Design Strategies for Branched and Highly Branched Macromolecular

Chapter 26

Design Strategies for Branched and Highly Branched Macromolecular Architectures Using NitroxideMediated Living Free-Radical Procedures Downloaded by NANYANG TECHNOLOGICAL UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch026

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Craig J. Hawker , Eva E . M a l m s t r ö m , Jean M. J. Fréchet , Marc R. Leduc , R. Bernard Grubbs , and George G . Barclay 2

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I B M Almaden Research Center, 650 Harry Road, San Jose, C A 95120-6099 Department of Chemistry, University of California, Berkeley, C A 94720-1460 Shipley Company, 455 Forest Street, Marlborough, MA 01752-3092 3

Free radical procedures play a dominant role i n the synthesis o f a wide variety o f commodity polymers due to the mild, non-demanding reaction conditions and compatibility with numerous functional groups. However, one o f the major disadvantages o f free radical polymerizations is the lack o f control over macromolecular structure. For example, the polymers obtained are frequently polydisperse and the technique provides poor control over molecular weight, chain ends, and architecture. Many o f these shortcomings have recently been overcome using nitroxide mediated 'living' free radical procedures, coupled with well defined unimolecular alkoxyamine initiators. In this report, various strategies for the control o f polymeric structure and macromolecular architecture using novel 'living' free radical procedures w i l l be discussed and compared with currently available techniques. The control and manipulation o f the physical and mechanical properties o f polymeric materials is an ongoing challenge i n polymer science. One method which has recently gained prominence as a means to obtain this goal is the controlled incorporation o f branching i n linear polymer structures. L o w levels o f branching gives graft copolymers, while increasing the amount o f branching leads to hyperbranched structures and eventually to dendritic macromolecules. These highly branched macromolecules, such as dendrimers and hyperbranched polymers, are a new class o f macromolecular architecture which have received considerable attention recently. One o f the primary reasons for this explosion o f interest i n branched macromolecules is the belief that new and/or improved properties w i l l be observed for these materials. In fact a number o f unique properties, such as intrinsic viscosity, 1

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© 1998 American Chemical Society

In Controlled Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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melt viscosity, molecular inclusion, etc. have already been observed for dendritic macromolecules and many more w i l l not doubt be realized as other macromolecular architectures become available. For example, globular dendrimers are believed to be essentially free o f chain entanglements, in direct contrast to most linear polymers where physical properties are dominated by chain entanglements. This leads to the observed differences in intrinsic and melt viscosity for dendrimers when compared to linear polymers. However one difficulty associated with dendritic and hyperbranched macromolecules is the limited range o f reaction types that can be used for their construction. B y their design, dendrimers and hyperbranched macromolecules are typically prepared by condensation chemistry. Since one o f the most intriguing and potentially useful applications for branched macromolecules is i n their blending with commodity linear polymers to improve the mechanical and physical properties o f the linear material, the restriction to polymers based on poly condensation chemistry severely limits the study and exploitation of these materials. In a similar vein, well defined graft copolymers are typically prepared by living techniques, such as anionic polymerization, which are synthetically demanding and incompatible with a wide range o f useful functional groups and monomer units. This results in a narrow range of possible structures with little opportunity for the incorporation o f functional groups. 6

The development o f an alternative approach to branched macromolecular architectures which relies on simple free radical chemistry is therefore highly desirable and would alleviate many o f the above difficulties. Such an objective would, under traditional free radical polymerization conditions, be extremely problematic, i f not impossible. This is due to the uncontrolled nature o f normal free radical polymerizations coupled with numerous termination reactions which leads to polydisperse, poorly defined materials. Also the tendency to undergo termination by radical-radical recombination is a serious drawback for branched systems since coupling o f the chain ends would soon result in crosslinking and gelation. The recent development o f a "living" free radical polymerization process based on stable nitroxidé radicals as reversible chain termination agents offers the possibility of reducing the influence o f termination reactions and therefore overcoming many o f the drawbacks traditionally associated with free radical chemistry. Nitroxide mediated 'living' free radical procedures may therefore have a number o f advantages when compared to traditional techniques for the preparation of novel branched polymer systems. The development o f a polymerization process, based on free radical chemistry, which displays many o f the characteristics of a living polymerization has long been a goal o f polymer chemists. Early attempts to realize a "living" free radical procedure involved the concept o f reversible termination o f growing polymer chains by iniferters, ' however this concept was plagued by high polydispersities and poor control over the functional groups at the chain ends. Following this approach M o a d and R i z z a r d o introduced the use o f stable nitroxide free radicals, such as 2,2,6,6tetramethylpiperidinyloxy ( T E M P O ) , as reversible terminating agents to "cap" the growing polymer chain. The advantages o f using nitroxide free radicals are their near diffusion controlled rate o f reaction with carbon centered free radicals which leads to 7

