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Synthesis, Characterization and Potential Applications of Functionalized Polyethylene Materials

Dissertation
Author: Yanika Schneider
Abstract:
Functionalized polyethylene (PE) materials were prepared using two different synthetic pathways. The first strategy involved the synthesis of PE copolymers bearing functionalities capable of atom transfer radical polymerization (ATRP), followed by grafting with (meth)acrylic monomers. A variety of PE macroinitiators were prepared by copolymerizing ethylene with an initiating comonomer, 5- norbornen-2-yl-2'-bromo-2'-methyl propanoate ( 3 ) using either Ni α-iminocarboxamidato complexes (1a-c ), activated with nickel bis(1,5-cyclooctadiene) (Ni(COD)2 ), or a Ni α-keto-β-diimine complex (7 ), activated with methylaluminoxane (MAO). PE macroinitiators were subsequently grafted with methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), n -butyl acrylate (nBA), tert -butyl acrylate (tBA) using ATRP. In addition to allowing control over comonomer incorporation and the molecular weight characteristics of the products, this strategy also made it possible to access higher-order architectures, such as triblock and pentablock structures. Grafting with MMA, HEMA, and tBA produced hydrophilic polyolefins, while grafting with nBA generated elastomeric materials with excellent mechanical properties. The second method for generating functionalized PE materials took advantage of ketene chemistry in crosslinking and grafting reactions. First, a Meldrum's acid functionalized norbornene ( 4 ) that generates ketenes at elevated temperatures was incorporated into PE using 1b /Ni(COD)2 . Efficient crosslinking of PE copolymers took place at 185 °C through ketene dimerization, while functionalization was performed in the presence of small molecule and polymer nucleophiles. Unlike other crosslinking techniques, this strategy did not require small molecule radical sources or catalysts, and provided tunable crosslinking densities in PE, as demonstrated by rheological and tensile experiments. Both synthetic pathways provide opportunities to prepare novel hybrid materials, with an unprecedented level of control over the polymer composition, crystallinity, and topology of the products.

x TABLE OF CONTENTS Literature Review and Research Motivation …………………………………… 1 Chapter 1: PE Macroinitiators Prepared u-Iminocarboxamidato Complexes ……..…………………………………………………….21 Introduction ………………….…………….………………………....22 Section 1.1: Inimer Synthesis and Reactivity …..…………………….26 -Iminocarboxamidato Initiators………..28 Section 1.3: Ethylene Homopolymerizations using Isobutyl -Iminocarboxamidato Analogue…………………..…...32 Section 1.4: Effect of Polymerization Conditions on the Properties of PE Macroinitiators ……………………………………… 34 Section 1.5: Novel Triblock Structures Prepared by Pressure Pulsing..39 Conclusion ………………………………………………….………...43 Experimental ……………………………………….………….……...45 References ……………………………………….………….………...49 Chapter 2: Hydrophilic Materials: Grafting with Methyl Methacrylate, 2-Hydroxyethyl Methacrylate, and tert-Butyl Acrylate 50 Introduction …………………………………….………………….… 51 Section 2.1: Synthesis and Characterization of PMMA Grafts ……… 56 Section 2.2: Synthesis and Characterization of PHEMA Grafts ……..70 Section 2.3: Synthesis and Characterization of PtBA Grafts and their Conversion into PAA Grafts ..………………………….80 Conclusion ………………………………….……………………......95 Experimental …………………………………..……………………..97 References ……………………………………….………….………..101 Chapter 3: Elastomeric Materials: Grafting with n-Butyl Acrylate............104 Introduction …….………………………………….……….…...........105 Section 3.1: Synthesis and Characterization of PE Macroinitiators....109 Section 3.2: Synthesis and Characterization of Graft Copolymers ......119 Section 3.3: Synthesis and Characterization of Elastomers Prepared using a Binuclear Initiator……………………………… 128 Conclusion…………………………………………………................144 Experimental …………………………………..……………………..146 References ……………….……………………….………….……….149

