Poly metal-free initiators

Reetz and co-workers " first used metal-free carbon, nitrogen, or sulfur nudeophiles as initiators for the controlled anionic polymerization of nBA. It was thought that repladng the metal counterion in the polymerization would reduce the problem assodated with aggregation and improve the control over the polymerization. Tetrabutylammonium salts of malo-nate derivatives provided poly(n-butyl acrylate) (PnBA) of rdativdy narrow MWD at room temperature (Scheme 14). Many metal-free initiators for the polymerization of alkyl (meth) acrylates using a variety of anions and cations have been reported (Scheme 15) 208,220-224... 

Pentadienyl-terminated poly(methyl methacrylate) (PMMA) as well as PSt, 12, have been prepared by radical polymerization via addition-fragmentation chain transfer mechanism, and radically copolymerized with St and MMA, respectively, to give PSt-g-PMMA and PMMA-g-PSt [17, 18]. Metal-free anionic polymerization of tert-butyl acrylate (TBA) initiated with a carbanion from diethyl 2-vinyloxyethylmalonate produced vinyl ether-functionalized PTBA macromonomer, 13 [19]. 

A metal-free REP method was developed by Kudo etal., by using the labile thioester bond of thiocarbamates to produce cyclic poly(sulfide)s (Scheme 12.14) [54]. For this, l-phenoxy-2-propane thiirane (50) was polymerized using the cyclic thiazolidine-2,4-dione initiator (51) and tetrabutylammonium chloride as a catalyst to produce cyclic poly(sulfide)s (52) having molecular weights below 8kDa and PDI-values of about 1.3. Attempts to increase the molecular weight to 10 kDa were successful, but yielded polymers with high PDI-values (ca. 1.9). 

The general results obtained with the present polymerization systems may be summarized as follows, (i) terpyridyl-based monoligated systems may be used in ATRP. Generally, the use of monoligated initiators results in comparably fast polymerizations, (ii) Complexes that contain the reduced species of the corresponding metal (preferably Cu) were found to work best and additionally avoid the use of a reducing agent (e. g. MAO, Al(/-OPr)3). (iii) poly (styrene) may be prepared up to a of 80,000 in 20 - 30 % yield. Polydispersities vary from 1.5 - 1.9. These polymers are virtually metal free as determined by atom absorption spectroscopy (AAS, metal content < 100 ng/g). (v) Polymerizations proceed fast, yet level off after approximately 2 hours at monomer consumption < 30 %. In principle, polymerization may cease due to deactivation or recombination of the radicals. Based on the fact that polymerization yields are neither influenced by the addition of further initiator... 

Last but not least, metal-free polyesters can be prepared by using all-organic initiators. Endo showed that a mixture of alcohol and HCl - C2H5O is a cationic catalyst for the controlled ROP of e-CL and 5-VL (105). One limitation of this system is that low Mn chains are usually prepared, ie, lower than 15,000 for poly( -CL) (106). Poly(S-VL) is an exception, with up to 50,000. 

However, all synthetic approaches involving ATRP rely on a metal catalyst. Full metal-free and thus greener approaches to block copolymers were realized by the combination of Upase ROP with nitroxide-mediated living free radical polymerization [44]. With this system it was also possible to successfully perform a one-pot chemoenzymatic cascade polymerization from a mixture containing a dual initiator, CL and styrene (Fig. 12). Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-methylcaprolactone for the synthesis of chiral block copolymers. A metal-free synthesis of block copolymers using a radical chain transfer agent as a dual initiator in enzymatic ROP to yield poly(CL-f -styrene) was also reported recently [119]. 

When living poly(methyl methacrylate) (PMMA) prepared by group transfer polymerization (GTP) is used as a macroinitiator for the ROP of cyclic carbonates, a site transformation from the silyl ketene acetal (GTP-mechanism) to an alcoholate (anionic ROP-mechanism) with a metal-free counterion occurs (Scheme 12.5). The GTP of PMMA was initiated with l-methoxy-l-trimethylsilyloxy-2-methyl-l-propene (MTS) in combination with catalytic amounts of tetrabutyl ammonium cyanide in THF as solvent. Towards the end of the reaction, DTC is dissolved in the reaction mixture and lequiv. of fluoride anions (e.g. tris(dimethylamino) sulfonium difluorotrimethylsilicate TASF), with respect to the active species, is added. In this way, good yields of the respective block copolymers were obtained. A model experiment for this site transformation is the polymerization of DTC with MTS as the initiator and TASF as the desilylating agent. The fluoride anion promotes desilylation of the silyl ketene acetal with formation of an enolate, which reacts as a carbon-centered nucleophile with the carbonyl carbon of DTC, thereby... 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

HPMA [36] and a vinyl metal-chelating monomer A-(A/, A/ -dicarboxy-methylaminopropyl)methacrylamide synthesized [35]. Chemical structures of HPMA and DAMA are given in Figure 4. Poly(HPMA-co-DAMA) was prepared by free radical copolymerization in methanol with AIBN as initiator. Molecular weight distribution was determined by size exclusion chromatography and content of side-chain carboxylic group by acid-base titration. 

Several papers have appeared in the literature in recent years showing that certain metal acetylacetonates can function as initiators for the polymerisation of vinyl and diene monomers in bulk and solution (1 - 12). Results for the kinetics of bulk and solution polymerisation are consistent with the view that the reaction occurs by a free-radical mechanism. The usual free-radical kinetics are operative, but an unusual feature is that, in some cases, certain additives such as chlorinated hydrocarbons have an activating effect upon the reaction by inducing more rapid decomposition of the initiator (2,11,12,13). Other additives which have been reported as promotors for the polymerisation include pyridlne(14) and aldehydes and ketones(15). The complexity of the reaction in the presence of such additives is evident from the fact that chloroform has been reported to be an inhibitor for the poly-merlsatlon(3). 

The main function of metal deactivators (MD) is to retard efficiently metal-catalyzed oxidation of polymers. Polymer contact with metals occur widely, for example, when certain fillers, reinforcements, and pigments are added to polymers, and, more importantly when polymers, such as polyolefins and PVC, are used as insulation materials for copper wires and power cables (copper is a pro-oxidant since it accelerates the decomposition of hydroperoxides to free radicals, which initiate polymer oxidation). The deactivators are normally poly functional chelating compounds with ligands containing atoms like N, O, S, and P (e.g., see Table 1, AOs 33 and 34) that can chelate with metals and decrease their catalytic activity. Depending on their chemical structures, many metal deactivators also function by other antioxidant mechanisms, e.g., AO 33 contains the hindered phenol moiety and would also function as CB-D antioxidants. 

There was a strong possibility that initiation occurred via photosensitized formation of hydrogen peroxide on the metal oxide surface followed by desorption and subsequent photolysis of hydrogen peroxide to form free radicals. Radicals formed by this means would lead to abstraction of hydrogen from the fiber surface and subsequent grafting of poly(methyl acrylate) at these sites. 

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