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efficient capping o f the growing chain end and their inability to self-initiate vinyl polymerizations. Subsequently, Georges refined the use o f T E M P O i n "living" free radical polymerizations and demonstrated that narrow molecular weight distribution polystyrene ( P D . = 1.10-1.30) could be prepared using bulk polymerization conditions. Recently, it has been shown that nitroxide mediated "living" free radical polymerizations can also be used to control molecular w e i g h t and chain end functional groups while maintaining low polydispersities and permitting the synthesis of block copolymers. These results, combined with the complementary work o f Sawamoto, Matyjaszewski, P e r c e c , and J e r o m e i n the related area o f metal assisted 'living' radical polymerization, have demonstrated that a free radical polymerization process having the characteristics o f a living polymerization can be achieved. 11

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We foresee that i n addition to the intrinsic scientific interest possessed by both the "living" free radical procedure and these new branched macromolecules based on addition monomers, a large number of potential advanced technological applications exist. This is due to the greatly improved properties, such as rheology and compatibilization, that we anticipate for these novel materials when compared to their linear or even star analogs as well as the ability to tailor and control the number and, in some cases, location o f reactive functionalities or catalytic sites. Such property enhancements and/or new properties may prove useful i n a variety o f advanced materials and technological applications such as compatibilizing agents, surface agents, polymeric additives, modifying agents for a variety o f polymer properties (melt rheology, compressibility, modulus, etc.), macromolecular catalysts and carriers (i.e. dye or drug applications), high performance crosslinking agents, etc. In this report, we detail the underlying features o f nitroxide mediated 'living' free radical procedures which permit numerous strategies to be developed for the synthesis o f branched polymers. These branched macromolecular architectures range from graft copolymers to hyperbranched macromolecules and hybrid dendritic-linear block copolymers. The advantages o f 'living' free radical strategies w i l l be compared and contrasted with traditional procedures used for the synthesis o f complex macromolecular architectures. It should be noted that many o f the strategies developed for nitroxide mediated systems can also be successfully applied to A T R P polymerizations. Experimental Section Nuclear magnetic resonance spectroscopy was performed on a Bruker A M 250 F T N M R spectrometer using deuterated chloroform as solvent and tetramethylsilane as internal reference. G e l permeation chromatography was carried out on a Waters chromatograph connected to a Waters 410 differential refractometer with T H F as the carrier solvent. Differential scanning calorimetry was performed on a Perkin Elmer D S C - 7 calorimeter using a scanning rate o f 10 C/minute under a nitrogen atmosphere. The glass transition temperature was defined as the half way point o f transition heat flow. Analytical T L C was performed on commercial Merck plates coated with silica gel GF254 (0.25 m m thick). Silica gel for flash chromatography


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was Merck Kieselgel 60 (230-400 mesh). A l l solvents used for synthesis were dried and distilled i n the appropriate manner before use.


1

f

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1 -Phenyl-1 -(2 * ,2 ,6* ,6 -tetramethy 1-1 -piperidiny loxy )ethane 1, To ethylbenzene (100 ml) was added di-t-butyl peroxide (5.0 g, 33.0 mmol) followed by T E M P O (10.5 g, 66.0 mmol). The reaction mixture was then heated at reflux under argon for 16 hours and evaporated to dryness. The crude product was purified by flash chromatography eluting with hexane gradually increasing to 1:1 hexane/dichloromethane. This gave the T E M P O derivative, 1, as a crystalline white solid which could be recrystallized from cold ethanol (-78°C), (7.20 g, 42%); m.p. 4647°C; IR (neat) 2950, 1490, 1390, 1375, and 1040 c m " ! ; l H N M R (CDCI3) δ 0.64, 1.05, 1.16, 1.36 (each br s, 12H, C//3), 1.23-1.58 (m, 6 H , CH ); 1.44 (d J= 7 H z , 3 2