xi Chapter 4: Meldrum’s Acid Functionalized PE: A Versatile System for Crosslinking and Functionalization ……………………………… 152 Introduction …….………………………………….……….….........154 Section 4.1: Synthesis and Characterization of Meldrum’s Acid Copolymers ………..…..…………...............................156 Section 4.2: Crosslinking Experiments…………………………….164 Section 4.3: Mechanical Properties of Crosslinked PE…………….167 Section 4.4: Synthesis of Graft Copolymers using Nucleophilic Addition …………..…..…………................................169 Conclusion ………………………………………………….............175 Experimental …………………………………..…………………….177 References ……………………………………….………….……….180 -keto-

-diimine Complex ….………………………………………………………....182 Introduction ………………………….……….…………………......183 Section 5.1: Synthesis of PE Homopolymers under Living and Quasi-Living Conditions ………………………….187 Section 5.2: Synthesis and Characterization of MA Copolymers…..196 Section 5.3: Synthesis and Characterization of PE Macroinitiators and their Grafts ……………………………………….200 Section 5.4: Chain Transfer Polymerizations using Main Group Metal Alkyls .…………………………........................210 Conclusion ………………………….……………………………….214 Experimental ……..……………………..…………………………..216 References ……………………………………….………….……… 219 Concluding Remarks …………………………………………………….…… 221

xii NOMENCLATURE All polymers are named according to the entry in which they are prepared. Polymer names are always provided in bold, according to the following designation: “C1T1E1” denotes a material prepared in Chapter 1, Table 1, in Entry 1 ABBREVIATIONS AA – acrylic acid ATR-IR – attenuated total reflectance infrared ATRP – atom transfer radical polymerization 2-BB – 2-bromoisobutylryl bromide DMA – dynamic mechanical analysis DSC – differential scanning calorimetry GC/MS – gas chromatography/mass spectrometry GPC – gel permeation chromatography HEMA – 2-hydroxyethyl methacrylate HMTETA – hexamethyldiethylenetriamine Inimer – initiating comonomer MAO – methylaluminoxane MMA – methyl methacrylate

xiii M n – number average molecular weight nBA – n-butyl acrylate Ni(COD) 2 – nickel bis(1,5-cyclooctadiene) NMR – nuclear magnetic resonance PDI – polydispersity index PE – polyethylene PMDETA – pentamethyldiethylenetriamine tBA – tert-butyl acrylate TGA – thermogravimetric analysis TEM – transmission electron microscopy T m – melting point T g – glass transition temperature WAXS – wide angle x-ray scattering X c – degree of crystallinity pTSA – p-toluene sulfonic acid

xiv LIST OF FIGURES Figure L.1 Three major grades of polyethylene……………………………….3 Figure L.2 Methodologies for preparing functionalized PE……………….….6 Figure L.3 Structures of late transition metal olefin polymerization initiators 7 Figure L.4 Higher-order architectures accessible via controlled radical polymerization methods………………………………………………………...9 Figure L.5 Synthetic strategy for preparation of functionalized PE materials...16 Figure 1.1 -iminocarboxamidato complexes 1a-g, Ni(COD) 2 , and 5-norbornen-2-yl acetate (2) …………………………………..22 Figure 1.2 GC-MS of inimer 3..........................................................................27 Figure 1.3 Reactivity of inimer (3) with pre-catalyst (1a) and co-activator Ni(COD) 2 ……………………………………………………………………… 28 Figure 1.4 1 H NMR spectrum of PE macroinitiator C1T1E5 ……………..… 30 Figure 1.5 Evolution of M n versus time for 1b/Ni(COD) 2 ...………………… 32 Figure 1.6 Evolution of M n and PDI over time ……………………………….35 Figure 1.7 Effect of polymerization temperature ……………………………...38 Figure 1.8 Cartoon representation and physical properties of a triblock copolymer…………………………………………………………………….… 40

