ATRP, macromonomers

As in the case of PS (Section 8.2.1) polymers formed by living radical polymerization (NMP, ATRP, RAFT) have thermally unstable labile chain ends. Although PMMA can be prepared by NMP, it is made difficult by the incidence of cross disproportionation.42 Thermal elimination, possibly by a homolysis-cross disproportionation mechanism, provides a route to narrow polydispersity macromonomers.43 Chemistries for end group replacement have been devised in the case of polymers formed by NMP (Section 9.3.6), ATRP (Section 9.4) and RAFT (Section 9.5.3). 

ATRP catalysts may be used to generate radicals and thus alkoxyamines can be produced from alkyl halides in high yield (Scheme 9.21).174 The alkoxyaminc 102 was obtained in 92% yield 174 whereas reaction of TEMPO with PMMA under ATRP conditions is reported to provide a macromonomer (Section 9.7.2.1). 

One of the major advantages of radical polymerization over most other forms of polymerization, (anionic, cationic, coordination) is that statistical copolymers can be prepared from a very wide range of monomer types that can contain various unprotected functionalities. Radical copolymerization and the factors that influence copolymer structure have been discussed in Chapter 7. Copolymerization of macromonomers by NMP, ATRP and RAFT is discussed in Section 9.10.1. 

A side reaction in NMP is loss of nilroxide functionality by thermal elimination. This may occur by disproportionation of the propagating radical with nitroxide or direct elimination of hydroxy lam ine as discussed in Section 9.3.6.3. In the case of methacrylate polymerization this leaves an unsaturated end group.1" The chemistry has also been used to prepare macromonomers from PMMA prepared by ATRP (Section 9.7.2.1),... 

The grafting through approach involves copolymerization of macromonomers. NMP, ATRP and RAFT have each been used in this context. The polymerizations are subject to the same constraints as conventional radical polymerizations that involve macromonomers (Section 7.6.5). However, living radical copolymerization offers greater product uniformity and the possibility of blocks, gradients and other architectures. 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

The grafting-through method has also been studied for ATRP. Vinyl chloroacetate is used as the initiator in ATRP of a monomer such as styrene to produce a macromonomer. Vinyl chloroacetate does not significantly copolymerize with styrene, and the result is a polystyrene vinyl macromonomer, which is then polymerized to a brush polymer [Davis and Matyjaszewski, 2002],... 

Initially, the polymerization of macromonomers was achieved by free radical polymerization reactions, which allowed only a limited control of the final properties. With the advent of ROMP and new free radical polymerization techniques, such as atom transfer radical polymerization (ATRP) the control of final properties became more facile (16). ATRP and ROMP techniques can be combined for the synthesis of macroinitiators (17). 

Kaneko et al. have reported the preparation of EPR macromonomers, which is useful to synthesize PMMA-g-EPR graft copolymers [106]. Graft copolymers possessing a poly(methacrylate) backbone and EPR branches were successfully synthesized by the copolymerization of EPR macromonomers with MMA. Methacryloyl terminated EPR, which were EPR macromonomers, were prepared by the sequential end functionalization of EPRs with vinylidene end group via hydroalumination, oxidation, and esterification (Fig. 20). The ATRP method can be used for the copolymeriza-... 

The preparation of PnBA-g-PE and PtBA-g-PE graft copolymers was reported using Fe-mediated olefin polymerization, chain shuttling with Zn and ATRP techniques [108]. Terminally hydroxyl PE was synthesized from Zn-terminated PE by oxidation and hydrolysis, as referred to above. It was converted to methacrylated PE, as PE macromonomer, using methacryloyl chloride. The resulting PE macromonomer was used for the copolymerization of nBA or tBA by ATRP using CuBr/tris((N,N-dimclhylamino)clhyl)aminc. The obtained graft copolymers were characterized by GPC, DSC, and JH NMR. 

Allyl PS macromonomers, which were synthesized by the ATRP of styrene with CuBr/bipyridine, have been used as comonomers in metallocene-catalyzed propylene copolymerizataions using Me2Si(2-Me-4,5-BzInd)2ZrCl2/ MAO [110]. It has been found that the incorporation of the PS macromonomers increases with a decrease in molar mass of the macromonomer and propylene concentration and increasing polymerization temperature. The highest comonomer incorporation (10.8 wt%) was achieved in the copolymerization at 70 °C. 