9

H , C H ( C / / ) ) , 4.76 (q, J= 7 H z , 1 H , C / / ( C H ) ) , and 7.25-7.35 (m, 5 H , A r # ) ; 1 3 c N M R (CDCI3) δ 17.22, 20.34, 23.55, 31.59, 34.49, 40.37, 59.66, 83.10, 126.59, 126.74, 127.97, and 145.84; mass spectrum (EI) m/z 261. 3

3

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l-Benzyloxy-2-phenyl-2-(2 ,2 ,6 ,6'-tetramethyl-r-piperidinyloxy)ethane 13, To a solution o f benzoyl peroxide (4.0 g, 16.5 mmol) i n distilled styrene (160 ml) was added 2,2,6,6-tetramethyl-l-piperidinyloxy ( T E M P O ) (5.68 g, 36.4 mmol) and the solution heated at 80 C under nitrogen for 20 hours. After cooling the solution was evaporated to dryness and purified by flash chromatography eluting with 1:1 hexane/dichloromethane, gradually increasing to 1:9 hexane/dichloromethane to give the modified T E M P O initiator, 13, as a pale yellow oil (2.64 g, 42%); IR (neat) 3100-2850, 1720, and 1200 c m " ; ^ H N M R (CDCI3) δ 0.75, 1.07, 1.21, 1.37 (each br s, 12H, C//3), 1.38-1.52 (m, 6 H , CH ); 4.53 ( A B q , J = 6 H z , 1 H , C / / H ) , 4.83 ( A B q , J= 6 H z , 1 H , C H / / ) , 5.06 ( A B q , J= 3 H z , 1 H , CH), 7.25-7.56 (m, 8 H , ArH) and 7.91 (B o f A B q , J = 6 H z , 2 H , ArH); 13c N M R (CDCI3) δ 17.09, 20.31, 34.00, 40.36, 60.01, 66.68, 83.90, 127.54, 127.97, 128.18, 129.48, 130.14, 132.72, 140.61, and 166.20; mass spectrum (EI) m/z 381; Anal. Calcd. for C 2 4 H 3 1 N O 3 : C , 75.6; H , 8.19; N , 3.67. Found: C , 76.0; H , 7.97; N , 3.86. 1

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l-Hydroxy-2-phenyl-2-(2 ,2 ,6 ,6 -tetramethyl-l -piperidinyloxy)ethane 7, To a solution o f the benzyl ester, 13, (3.2 g, 8.4 mmol) i n ethanol (100 ml) was added aqueous sodium hydroxide (10 m l o f a I N solution, 10.0 mmol) and the solution heated at reflux under nitrogen for 2 hours. After cooling, the solution was evaporated to dryness, partitioned between water (200 ml) and dichloromethane (200 ml) and the aqueous layer extracted with dichloromethane (2 χ 100 ml). The combined organic layers were dried with magnesium sulfate, evaporated to dryness and the crude product purified by flash chromatography eluting with 1:4 hexane/dichloromethane, gradually increasing to 1:9 hexane/dichloromethane to give the hydroxy derivative, 7, as a pale yellow oil (2.01 g, 87%); ; l H N M R (CDCI3) δ I. 14, 1.21, 1.33, 1.50 (each br s, 12H, C//3), 1.38-1.72 (m, 6 H , CH ); 3.71 (br d, J = 2


437

9 H z , 1 H , C i / H ) , 4.21 (d o f d, J= 2 and 6 H z , 1 H , C H / / ) , 5.29 (d o f d, J = 2 and 3 H z , 1 H , C i / ) , 5.88 (br s, O / / ) ; and 7.25-7.56 (m, 5 H , A r i / ) ; C N M R (CDC1 ) δ 17.15, 20.41, 20.73, 32.76, 34.61, 40.23, 40.41, 60.38, 61.69, 69.73, 83.59, 126.20, 127.89, 128.34, and 138.92; mass spectrum (EI) m/z 277; A n a l . Calcd. for C H 7 N 0 : C , 73.6; H , 9.81; N , 5.05. Found: C , 73.8; H , 10.05; N , 5.08. 1