xv Figure 2.1 ATRP Mechanism…………………………………………..……..52 Figure 2.2 Cartoon representation of grafting strategies …………………….54 Figure 2.3 GPC traces of PE macroinitiator C1T4E1 and PMMA graft C2T1E1 ………………………………….……………………………… 59 Figure 2.4 Molecular structures of Cu ligands PMDETA and HMTETA…...61 Figure 2.5 GPC trace of PMMA side chain obtained via hydrolysis of C2T2E1 ………………………………………………………..64 Figure 2.6 Solution 1 H NMR spectrum of MMA graft copolymer C2T2E1..65 Figure 2.7 DSC thermograms of MMA graft copolymers…………………...66 Figure 2.8 ATR-IR spectra of MMA graft copolymers……………………...67 Figure 2.9 Thermogravimetric analysis of MMA graft copolymers………...68 Figure 2.10 Solution 1 H NMR spectrum of soluble fraction of C2T9E1…..75 Figure 2.11 Solid-state 13 C NMR of HEMA graft copolymer C2T7E1…….76 Figure 2.12 13 C NMR spectra of 1h and 3h HEMA graft copolymers C2T7E1-2………………………………………………………..77 Figure 2.13 ATR-IR spectra of HEMA graft copolymers……………….…..78 Figure 2.14 Thermogravimetric analysis of HEMA graft copolymers ….….79 Figure 2.15 1 H NMR spectrum of tBA graft copolymer C2T11E2................84 Figure 2.16 GPC traces of PE macroinitiator C2T10E3 and tBA graft C2T11E5 ………………………………………………………..… 85 Figure 2.17 DSC thermogram of tBA graft copolymer C2T11E1 ……….… 86 Figure 2.18 Solid-state 13 C NMR spectra of PtBA graft C2T11E4 and PAA graft C2T12E6 …...…………………………………………………….90 Figure 2.19 Water contact angle measurements on thin films of PE macroinitiator C2T10E1, PtBA graft C2T11E3 and PAA graft C2T12E5 90 Figure 2.20 ATR-IR spectra of tBA and AA graft copolymers…………......92 Figure 2.21 Thermogravimetric analysis tBA and AA graft copolymers …………………………….….……………………….…… 94

xvi Figure 3.1 Cartoon representation of block copolymer phase separation that gives rise to physical crosslinks in thermoplastic elastomers 105 Figure 3.2 Degree of crystallinity and 3 incorporation as a function of [3] 0 111 Figure 3.3 Evolution of thermal properties of PE macroinitiators………… 113 Figure 3.4 Wide-angle X-ray scattering of macroinitiator C3T1E4…….… 114 Figure 3.5 Dynamic mechanical analyses of PE macroinitiators………….115 Figure 3.6 Monotonic stress-strain curves of PE macroinitiators………….116 Figure 3.7 Comparison of elastic recoveries of PE macroinitiators…….… 118 Figure 3.8 1 H NMR spectrum of nBA graft C3T4E4………………..……...120 Figure 3.9 Monotonic stress-strain curves of graft copolymers prepared from macroinitiators with low inimer content……………………………………… 123 Figure 3.10 Monotonic stress-strain curves of graft copolymers prepared from macroinitiators with high inimer content……………………………………...124 Figure 3.11 Comparison of elastic recoveries of graft copolymers prepared from macroinitiators with low inimer content………………………………… 126 Figure 3.12 Comparison of elastic recoveries of graft copolymers prepared from macroinitiators with high inimer content………………………………..127 Figure 3.13 Cartoon representations of elastomeric graft copolymers prepared using initiators 1b, and 2b ………………………………...………...128 Figure 3.14 GPC traces of triblock and pentablock PE macroinitiators ….....132 Figure 3.15 Monotonic stress-strain curves C3T6E1-5……………………… 136 Figure 3.16 True stress versus true strain curves of graft copolymers C3T7E1-3………………………………………………………...137 Figure 3.17 Elastic recoveries of graft copolymers C3T7E1-3…………….… 139 Figure 3.18 Monotonic stress-strain curves of pentablock macroinitiators…..141 Figure 3.19 True stress versus true strain curves of graft copolymers C3T7E3-6………………………………………………………..142 Figure 3.20 Elastic recoveries of graft copolymers C3T7E3-6 …………..143

