CONTROLLING POLYMERIZATION

Radical polymerization is often the preferred mechanism for forming polymers and most commercial polymer materials involve radical chemistry at some stage of their production cycle. From both economic and practical viewpoints, the advantages of radical over other forms of polymerization are many (Chapter 1). However, one of the often-cited problems with radical polymerization is a perceived lack of control over the process the inability to precisely control molecular weight and distribution, limited capacity to make complex architectures and the range of undefined defect structures and other forms of structure irregularity that may be present in polymers prepared by this mechanism. Much research has been directed at providing answers tor problems of this nature. In this, and in the subsequent chapter, we detail the current status of the efforts to redress these issues. In this chapter, wc focus on how to achieve control by appropriate selection of the reaction conditions in conventional radical polymerization. 

Minor (by amount) functionality is introduced into polymers as a consequence of the initiation, termination and chain transfer processes (Chapters 3, 5 and 6 respectively). The.se groups may either be at the chain ends (as a result of initiation, disproportionation, or chain transfer,) or they may be part of the backbone (as a consequence of termination by combination or the copolymerization of byproducts or impurities). In Section 8,2 wc consider three polymers (PS, PMMA and PVC) and discuss the types of defect structure that may be present, their origin and influence on polymer properties, and the prospects for controlling these properties through appropriate selection of polymerization conditions. 

The functional groups introduced into polymer chains as a consequence of the initiation or termination processes can be of vital importance in determining certain polymer properties. Some such functionality is generally unavoidable. However, the types of functionality can be controlled through selection of initiator, solvent and reaction conditions and should not be ignored. 

Such functionality can also be of great practical importance since functional initiators, transfer agents, e(c. are applied to prepare end-functional polymers (see Section 7.5) or block or graft copolymers (Section 7.6). In these cases the need to maximize the fraction of chains that contain the reactive or other desired functionality is obvious. However, there are also well-documented cases where weak links formed by initiation, termination, or abnormal propagation processes impair the thermal or photochemical stability of polymers. 

it is important to know, understand and control the kinetics and mechanism of the entire polymerization process so that desirable aspects of the polymer structure can be maximized while those reactions that lead to an impairment of properties or a less than ideal functionality can be avoided or minimized. A corollary is that it is important to know how a particular polymer was prepared before using it in a critical application. 

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When exposed to sunlight, it is converted to a white insoluble resin, disacryl. Oxidized by air to propenoic acid small amounts of hy-droquinone will inhibit this. Bromine forms a dibromide which is converted by barium hydroxide into DL-fructose. The acrid odour of burning fats is due to traces of propenal. It is used in the production of methionine and in controlled polymerization reactions to give acrolein polymers. ... 

Polybutenes. Polybutenes are produced by controlled polymerization of butenes and isobutene (isobutylene) (see Butylenes). A typical polyisobutylene stmcture is... 

In this chapter, we restrict discussion to approaches based on conventional radical polymerization. Living polymerization processes offer greater scope for controlling polymerization kinetics and the composition and architecture of the resultant polymer. These processes are discussed in Chapter 9. 

Dithiocarbamatc 16 has been used to prepare low dispersity PMAA ( Mw 1 Mn-1.2).52 Photopolymerization of S in the presence of dithiocarbamate 16 also displays some living characteristics (molecular weights that increase with conversion, ability to make block copolymer). However, 17 appears to behave as a conventional initiator in S polymerization.53 The difference in behavior was attributed to the relatively poor leaving group ability of the 2-carboxyprop-2-yI radical. This hypothesis is supported by MO calculations. Dithiocarbamatc 17 was used to control polymerizations of MMA,54 HEMA54 and NIPAM.5... 

MARTEN AND HAMiELEC Diffusion-Controlled Polymerization Nomenclature... 

Thermal Effects in Addition Polymerizations. Table 13.2 shows the heats of reaction (per mole of monomer reacted) and nominal values of the adiabatic temperature rise for complete polymerization. The point made by Table 13.2 is clear even though the calculated values for T dia should not be taken literally for the vinyl addition polymers. All of these pol5Tners have ceiling temperatures where polymerization stops. Some, like polyvinyl chloride, will dramatically decompose, but most will approach equilibrium between monomer and low-molecular-weight polymer. A controlled polymerization yielding high-molecular-weight pol)mier requires substantial removal of heat or operation at low conversions. Both approaches are used industrially. 

The (a-sulfanylalkyl)phosphonates and their difluorinated derivatives are also, by easy radical cleavage of the C-S bond, useful precursors of phospho-nomethyl or phosphonodifluoromethyl radicals, which can be added to double bonds and so, introduced in a variety of structures. Besides, the use of phospho-nodithioformates as radical trapping agents and their use as RAFT reagent for controlled polymerization open a new interesting field of appHcation for these dithioesters. 

Occasionally in the synthesis of the copolymers, insoluble material is produced. This results from polymer containing blocks of polyglycolide rather than the desired random structure. Obviously, such compositions would have considerable effect on the performance of controlled release formulations utilizing those polymers. This problem is particularly evident when one is seeking to utilize the 50 50 glycolide/lactide copolymer as a biodegradable excipient. However, with carefully controlled polymerization conditions, useful 50 50 polymer is readily produced. 

Although more studies need to be performed to study the scope and generality of this system, the use of amine hydrochloride salts as initiators for controlled NCA polymerizations shows tremendous promise. Fast, reversible deactivation of a reactive species to obtain controlled polymerization is a proven concept in polymer chemistry, and this system can be compared to the persistent radical effect employed in all controlled radical polymerization strategies [37]. Like those systems, success of this method requires a carefully controlled matching of the... 

The polymerizations initiated by HMDS and N-TMS amines usually complete within 24 h at ambient temperature with quantitative monomer consumption. These polymerizations in general are slower than those mediated by Deming s Ni(0) or Co (0) initiators (about 30-60 min at ambient temperature) [19, 24, 25], but are much faster than those initiated by amines at low temperature or using amine hydrochloride initiators [20]. These HMDS and N-TMS amine-mediated NCA polymerizations can also be applied to the preparation of block copolypeptides of defined sequence and composition [22]. This organosilicon-mediated NCA polymerization, which was also shown by Zhang and coworkers to be useful for controlled polymerization of y-3-chloropropanyl-L-Glu NCA [43], offers an advantage for the preparation of polypeptides with defined C-terminal end-groups. 

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