Recently, Muller et al. studied block and graft copolymers poly(n-butyl acrylate)-Wocfc/gra/f-poly(acrylic acid), PnBA-h/g-PAA [136]. The non-polar block/backbone has a low glass transition temperature, thus dynamic micelles were expected the ionic block/side-chains are weak anionic polyelectrolytes, thus a strong dependence of micellization on pH could be expected. The graft copolymers were synthesized by ATRP copolymerization of poly(-ferf-butyl acrylate) macromonomers with n-butyl acrylate, followed by hydrolysis of the terf-butyl acrylate side-chains to PAA [137]. The length of the PAA side chains was varied from 20 to 85 monomer units and their number from 1.5 to 10, whereas the length of the backbone was kept at ca. 130 units. 

Fig. 41. Molecular weight distribution change for the copolymerization of MMA and pDMS macromonomer (pDMS-MA) (Mn=2370, F= 1.0) in xylene solution (xylene=3lwt%). Conditions for RAFT [MMA]o/[pDMS-MA]o/[CDB]o/[initiator]o=285/15/l/0.5, initiator/tem-perature BPO/75°C (closed symbol), AIBN/60°C (open symbol). Conditions for the conventional AIBN polymerization [MMA]0/[pDMS-MA]o/[AIBN]o=380/20/l, temperature= 75°C. Conditions for the ATRP [MMA]o/[pDMS-MA]o/[EBiB]0/[CuCl]o/[dnNbpy]o= 285/15/1/1/2, temperature=75°C. Reprinted with permission from [320], Copyright (2001) American Chemical Society.

Fig. 44. Right copolymers of pDMS methacrylate macromonomers with different branch distributions prepared by different methods RAFT (top, solid line), FRP (middle, dotted line), ATRP (bottom, broken line). Left impact of branch distribution on mechanical properties (stress vs draw ratio) [320]...

In a similar way, n-butyl acrylate was copolymerized by ATRP with methacrylate macromonomers containing highly branched polyethylene prepared by Pd-catalyzed living ethylene polymerization. The observed reactivity ratios depend on the molecular weight and concentration of the macromonomer. The resulting graft copolymers showed microphase separation by AFM [304]. 

Like the telomerization process, ATRP enables the synthesis of two different types of macromonomers either with a polymerizable double bond or with a polycondensable group. As depicted in Sect. 3.1 on the design of the macromonomers, the polycondensable groups also comprise groups that afford ring-opening polymerization. 

According to Scheme 64, the resulting oligomers bear an R group (provided by the initiator) in the a position and a halogen atom (provided by the initiator) in the a> position. The macromonomers can also be obtained by ATRP, using three different concepts (Scheme 64) ... 

Matyjaszewski et al. first used initiators bearing an unsaturated group for the ATRP process of styrene. In 1998, they [318] used vinyl chloroacetate as the initiator for the ATRP of styrene. As VAc was unreactive towards styrene in radical copolymerization, vinyl chloroacetate was able to initiate the ATRP of styrene (Scheme 65). The resulting PS macromonomers, with molar mass ranging from 5 x 103 to 15 x 103 gmol, were copolymerized with N-vinylpyrrolidinone. The amphiphilic copolymers obtained were used as hydrogels. 

Several ligands were used with allyl-type and vinyl-type initiators, such as 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMDETA), NJJyN JJ jN -pentamethyldiethylenetriamine (PMDETA), or compound 7 in Scheme 66. Zeng et al. showed that the combination of initiator 1 in Scheme 66 with BA6TREN or initiator 4 in Scheme 66 with BA6TREN gave the best control of the molar mass for the ATRP of 2-(dimethylamino)ethyl methacrylate. These allylic macromonomers are then able to copolymerize with acrylamide. 

In a previous work, Cheng et al. [322] performed the same synthesis but without any ligand excess. The resulting macromonomer was similar to that described in Scheme 69 but with a chain-end bromine atom. This macromonomer was polymerized by the ATRP process, leading to a hyper-grafted polymer. The Mark-Houwink coefficient was 0.47, which characterized the hyperbranched structure. Hydrolysis of such a polymer led to the corresponding poly(acrylic acid). Similarly, Hua et al. [323] performed the synthesis of brushlike poly(acrylic acid). 

In a similar way, Norman et al. [331] synthesized PMMA oligomers by ATRP. The methacrylic double bond of the resulting macromonomer was directly obtained by elimination of the terminal halogen by catalytic CTAs, such as 5,10,15,20-lclraphcnyl-21 Lf,23f/-porphine cobalt(II) [Co(tpp)]... 

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