xvii Figure 4.1 The chemistry of ketenes ...………………………………………..154 Figure 4.2 Molecular structure of Meldrum’s acid functionalized norbornene 4 ………………………………...………………………………… 155 Figure 4.3 1 H NMR spectrum of PE-co-MelA copolymer C4T2E5 ….….…..157 Figure 4.4 DSC thermograms of PE-co-MelA copolymers C4T1E1-4.……..162 Figure 4.5 ATR-IR spectra of PE and PE-co-MelA copolymer C4T2E5.…...163 Figure 4.6 Kinetics of ketene formation of 5 at 185 °C ………………….…...164 Figure 4.7 Gel content as a function of 4 incorporation ……………………...166 Figure 4.8 Stress-strain curves of crosslinked materials …………….….…… 168 Figure 4.9 Structures of nucleophiles utilized in grafting reactions …….…..170 Figure 4.10 1 H NMR spectrum of PE-co-MelA copolymer C4T1E3 and pyrene nucleophile 6b ………………………………………………..…… 172 Figure 4.11 1 H NMR spectrum of PEG graft C4T6E6 …..……………….…...173

xviii Figure 5.1 Molecular structure of complex 7……………………………….183 Figure 5.2 GPC traces of C5T1E6 (methanol quench) and C5T1E7 (Et 3 SiH quench)……………………………………………………………… 190 Figure 5.3 Evolution of M n as a function of time ………………………..… 191 Figure 5.4 GPC trace of PE homopolymer C5T3E1……………………..… 195 Figure 5.5 DSC thermogram of PE homopolymer C5T3E1 …………..…...195 Figure 5.6 1 H NMR spectrum of PE-co-MA copolymer C5T5E4 ………...198 Figure 5.7 DSC thermogram of PE-co-MA copolymer C5T3E1 ……….… 198 Figure 5.8 1 H NMR spectrum of PE macroinitiator C5T6E5 ………..…….203 Figure 5.9 DSC thermogram of PE macroinitiator C5T6E1 …………..…...203 Figure 5.10 NMR spectrum of MMA graft copolymer C5T6E1 …….….… 205 Figure 5.11 GPC traces of PE macroinitiator C5T5E8 and PMMA graft C5T6E3 …………………………………………………………….…..207 Figure 5.12 NMR spectrum of nBA graft copolymer C5T7E1 ………..…… 208 Figure 5.13 Coordinative chain transfer polymerization …………………….210

1 Literature Review and Research Motivation

2 Synthesis of Functionalized Polyethylene Polyethylene (PE) is the largest volume commodity plastic, with the world- wide production exceeding 100 million tons annually. 1 This is mainly due to its excellent physical, mechanical and chemical properties, as well as the relative low cost of its monomer (ethylene),which is a byproduct of oil and natural gas refinement.PE is significantly less dense than other materials, and can be easily melt- processed and recycled. 2 Stable against acid, moisture and UV degradation, PE also exhibits improved crack resistance over other commodity plastics. 3 The microstructure of PE has a profound effect on its physical properties, and the three main PE architectures are illustrated in Figure L.1. The degree of branching influences the density of the material, which in turn determines its commercial application. At one extreme is high density polyethylene (HDPE), which contains a few to no branches,and thus has the highest relative density. This material is prepared by coordination insertion polymerization, and exhibits the highest melting point (T m ), degree of crystallinity (X c ) and tensile strength . HDPE is employed in milk jugs, detergent bottles, garbage containers and water pipes. At the other extreme is the low density polyethylene (LDPE), which is a highly branched material prepared via a high temperature, high pressure radical polymerization. This material has a T m that is typically 15-20 °C below that of HDPE. As a consequence of its branched microstructure, LDPE has a lower tensile strength, but also increased ductility relative to HDPE. This material is typically used in plastic bags and food packaging. 4

