CHAPTER 2 LITERATURE REVIEW
CHAIN GROWTH POLYMERIZATION
Polymerization reactions are classified into two major groups based on the polymerization mechanism, namely, step growth and chain growth processes29-30. There are a variety of different chain polymerization mechanisms including free radical, ionic, complex and ring opening polymerization. In chain growth polymerization, the growth of the chain is associated with the addition of consecutive monomer units to a single molecule. The features of chain growth polymerization are as follows29-31:
- The chain growth reaction involves the addition of a monomer at one end of the chain. The polymerization reaction is effected by a kinetic chain of reactions. Thus, different mechanisms operate at different stages of the reaction.
- The polymerization rate initially increases and becomes constant with polymerization time.
- An initiator is required to start the reaction.
- Chain growth polymerization involves the reaction of a monomer with active centers that may be free radicals, ions and polymer-catalyst bonds.
CONVENTIONAL FREE RADICAL POLYMERIZATION
Mechanism of Free Radical Polymerization
A typical chain growth polymerization reaction consists of three major kinetic steps, namely, initiation, propagation and termination. The free radical polymerization process is one of the most important chain growth polymerization methods. Conventional free radical polymerization is a rapid chain reaction which proceeds via the following characteristic kinetic steps.
Organic compounds with a half life t1/2 ≈ 10 hours, such as benzoyl peroxide (BPO) and azobisisobutyronitrile (AIBN) are commonly employed as initiators for production of free radicals (R●) at a rate constant kd31. The formation of free radicals is followed by the subsequent one step addition to the monomer molecule to form a new free radical (monomer radical, R-M1●) at a rate constant of ki. The rate of initiation is given by the following equation and efficiency of chain end functionalization6-7,31. The poor control of the polymerization process is due to the inherent slow initiation relative to fast propagation and the inevitable near diffusion controlled termination reactions of growing radical species7. In addition, the life span of a propagating chain is too short for any synthetic manipulation such as chain end functionalization reactions or addition of a second monomer to form a block copolymer7.
Thus, there is a need for the development of controlled/”living” synthesis methods for the preparation of well-defined polymer structures. Thus, several new controlled/”living” free radical polymerization methods were developed independently by different research groups worldwide. The most powerful controlled/”living” free radical polymerization techniques are nitroxide mediated radical polymerization3 (NMP), reversible addition fragmentation chain transfer radical polymerization3 (RAFT) and atom transfer radical polymerization3,7,8 (ATRP).
CONTROLLED/“LIVING“ FREE RADICAL POLYMERIZATION (CRP)
The term controlled/”living” polymerization refers to a polymerization reaction in which termination reactions occur at a minimal rate of less than ten percent. Truly living polymerization cannot be achieved in radical polymerization because of unavoidable fast, irreversible termination reactions of growing radicals induced by a high concentration of highly reactive radicals7,8. However, if the ratio of terminated chains to total number of chains is kept minimal, the polymerization reaction can proceed in a controlled manner. The ability to allow the synthesis of well defined polymers with predetermined number average molecular weight, composition, topology and chain end functionality, while retaining much of the versatility of radical polymerization, has resulted in the widespread adoption of controlled/”living” radical polymerization in research laboratories7,9.
Several controlled/”living” free radical polymerization methods, which allow the preparation of well defined polymers by free radical mechanism, have been reported in the literature. Otsu and Yoshida32 first published a report on the controlled free radical polymerization process by outlining a method for the preparation of well defined polymers using the Initiator-Transfer-Agent-Termination (INIFERTER) method.
CHAPTER 1: INTRODUCTION
CHAPTER 2: LITERATURE REVIEW
2.1 CHAIN GROWTH POLYMERIZATION
2.2 CONVENTIONAL FREE RADICAL POLYMERIZATION
2.3 CONTROLLED/”LIVING” FREE RADICAL POLYMERIZATION (CRP)
2.4 NITROXIDE MEDIATED RADICAL POLYMERIZATION (NMP)
2.5 REVERSIBLE ADDITION FRAGMENTATION CHAIN TRANSFER RADICAL POLYMERIZATION (RAFT)
2.6 ATOM TRANSFER RADICAL POLYMERIZATION (ATRP)
2.7 FUNCTIONALIZED POLYMERS BY ATOM TRANSFER RADICAL POLYMERIZATION
2.8 FUNCTIONALIZED POLYMERS VIA 1,1-DIPHENYL- ETHYLENE DERIVATIVES
CHAPTER 3: EXPERIMENTAL
3.1 MATERIALS AND GLASSWARE
3.3 ATOM TRANSFER RADICAL POLYMERIZATION: SYNTHESIS OF CHAIN END FUNCTIONALIZED POLYMERS
CHAPTER 4: RESULTS AND DISCUSSION
4.1 ATOM TRANSFER RADICAL POLYMERIZATION: SYNTHESES OF CHAIN END FUNCTIONALIZED POLYMERS
4.2 ATOM TRANSFER RADICAL POLYMERIZATION: POLYMERIZATION KINETIC STUDIES.
CHAPTER 5: SUMMARY
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