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2-Pheny l-2-(2 ,2 ,6 ,6 * -tetramethy 1-1 -piperidiny loxy)-1 -(4 -viny lbenzy loxy)ethane 4, To a solution o f the alcohol, 7, (1.0 g, 3.6 mmol) i n dry tetrahydrofuran (20 ml) was added sodium hydride (200 mg, 5.0 mmol) and the mixture stirred at room temperature for 10 minutes. A solution o f p-vinylbenzyl chloride (1.52 g, 10.0 mmol, 3.0 equiv.) was added and the mixture stirred at room temperature for 1 hour, then heated at reflux for 16 hours. The reaction mixture was then evaporated to dryness and partitioned between dichloromethane (50 ml) and water (50 ml) and the aqueous layer extracted with dichloromethane (2 χ 50 ml). The combined organic layers were dried, evaporated to dryness and purified by flash chromatography eluting with 1:1 hexane/dichloromethane increasing to dichloromethane. This gave the styrene derivative, 4, as a pale yellow oil. Y i e l d 71%; ! H N M R ( C D C l 3 ) δ 0.63,1.01 (each br s, 6 H , Ci/3), 1.15-1.55 (m, 12H, 3 χ CH and 2 χ C / / ) , 3.65 ( A B q , J= 6 H z , 1 H , C i / H ) , 3.95 ( A B q , J= 6 H z , 1 H , C H / / ) , 4.41 (s, 2 H , CH ), 4.84 (d o f d, J= 2 and 6 H z , 1 H , C i / H ) , 5.20 (d o f d, J= 2 and 7 H z , 1 H , =CUH), 5.71 (d o f d, J= 2 and 6 H z , 1 H , =CH#), 6.66 (d o f d, J = 6 and 7 H z , 1 H , =Ci/), 7.08 (d, 2 H , AiH) and 7.25-7.52 (m, 7 H , ArH); C N M R (CDCI3) δ 17.17, 20.56, 33.87, 40.48, 59.36, 72.74, 72.83, 85.41, 113.52, 126.04, 127.24, 127.53, 127.82, 127.88, 136.62, 138.14, and 141.79; mass spectrum (EI) m/z 393. 2

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General Procedure for Synthesis of Dendritic Initiators, [G-4]-init, 11, To a solution o f the alcohol, 7, (392 mg, 1.50 mmol) i n dry tetrahydrofuran (25 ml) was added sodium hydride (80 mg, 60% dispersion i n oil, 2.0 mmol) and the reaction mixture was stirred under argon at room temperature for 15 minutes. The dendritic bromide, [G-3]-Br, 10, (1.70 g, 0.51 mmol) dissolved in dry tetrahydrofuran (10 ml) was then added dropwise and the reaction mixture heated at reflux under argon for six hours. The reaction mixture was then cooled, evaporated to dryness, and partitioned between dichloromethane (100 ml) and water (100 ml). The aqueous layer was extracted with dichloromethane (2 χ 50 ml) and the combined organics dried and evaporated to dryness. The crude product was purified by flash chromatography eluting with dichloromethane gradually increasing to 1:9 diethyl ether/dichloromethane to give the dendritic initiator, 11, as a colorless foam (1.61 g, 86%); I R (neat) 2950, 1500, 1380, and 1030 c m " ; H N M R (CDCI3) δ 0.54 (br s, 3 H , Ci/3), 0.91-1.50 (br m, 15H, CH and C//3), 3.55 ( A B q , 1H, C H / / ) , 3.88 ( A B q , 1H, C H / / ) , 4.36 (s, 2 H , CH ), 4.85, 4.92 and 5.00 (each s, 2 8 H , A r C / / 0 ) , 4.89 ( A B q , 1H, CH), 6.35-6.60 (complex m, 2 1 H , ArH), and 7.25-7.35 (m, 45 H , ArH); C N M R (CDCI3) δ 17.30, 20.49, 34.10, 39.76, 40.58, 60.06, 70.09, 70.18, 72.93, 1

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85.51, 101.35, 101.71, 106.00, 106.50, 127.18, 128.05, 128.70, 136.90, 139.37,