3 Figure L.1.Three major grades of polyethylene 5 T m is melting temperature; In between the two extremes we find linear low density polyethylene (LLDPE), which -olefins prepared by coordination-insertion polymerization.The branches in this material are uniform in length and evenly distributed along the backbone,which results in improved mechanical properties and chemical resistance relative to LDPE. Because of its toughness, flexibility and transparency, LLDPE is predominantly employed in film applications, including stretch wrap, bubble wrap, and multi-layer composite films. 6 LDPE LLDPE HDPE T m

= 0.94 g/cm 3 = 32 MPa branches 5 T m = 115-130 °C = 0.92-0.93 g/cm 3 = 17-27 MPa branches = 10-30 T m °C = 0.91 g/cm 3 = 14 MPa branches

4 First synthesized and characterized in the 1930s,PE in the low density form became a commercially relevant product during World War II, when the British used it for insulation of radar cables. 7 In 1953, Karl Ziegler and Erhard Holzkamp demonstrated the synthesis of HDPE at reduced pressures and temperatures using a variety of titanium catalysts activated with alkyl aluminums. 8 Several years later, Giulio Natta utilized this catalyst/cocatalyst combination to prepare materials with controlled tacticity, and together with Ziegler he won the 1963 Nobel Prize in chemistry for the development of olefin polymerization catalysts. 9 With the advent of single-site metallocene catalysts, 10 new grades of PE could be attained. These include materials ranging from very low density polyethylene (VLDPE),utilized in various tubing and packaging applications, to ultra-high molecular weight polyethylene (UHMWPE),employed in bullet-proof vests and hip replacement components. 11 Despite its superior properties, PE has a number of disadvantages that are associated with the simplicity of its repeat unit. Lack of polar functionality leads to low surface energy, which causes problems with adhesion, dyeability, printability, and compatibility with polar materials. 12 Over the last thirty years, research efforts in industry and academia have focused on finding efficient strategies for incorporating polar functionality into PE by copolymerizing ethylene with polar comonomers. However, because traditional olefin polymerization systems employ early transition metals that are highly oxophilic, they are readily deactivated in the presence of commodity polar monomers, such as (meth)acrylates. 13 Indeed, the only way to produce functionalized PE using early transition metal complexes is by employing

5 special monomers containing methylene spacers between a C-C double bond and polar functionality, which introduces significant extra costs. 14 An alternative strategy for preparing PE containing polar groups involves the use of late transition metal complexes that are more tolerant of functionality. 15 There are three main families of late transition metal initiators capable of copolymerizing ethylene with polar monomers,and their structures are provided in Figure L.2. For example, salicylaldimine-based complexes have been successful in synthesizing copolymers of ethylene and functionalized norbornenes in a controlled fashion, producing materials with high molecular weights and moderate polydispersity indices (PDIs). 16 Comonomer incorporation is between 10-31 mol %, and the products have a linear structure,with high melting points and moderate degrees of crystallinity. In contrast,Pd and Ni -diimine complexes typically generate hyperbranched, amorphous materials,with a structure somewhere in between LDPE and VLDPE,due to “chain walking.” 17 P -diimine initiators are capable of copolymerizing olefins with methyl acrylate, methyl vinyl ketone,and silyl vinyl ethers. 18 Although high incorporation of comonomer is reported, the functional groups are generally located at chain ends,due to chain walking after comonomer insertion. In contrast, there are few -diimine complexes that are capable of copolymerizing ethylene with a polar monomer like methyl acrylate (MA) because the catalyst is significantly poisoned by the O-binding mode of the monomer. Thus, high pressures and temperatures must be employed, and even these conditions result in low molecular weights and less than 1 mol % incorporation of functionality. 19

