and Derivatives. Telechelic polystyrene, poly(2,4,6-trimethylst5Tene), poly(p-methylstyrene), and poly(p-chlorostyrene) can be prepared by living carbocationic polymerization (269-271) or by inifer method (272). While end-quenching the living carbocationic polsrmerization gave quantitatively polymers with sec-benzylic termini, the diciunyl chloride/BClg inifer system yielded a,with olefine end groups were also prepared by dehydrochlorination (272,273). 

From the viewpoint of polymer synthesis, the multiplicity of the propagating species provides a possibility to synthesize polymers with varying structures from the different intermediates. In fact, the steric structure of, e.g., polystyrene, polyfvinyl ether)s, and poly(a-methylstyrene) can be controlled by selecting counteranions Recently, Kennedy and his associates studied in detail the control of chain transfer and termination by counteranions (initiators) in isobutene polymerization, and opened a new field of block and graft polymer synthesis (especially by their inifer method)... 

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

Before the development of living cationic polymerization in the 1980s, Kennedy and his co-workers devised another way to synthesize end-functionalized polymers, which uses special reagents called inifer, or initiator-chain transfer agents [129]. The method is primarily for the synthesis of polyisobutene with a tertiary chlorine terminal, which is, however, a synthon for a variety of other functional groups. These developments have been reviewed extensively [1,3,130] and fall outside the scope of this chapter. 

Fundamental studies directed toward the elucidation of the mechanism of olefin i.e.f isobutylene, polymerizations yielded a new method for the synthesis of novel linear and tri-arm star telechelic polymers and oligomers [1,2]. The synthesis involves the use of bi- or tri-functional initiator/transfer agents, so called inifers (binifers and trinifers), in conjunction with BCI3 coinitiator and isobutylene, and gives rise to polyisobutylenes carrying exactly two or three terminal -CH2-C(CH3)2Cl groups. These liquid telechelic polyisobutylene chlorides can be readily and quantitatively converted to telechelic polyisobutylene di- or tri-olefins [2,3] which in turn can quantitatively yield by hydroboration/oxidation telechelic polyisobutylene di- and triols [4,5]. 

To fully exploit the beneficial properties of PIB in, for example, polyurethanes, thermoplastic elastomers, and sealants, controlled polymerization methods are required that provide access to controlled end-group functionalities. The controlled polymerization of isobutene was first reported by Kennedy, which was based on the inifer technique. This so-called inifer that is added to provide control over the polymerization is a compound that acts both as the initiator and the transfer agent. The inifer deliberately induces chain transfer to control the molecular weight of the PIB, whereby each transfer step generates... 

Already before reporting this combined inifer and living polymerization approach, Kennedy and coworkers developed a controlled isobutene polymerization method based on cumyl ester initiators (Scheme 8.6) with boron trichloride as activator and incremental monomer addition [28], The livingness of the polymerization was demonstrated by the linear increase of number-average molar mass and the constant number of polymer chains (A) with the amount of PIB obtained (wp, as measure for conversion) as well as the narrowing of the molar mass distribution with conversion (Fig. 8.1) [28]. 

A telechelic polymer is defined as a relatively low-molar-mass spedes (M < 20,000), with functional end groups that can be used for further reaction to synthesize block copolymers or for network formation. Cationic polymerization methods can be used to prepare these fimctionalized polymers using the initiator-transfer, or Inifer, technique perfected by Kennedy. If the initiating catalyst-cocatalyst system is prepared from a Lewis acid and an alkyl or aryl halide, i.e.. 

---