139.50, 141.31, 142.10, 159.96, 160.17, and 160.28. Copolymer of 4 with styrene, 5, A solution o f the styrene-TEMPO monomer, 4, (450 mg, 1.15 mmol), styrene (2.40 g, 23.0 mmol, 20 equiv.) and A I B N (40 mg, 0.23 mmol) i n dry tetrahydrofuran (20 ml) was heated at reflux under argon for 24 hours. The reaction mixture was evaporated to dryness, redissolved in dichloromethane (10 ml) and precipitated into methanol (500 ml) followed by hexane (500 ml). The copolymer, 5, was isolated as a white powder. Y i e l d 72%; M = 12 000 and P D = 1.80; ! H N M R (CDCI3) δ 0.65, 0.90-1.70, 3.65, 3.95, 4.30, 4.82, and 6.40-7.25 (br m); C N M R (CDCI3) δ 17.25, 39.0-43.5, 125.3 (br), 127.5 (br), 128.30, and 144.7-145.8 (a number o f resonances were too small to observe). n

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Preparation of graft polymers, 8, A solution o f the polymeric initiator, 5, (200 mg, 0.085 mmol equiv.) in styrene (1.82 g, 17.5 mmol, 200 equiv.) was heated at 130°C with stirring under argon for 72 hours. During this time the viscosity o f the solution was observed to gradually increase

and the clear reaction mixture eventually

solidified. The reaction mixture was then dissolved i n dichloromethane (25 ml) and precipitated into hexane (1 1) followed by re-precipitation into methanol (1 1). The graft polystyrene, 8, was isolated as a white solid after drying. Y i e l d 80%; M 000 and P D = 2.01;

N M R (CDCI3) δ 0.90-1.70 (br m), and 6.40-7.25;

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(CDCI3) δ 39.0-44.5, 125.0 (br), 127.5 (br), and 144.5-146.0.8 General Procedure for Macromonomer Copolymerization A mixture o f the initiator, 1, (65.0 mg, 0.25 mmol) and poly(caprolactone) macromonomer, 2, ( M = 3 500, P D . = 1.10) (2.00 g, 0.6 mmol) was dissolved i n styrene (7.02 g, 67.5 mmol) and the reaction mixture heated at 125°C for 48 hours. During this time the polymerization mixture became progressively more viscous and eventually solidified. The reaction mixture was then cooled to room temperature and redissolved in tetrahydrofuran (25 ml) and precipitated (2x) into methanol (1 1) to give the graft copolymer, 13, as a white solid; (8.24 g, 82 %); IR. 3200-3000, 2920, 2850, 1720, 1600, 1490, 1470 c m " ; Η N M R (CDCI3) δ 1.28-2.05 (complex m), 2.30 (t, C # C O ) ; 4.10 (t, C # O C O ) ; and 6.40-7.22 (complex m); C N M R (CDCI3) δ 22.30, 40.20, 40.35-45.0 (broad multiplet), 53.20, 125.40, 127.62, 127.98, 144.50145.5 (br), and 173.5. n

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Results and Discussion One o f the unique features associated with 'living' free radical procedures which permit the synthesis o f numerous complex macromolecular architectures is the compatibility o f the alkoxyamine initiating center with a wide variety o f reaction conditions. This allows functionalized unimolecular initiators to be readily prepared and then polymerized, or coupled with a preformed polymer, to give well 17