6 Figure L.2.Structures of late transition metal olefin polymerization initiators Palladium phosphine-sulfonate complexes have demonstrated a great deal of success in copolymerizing a variety of polar monomers, including acrylates, vinyl acetate, vinyl halides, and vinyl ethers. 20 The microstructure of the products is highly linear, and the monomer is incorporated into the PE backbone, not at chain ends, as has been repor -diimine initiators. The main limitation with this catalytic system is the moderate polymerization activity, and the low number average molecular weight (M n ) of the products, which is typically between 10-20 kg/mol. However, one recent report demonstrated the synthesis of a MA copolymer with M n = 41 kg/mol using a Pd sulfonate complex bearing bulky methoxy groups. 21 Despite these recent advances in late transition metal catalysis, it is challenging to obtain semicrystalline PE copolymers with high molecular weights and substantial incorporation of polar functionality via the direct copolymerization technique.Other strategies for generating functionalized PE materials have been reported, and the three main methods are illustrated in Figure L.3. For example, post- -Diimine Salicylaldimine Phosphine-Sulfonate

7 functionalization reactions can be performed on pristine PE, in the presence of a radical source and polar monomers. 22 Although this method is utilized in industry, it affords little control over the final polymer structure and composition due to the use of harsh reagents, such as peroxides. 23 Similarly, functionalized PE can be prepared through the use of a reactive “intermediate” that can be subsequently converted into block or graft copolymers. This strategy is particularly promising because it allows for high incorporation of functionality, while preserving the original excellent physical properties of PE. Because the second polymerization step takes advantage of controlled free radical techniques, the next section will highlight the development and application of this polymerization methodology. Figure L3.Methodologies for preparing functionalized PE

8 Controlled Free Radical Polymerizations Until recently, the synthesis of polymers with higher-order architectures was only possible using living polymerization techniques.This is because living polymerizations are distinguished by lack of termination or chain transfer reactions that enable the synthesis of well-defined materials, with predictable molecular weights,and PDIs close to unity. The controlled propagation mechanism allows for the preparation of a variety of polymer architectures, as shown in Figure L.4, including block, graft, segmented, gradient, and end-functionalized structures. Unfortunately, living polymerizations typically employ highly reactive initiators, such as sec-butyl lithium, and thus must be carried out with complete exclusion of oxygen and moisture, and at lower temperatures. 24 Further, monomers with polar functionalities typically lead to loss of control over the polymerization, so the range of monomers that can be targeted via this method is quite limited. In contrast, conventional free radical polymerization is the most widely used method of polymer synthesis because it can be employed on a large variety of monomers, and requires less stringent reaction conditions.For example, water is well-tolerated in these reactions,and a more convenient temperature range (0-100 °C) can be employed. Unfortunately,despite their versatility,conventional free radical polymerizations suffer from slow initiation, very fast propagation and fast termination, all of which lead to poor control over the resulting polymer structure. Because the lifetime of a propagating chain is typically very short, and the chain is

9 essentially ‘dead’ only a fraction of a second after it has been formed, higher-order architectures cannot be achieved using conventional free radical techniques. In the early 1990s, a new polymerization methodology was introduced called “controlled free radical polymerization” or CRP, which combines the functional group tolerance of free radical polymerizations with the controlled reaction kinetics of anionic polymerizations. 25 CRPs are distinguished from conventional free radical methods by much faster initiation rates, and the presence of a dynamic equilibrium between propagating and dormant species, which decreases the amount of termination reactions from 100 to 10%. In this manner, the lifetime of the growing chain Figure L.4.Higher-order architectures accessible via controlled radical polymerization methods (adapted from ref. 25a)

