Direct free radical polymerization

Hyperbranched polymers were synthesized by direct free-radical polymerization of ethylene glycol dimethacrylate monomer in the presence of a CCT catalyst. The free-radical homopolymerization of divinyl monomers is thought to selectively yield trimer 96,32i 322 though previous work on oligomer distributions would indicate that this is unlikely. 

CONJUGATED OILS AS COATINGS DIRECT FREE RADICAL POLYMERIZATION... 

Noda and Watanabe [42] reported a simple synthetic procedure for the free radical polymerization of vinyl monomers to give conducting polymer electrolyte films. Direct polymerization in the ionic liquid gives transparent, mechanically strong and highly conductive polymer electrolyte films. This was the first time that ambient-temperature ionic liquids had been used as a medium for free radical polymerization of vinyl monomers. The ionic liquids [EMIM][BF4] and [BP][Bp4] (BP is N-butylpyridinium) were used with equimolar amounts of suitable monomers, and polymerization was initiated by prolonged heating (12 hours at 80 °C) with benzoyl... 

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

Also very promising are the monolithic separation media prepared directly in situ within the confines of the capillary by a free-radical polymerization of liquid mixtures [44]. They are easy to prepare and completely eliminate packing of beads which, for the very small beads, might require new technical solutions. In addition, the in situ prepared monoliths appear to be the material of choice for the fabrication of miniaturized microfluidic devices that represent the new generation of separation devices for the twenty-first century [202,203]. 

Case 1 appears to accurately predict the observed dependence on persulfate concentration. Furthermore, as [Q]+otal approaches [KX], the polymerization rate tends to become independent of quat salt concentration, thus qualitatively explaining the relative insensitivity to [Aliquat 336]. The major problem lies in explaining the observed dependency on [MMA]. There are a number of circumstances in free radical polymerizations under which the order in monomer concentration becomes >1 (18). This may occur, for example, if the rate of initiation is dependent upon monomer concentration. A particular case of this type occurs when the initiator efficiency varies directly with [M], leading to Rp a [M]. Such a situation may exist under our polymerization conditions. In earlier studies on the decomposition of aqueous solutions of potassium persulfate in the presence of 18-crown-6 we showed (19) that the crown entered into redox reactions with persulfate (Scheme 3). Crematy (16) has postulated similar reactions with quat salts. Competition between MMA and the quat salt thus could influence the initiation rate. In addition, increases in solution polarity with increasing [MMA] are expected to exert some, although perhaps minor, effect on Rp. Further studies are obviously necessary to fully understand these polymerization systems. 

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

For classical free radical polymerizations the rate of propagation is proportional to the concentration of monomer and the square root of the initiator concentration. Termination usually occurs through a coupling of two live radical chains but can occur through disproportionation. The rate of termination for coupling is directly proportional to initiator concentration. The DP is directly proportional to monomer concentration and inversely proportional to the square root of the initiator concentration. 

There were several attempts to gain better control on the free radical polymerization process [18, 19], One of these methods was named the iniferter method. The compounds used in this technique can serve as m/tiator, trans/er agent and terminating agent [20-22], Another technique is based on the use of bulky organic compounds such as diaryl or triarylmethyl derivatives [23-25], The main disadvantages of these systems comprise slow initiation, slow exchange, direct reaction of counter radicals with monomers, and their thermal decomposition. Therefore, these techniques did not offer the desired level of control over the polymerization. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

Fig. 39 Micrograph of polymerized uniform submicrostructure with 9 p.m line width and 50 xm line spacing. The structure was written by two-photon initiated electron-transfer free radical polymerization of diacrylate monomer Sartomer SR 349 at 775 nm via direct excitation of dye 5,7-diiodo-3-butoxy-6-fluorone(H-Nu 470) for (A) and dye 3 for (B) in the presence of Ar,Ar-dimethyl-2,6-diisopropylaniline...

Photopolymerization. In many cases polymerization is initiated by irradiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directly to form active free radicals, or it may first undergo a bimolecular electron-transfer reaction, the products of which initiate polymerization (14). Triphenylalkylborate salts of polymethines such as (23) are photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

It is not possible at our present stage of knowledge to place all of the catalysts in exact position relative to their ionic nature. The "mid point may be displaced some to either direction. Most catalysts contain several different components with different degrees of ionicity. Which component acts as the active catalyst for a particular double bond is unknown in most cases. Only crude presentations are possible until techniques have been developed to determine the actual ionic nature of the propagating species in isotactic ionic polymerization s such as ESR is capable of in free radical polymerizations. 

Fig. 56. Directed graph of a rooted tree from a linear chain polymerized by a chain reaction (free radical polymerization). I denotes the initiator, E the terminated chain end106)...

They are discrete transforms and can therefore operate directly on the separate equations for each species, reducing them to one expression. Nonlinear terms arising from condensation polymerization can be handled and, with some difficulty, so can realistic terminations in free radical polymerization. They are a special case of the generating functions and can be used readily to calculate directly the moments of the distribution, and thus, average molecular weights and dispersion index, etc. Abraham (2) provided a short table of Z-transforms and showed their use with stepwise addition. 

The use of iV-alkoxy pyridinium salts is not limited to cationic polymerization. Since, in addition to cationic species, ethoxy radicals are also formed upon direct and sensitized irradiation of pyridinium salts (see above), pyridinium salt based photoinitiating systems may be used to initiate the polymerization of vinyl monomers that are prone to free radical polymerization. Kayaman et al. [71] recently polymerized mono- and bi-functional acrylate monomers by photosensitization of pyridinium salts. It therefore appears that pyridinium salts can promote both cationic and free radical polymerization and are, thus, eminently suitable for use in hybrid systems. 

Another synthetic approach based on pyridium salt photochemistry involves the use of alkoxy radicals which are formed in both direct and sensitized decomposition of pyridinium ions in free radical polymerization [78]. Obviously, polytetrahydrofuran (PTHF), terminated by JV-alkoxy pyridinium ions, can act as macrophotoinitiator for the polymerization of monomers such as methyl methacrylate (MMA) that readily polymerize by a free-radical mechanism. PTHF macrophotoinitiators were prepared by termination of living polymerization of THF by the corresponding IV-oxides, The well-defined macrophotoinitiators with exact functionalities, confirmed by H-NMR, UV-visible and g.p.e. analysis, were obtained. Upon irradiation of macroinitiators at suitable wavelengths, polymeric alkoxy radicals are produced. The overall process is shown for the pyridinium macrophotoinitiator in the following Scheme 21. 

Figure 6.2-24 Near-infrared absorbance spectra, recorded during free-radical polymerization of ethene at 190 °C and 2630 bar initial pressure (the arrows indicate the direction of the absorbance change with the reaction time).

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