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defined multifunctional or polymeric initiators. This feature, coupled with the low occurrence o f radical-radical coupling reactions, allows a wide variety o f complex macromolecular architectures to be prepared. Alternatively, the tolerance o f nitroxide mediated 'living' free radical polymerizations to various functional groups permits well defined linear polymers with functionalized chain ends or backbone units to be prepared which can then be coupled to give unusual graft, star, and block polymers. G r a f t Copolymers. The high degree o f latitude available in the design o f synthetic strategies for the preparation o f complex macromolecular architectures is perhaps best appreciated by examining the synthesis o f graft copolymers using 'living' free radical procedures. Essentially all possible strategies for the construction o f graft copolymers can be, or have been, accomplished using 'living' free radical chemistry. For example, the polymerization o f macromonomers has been demonstrated using the alkoxyamine, 1, as unimolecular initiator. A s shown in scheme 1, copolymerization o f methacrylate terminated poly(caprolactone), 2, with styrene using the unimolecular 1 gives the graft copolymer, 3, which was shown to have the anticipated structure by a variety o f spectroscopic and chromatographic techniques. Significantly, even under standard bulk polymerization conditions a variety o f macromonomers could be readily incorporated with the weight percentage o f macromonomer in the final graft copolymer being essentially the same as the feed percentage. The degree o f control over the structure o f the graft copolymers was also not affected by the bulk polymerization conditions and low polydispersity materials with controlled molecular weights were routinely obtained. It should be noted that the preparation o f well defined graft copolymers o f styrene and poly(caprolactone) by other techniques is not feasible due to the reactivity o f the ester linkages in poly(caprolactone) and further demonstrates the versatility o f 'living' free radical procedures. 18

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Alternatively, a polymeric initiator containing numerous initiating sites along the backbone can be prepared and then arms grown from the backbone to give the desired graft copolymer. The versatility of 'living' free radical procedures is further exemplified in this instance since the two possible approaches to polymeric initiators can be realized using alkoxyamine derivatives. In the first example, a monomer unit, 4, containing a single alkoxyamine group is polymerized under normal free radical conditions to give the desired linear polymeric initiator, 5 (Scheme 2 ) . This result is noteworthy since it demonstrates the stability o f the initiating alkoxyamine group to standard free radical conditions and may indicate that the initiating center is stable to a variety o f other polymerization conditions. If this is the case, new graft copolymers containing vinyl grafts and backbones formed from condensation, metathesis, or ring opening polymerizations may be easily prepared. Polymeric initiators, such as 5, can also be prepared by coupling o f reactive polymers with a functionalized alkoxyamine. For example, a mixture o f styrene and p-chloromethylstyrene can be polymerized using the unimolecular initiator, 1, to give a well defined linear polymer, 6, which contains numerous chloromethyl groups along the backbone. Reaction o f 6 with the sodium salt o f the hydroxy functionalized alkoxyamine, 7, then gives the polymeric initiator, 5 (Scheme 2 ) . The advantage o f this route is that the molecular weight and polydispersity o f the backbone polymer 2 0

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Scheme 1



7 4

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Scheme 2


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can be readily controlled by the 'living' free radical procedure and also demonstrates the compatibility o f nitroxide chemistry with reactive functional groups. In both cases a range o f graft copolymers could be readily grown from the backbone and it was found that the individual grafts grow at approximately the same rate with little or no crosslinking due to radical coupling reactions. H y p e r b r a n c h e d Macromolecules. The ability to introduce polymerizable groups into the alkoxyamine structure also raises the possibility o f preparing hyperbranched macromolecules by nitroxide mediated 'living' free radical procedures. The success o f this approach is based on the recent report by Frechet o f a novel self-condensing vinyl polymerization which, under cationic conditions, affords hyperbranched p o l y m e r s . The important feature o f this self-condensing strategy is a single monomer unit which contains a polymerizable double bond as well as an initiating center. Application to alkoxyamines then gives self-condensing monomers such as 4 which contain a polymerizable styrene group as well as an initiating/propagating alkoxyamine group. Therefore, polymerization o f 4 at 130°C was found to give the hyperbranched macromolecule, 9, whose structure was confirmed by a combination o f nmr and infrared spectroscopy (Scheme 3 ) . Significantly, no insoluble or cross-linked material was observed, which provides further support for the low occurrence o f termination by radical-radical coupling i n nitroxide mediated systems. Interestingly, when the kinetics o f the reaction are investigated it is apparent that the polymerization involves the step-wise coupling o f vinyl monomers and oligomers, typical o f condensation polymerizations, yet it proceeds by 'living' free radical vinyl chemistry. This can be better appreciated i f the evolution o f molecular weight is followed with time which clearly shows a gradual build up, and then disappearance, o f dimers, trimers, and low molecular weight oligomers (Figure 1). It should be noted that this system is different from the Cu(I) assisted 'living' free radical polymerization o f pchloromethylstyrene as reported by Matyjaszewski. In contrast to the above case, where propagation occurs from only secondary benzylic radical intermediates, the polymerization o f p-chloromethylstyrene involves both primary and secondary radical intermediates. Since it has been recently shown that propagation from a primary radical is a much less efficient process than from a secondary r a d i c a l , the structure of both hyperbranched systems should be different. In the alkoxyamine case, the polymerization is a true self-condensing vinyl polymerization leading to a hyperbranched macromolecule with a higher degree o f branching than the branched macromolecule obtained from p-chloromethylstyrene. 22