10 is extended from less than one second to several hours, allowing for the synthesis of sophisticated polymer structures. The three main CRP methodologies are classified by their mechanism of intermittent activation: nitroxide-mediated polymerization (NMP), 25b atom transfer radical polymerization (ATRP), 25a and reversible addition-fragmentation chain transfer (RAFT). 25c In the case of NMP,the equilibrium between propagating radicals and dormant chains is mediated by nitroxide-based stable free radicals, such as 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO). Both NMP and ATRP take advantage of the persistent radical effect,wherein radical-radical termination events produce an irreversible accumulation of deactivator, which shifts the equilibrium towards dormant species, and thereby decreases the probability of additional terminations. However, unlike NMP,ATRP employs a transition metal complex to facilitate the transfer of a pseudohalide “capping agent,” which maintains an equilibrium between growing and dormant chains. In contrast to the two above methods, RAFT mediates the polymerization using a reversible addition-fragmentation chain transfer process. This occurs with the assistance of a RAFT agent, which initiates the polymerization, and then reversibly reacts with the propagating chain to produce a dormant species.Common RAFT agents include dithioesters, dithiocarbamates, and xanthates.While RAFT can be employed on a greater variety of monomers,ATRP is the more widely utilized technique because it does not require the multi-step synthesis of the RAFT agent,and can be applied to a wide variety of systems.

11 Functionalized PE Prepared by Controlled Free Radical Polymerizations Although it is a relatively nascent area of research, there are several reports that have demonstrated successful synthesis of functionalized PE materials using the reactive “intermediate” strategy. 26 This involves the coordination insertion copolymerization of ethylene with a comonomer bearing a functional group capable of undergoing a subsequent polymerization, typically via a controlled free radical technique. If the functional group is incorporated into the PE backbone, graft copolymers can be prepared using this strategy. Alternatively, if the functionality is incorporated at the chain ends, the resulting structure is that of a block copolymer. In 2003, Matsugi et al.demonstrated the first controlled synthesis of poly(ethylene)-b-poly(methyl methacrylate) materials, by preparing hydroxyl- terminated PE using a zirconium metallocene complex in the presence of an allyl alcohol. 27 PE-OH was then treated with 2-bromoisobutyryl bromide (2-BB) to produce an end-functionalized macroinitiator, which was able to undergo ATRP with methyl methacrylate (MMA) to produce a block copolymer containing up to 75 wt % of PMMA. In the presence of homopolymers of PMMA and PE, PE-b-PMMA materials significantly reduced phase separation between the two immiscible homopolymers, thereby demonstrating their effectiveness as blend compatibilizers. Matyjaszewski et al.employed a similar methodology for preparing block copolymers of ethylene and MMA, n-butyl acrylate (nBA), and styrene (St). 28 A low molecular weight vinyl-terminated polymer was synthesized using a phenoxyimine

12 zirconium complex,and then functionalized with an ATRP initiator. In the presence of commodity monomers, this macroinitiator generated well-defined block copolymers with linearly increasing molecular weights and PDIs below 1.2. The same group also produced a Zn-terminated PE by a degenerative transfer coordination polymerization using bis(imino)-pyridine iron complex in the presence of diethyl zinc as the chain transfer agent. 29 PE-Zn was converted into PE-OH by oxidation and hydrolysis, followed by functionalization with 2-BB to prepare a PE macroinitiator. Subsequently, the macroinitiators were treated with nBA and tert- butyl acrylate (tBA) to produce block copolymers with controlled structure and composition. In a more recent report,a -diimine catalyst was used to prepare hyperbranched PE containing ATRP initiators at the chain ends. 30 This was accomplished by treating a Pd diimine complex shown in Figure L.3 with a functional acrylate monomer, 2-(2-bromoisobutyryloxy) ethyl acrylate,to generate a novel initiator capable of polymerizing ethylene in a living fashion.Subsequently, well-defined block copolymers were synthesized via ATRP in the presence of St and nBA.This was one of the first reports that demonstrated the direct incorporation of the reactive moiety into PE without requiring additional functionalization steps. Other CRP strategies have been employed to generate functionalized PE block copolymers. For example, D’Agosto and co-workers prepared an alkoxy-amine terminated PE that could subsequently undergo NMP to produce PE-b-PnBA. 31 Similarly, the reaction between dipolyethylenyl magnesium (PE-Mg-PE) and

