Functional polymers, syntheses initiation

Scheme 20 Synthesis of end-functionalized polymer by initiation method.

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

End-functional polymers, including telechelic and other di-end functional polymers, can be produced by conventional radical polymerization with the aid of functional initiators (Section 7,5.1), chain transfer agents (Section 7.5.2), monomers (Section 7.5.4) or inhibitors (Section 7.5.5). Recent advances in our understanding of radical polymerization offer greater control of these reactions and hence of the polymer functionality. Reviews on the synthesis of end-functional polymers include those by Colombani,188 Tezuka,1 9 Ebdon,190 Boutevin,191 Heitz,180 Nguyen and Marechal,192 Brosse et al.rm and French.194... 

Several pathways have been explored for their synthesis sequential addition of monomers to an initiator solution, reaction between co-functional polymers and more recently site transformation techniques. Each of these methods has advantages and drawbacks. 

Terminal-functionalized polymers such as macromonomers and telechelics are very important as prepolymer for construction of functional materials. Single-step functionalization of polymer terminal was achieved via lipase catalysis. Alcohols could initiate the ring-opening polymerizahon of lactones by lipase catalyst. The lipase CA-catalyzed polymerizahon of DDL in the presence of 2-hydroxyethyl methacrylate gave the methacryl-type polyester macromonomer, in which 2-hydroxyethyl methacrylate acted as initiator to introduce the methacryloyl group quanhtatively at the polymer terminal ( inihator method ).This methodology was expanded to the synthesis of oo-alkenyl- and alkynyl-type macromonomers by using 5-hexen-l-ol and 5-hexyn-l-ol as initiator, respechvely. 

Some particularities of the extraction of ions from an aqueous organic phase, and of the phase catalyzed polyetherification will be summarized. These will represent the fundamentals of our work on the synthesis of some novel classes of functional polymers and sequential copolymers. Examples will be provided for the synthesis of functional polymers containing only cyclic imino ethers or both cyclic imino ethers as well as their own cationic initiator attached to the same polymer backbone ABA triblock copolymers and (AB)n alternating block copolymers and a novel class of main chain thermotropic liquid crystalline polymers containing functional chain ends, i.e., polyethers. 

Initial reports on chemoenzymatic block copolymer synthesis focus on the enzymatic macroinitiation from chemically obtained hydroxy-functional polymers (route A in Fig. 4). The first report on enzymatic macroinitiation was published by Kumar et ah, who used anionically synthesized hydroxy-functional polybutadiene of various molecular weights ranging from 2600 to 19,000Da (Fig. 5) [16]. In a systematic study, the authors investigated the efficiency of the macroinitiation of CL and PDF by Novozym 435 as a function of the polybutadiene macroinitiator. The reaction profile showed that polybutadiene consumption steadily increased with the reaction... 

Since its discovery more than 50 years ago, olefin metathesis has evolved from its origins in binary and ternary mixtures of the Ziegler-Natta type into a research area dominated by well-defined molecular catalysts. Surveys of developments up to 1993 were presented in COMC (1982) and COMC (1995). Major advances in ROMP over the last 10 years include the development of modular, stereoselective group 6 initiators, and easily handled, functional-group tolerant ruthenium initiators. The capacity to tailor polymer functionality, chain length, and microstructure has expanded applications in materials science, to the point where ROMP now constitutes one of the most powerful methods available for the molecular-level design of macromolecular materials. In addition to an excellent and comprehensive text on olefin metathesis, a three-volume handbook s has recently appeared, of which the third volume focuses specifically on applications of metathesis in polymer synthesis. 

The phenomenal growth in commercial production of polymers by anionic polymerization can be attributed to the unprecedented control the process provides over the polymer properties. This control is most extensive in organolithium initiated polymerizations and includes polymer composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even monomer sequence distribution in copolymers. Furthermore, a judicious choice of process conditions affords termination and transfer free polymerization which leads to very efficient methods of block polymer synthesis. 

The second approach to the synthesis of tele-chelic polymers involves the use of functionally substituted anionic initiators. An example of a functional organolithium initiator is p-lithiophenoxide (40). Unfortunately, p-lithiophenoxide shows low solubility even in polar solvents like THF. It is difficult to control polymer molecular weights with such a heterogeneous initiator (41). Trepka (42) has improved the solubility of D-lithiophenoxide by adding alkyl groups to the ring, nevertheless, only moderate yields of the alkylated initiator have been reported. Hirshfield... 

Synthesis of well defined functionalized (- telechellc or multifunctional-) macromolecules Is an Important task for polymer chemists. The polymers with P0(0R)2, - Si(0R)3, -OH, - . .. functional groupslrS. are produced In limited quantities. The need for polymeric materials possessing specific properties has led to a renewed Interest Is functional polymers, especially if the initial material Is a common hydrocarbon polymer. One of the techniques that we use in our laboratory to prepare these new molecules Is based on anionic processes. This anionic technique is best suited to control the length of the chains prepared and to obtain samples with low polydlsperslty. Although the functionalization of carbanionic sites with various deactivating reagents Is easier than with other methods because of the long lived species, It Is still necessary to carefully control the deactivation reaction to prevent secondary reactions. 

A special case of controlled initiation is the inifer method. The word inifer (front initiator transfer agentsl describes compounds that function simultaneously as initiators and as chain transfer agents. The inifer technique provided the first carbocationic route toward the synthesis of telechelic ta. to functional) polymers. 

It can be asserted that the synthesis of well-defined functional polymers by means of functional free-radical initiators is far from being satisfactory. To our knowledge, this pathway was never used in this simple form as a step in the macromonomer synthesis. 

The scope of the living cationic polymerizations and synthetic applications of these functionalized monomers will be treated in the next chapter on polymer synthesis (see Chapter 5, Section III.B). One should note that the feasibility of living processes for these polar monomers further attests to the formation of controlled and stabilized growing species. Conventional nonliving polymerizations, esters, ethers, and other nucleophiles are known to function as chain transfer agents and sometimes as terminators. In addition, the absence of other acid-catalyzed side reactions of the polar substituents, often sensitive to hydrolysis, acidolysis, etc., demonstrates that these polymerization systems are free from free protons that could arise either from incomplete initiation (via addition of protonic acids to monomer) or from chain transfer reactions (/3-proton elimination from the growing end). 

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. 

A class of end-functionalized polymers with polymerizable terminal groups are generally called macromonomers. By both functional initiator and terminator methods, a variety of macromonomers have been synthesized in living cationic polymerization of vinyl ethers, styrenes, and isobutene, as summarized in Table 3 [16,31,147,149-151,155,158-171]. Some of these macromonomers are used in the synthesis of graft polymers (Section VI.C). 

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