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The versatility o f 'living' free radical systems can also be used to extend this concept o f vinyl based hyperbranched macromolecules to hyper-star and randomly branched macromolecular architectures. The presence o f bound alkoxyamine groups at the numerous chain ends o f 9 permits further chain growth to be initiated and star branched polymers with a hyperbranched core to be obtained. Alternatively the selfcondensing monomer, 4, can be copolymerized with normal vinyl monomers to give novel randomly branched macromolecular architectures. In this case, 4, acts as both the initiating group for the formation o f linear units as well as branch points for the assembling o f these linear units into a randomly branched structure. The 20



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Scheme 3



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Elution volume (ml)

F i g u r e 1. Gpc traces for the homopolymerization o f the self-condensing monomer, 4, at (a) 8 hours; (b) 16 hours; (c) 30 hours; and (d) 44 hours.


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interesting feature o f these novel macromolecular architectures is that the linear units have low polydispsersities and controlled molecular weights which further reinforces the degree o f control available in 'living' free radical polymerizations. Hybrid Dendritic-Linear Macromolecules. The compatibility o f the alkoxyamine functionality with a wide range o f reaction conditions also allows their incorporation into a variety o f macromolecular architectures at predetermined locations. One example o f this modular approach to the preparation o f complex structures is the synthesis o f hybrid dendritic-linear block copolymers by 'living' free radical procedures. In this case, dendritic poly ether macromolecules containing a single bromomethyl group at the focal point are prepared by the convergent growth approach. The focal point group is then used as the attachment point for the functionalized alkoxyamine by reaction o f 10 with 7 in the presence o f sodium hydride. This gives the dendritic initiator, 11, which has a single alkoxyamine initiating group at its focal point (Scheme 4). A variety o f monomers and comonomer mixtures can then be polymerized under standard 'living' free radical conditions using 11 to give a series o f novel hybrid dendritic-linear block copolymers, 12, i n which the molecular weight o f the linear block could be accurately controlled by the ratio o f 11 to monomer. For example, polymerization o f a 4:1 mixture o f styrene and methyl methacrylate (150 equivalents) with 11 gave the block copolymer, 12, which was shown to have a M o f 18 000 and a polydispersity o f 1.10. This compares favorably with the theoretical molecular weight o f 17 500 (Scheme 5). The structure o f 12 was confirmed by Η and C nmr spectroscopy and G P C which showed the total absence o f starting dendrimer in the final hybrid block copolymer (Figure 2). It should be noted that the synthesis o f hybrid dendritic-linear block copolymers by other techniques is either difficult, or in the case o f 12 not possible. 25

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Conclusion It is apparent from the above discussion that the versatility o f 'living' free radical procedures, coupled with their synthetic ease, opens up a number o f unique possibilities i n polymer synthesis, especially in the preparation o f complex macromolecular architectures. Unique polymeric architectures, such as hybrid dendritic-linear block copolymers, which are difficult, i f not impossible, to prepare using known techniques can now be prepared readily. A l s o known macromolecular architectures such as graft copolymers can be prepared with significantly greater synthetic ease while permitting an improvement in structural control. It is envisaged that the increased availability o f these complex macromolecular architectures, coupled with the ability to readily incorporate reactive functional groups, w i l l lead to advanced materials for a variety o f technological applications ranging from adhesives to advanced coatings. 26

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Acknowledgments. Financial support from the National Science Foundation under Grant N o . DMR-9400354 which supports the Center for Polymeric Interfaces and



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Scheme 5



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Elution volume (ml) F i g u r e 2. Comparison o f the G P C traces for the starting (a) dendritic initiator, 11, and (b) the final hybrid dendritic-linear block copolymer, 12.


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Macromolecular Assemblies, NSF grant DMR-9641291, and by the IBM Corporation is gratefully acknowledged.

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