13 thiocarbonylated compounds produced robust “macro” RAFT agents that were used to prepare low molecular weight PE-b-PMMA materials with narrow PDIs. 32 Although the grafting of functional monomers onto PE has been researched since 1960s, the first report of a controlled incorporation of functionality did not appear in the literature until the 1990s. Using constrained geometry zirconocene and titanocene catalysts, T. C. Chung was able to copolymerize ethylene and -olefins with three different reactive comonomers: borane-functionalized olefin, p-methyl styrene and divinyl benzene. 33 After a series of functionalization steps similar to those described above, the reactive monomer was transformed into an ATRP initiator. Subsequently, acrylates, methacrylates and styrenes were grafted from the macroinitiators to afford materials with controlled structure and composition. Likewise, Matyjaszewski and co-workers employed a zirconium catalyst to copolymerize ethylene and an unsaturated long-chain alcohol, protected with alkyl aluminums. 34 In a post-polymerization procedure, the hydroxyl groups were converted into ATRP initiators, followed by grafting of acrylates and methacrylates from the PE backbone.Because the graft side chains are attached to the PE backbone via ester linkages, base-catalyzed hydrolysis of the graft copolymers can provide useful insight into the nature of the ATRP reaction. Upon hydrolysis,the detached side chains demonstrated a linear increase in molecular weight and narrow PDIs, indicating a well-controlled grafting process. In a more recent report, an ATRP initiator was incorporated directly into the PE backbone using a Pd diimine catalyst, which produced hyperbranched PE

Full document contains 246 pages
Abstract: Functionalized polyethylene (PE) materials were prepared using two different synthetic pathways. The first strategy involved the synthesis of PE copolymers bearing functionalities capable of atom transfer radical polymerization (ATRP), followed by grafting with (meth)acrylic monomers. A variety of PE macroinitiators were prepared by copolymerizing ethylene with an initiating comonomer, 5- norbornen-2-yl-2'-bromo-2'-methyl propanoate ( 3 ) using either Ni α-iminocarboxamidato complexes (1a-c ), activated with nickel bis(1,5-cyclooctadiene) (Ni(COD)2 ), or a Ni α-keto-β-diimine complex (7 ), activated with methylaluminoxane (MAO). PE macroinitiators were subsequently grafted with methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), n -butyl acrylate (nBA), tert -butyl acrylate (tBA) using ATRP. In addition to allowing control over comonomer incorporation and the molecular weight characteristics of the products, this strategy also made it possible to access higher-order architectures, such as triblock and pentablock structures. Grafting with MMA, HEMA, and tBA produced hydrophilic polyolefins, while grafting with nBA generated elastomeric materials with excellent mechanical properties. The second method for generating functionalized PE materials took advantage of ketene chemistry in crosslinking and grafting reactions. First, a Meldrum's acid functionalized norbornene ( 4 ) that generates ketenes at elevated temperatures was incorporated into PE using 1b /Ni(COD)2 . Efficient crosslinking of PE copolymers took place at 185 °C through ketene dimerization, while functionalization was performed in the presence of small molecule and polymer nucleophiles. Unlike other crosslinking techniques, this strategy did not require small molecule radical sources or catalysts, and provided tunable crosslinking densities in PE, as demonstrated by rheological and tensile experiments. Both synthetic pathways provide opportunities to prepare novel hybrid materials, with an unprecedented level of control over the polymer composition, crystallinity, and topology of the products.