Article pubs.acs.org/Macromolecules
Synthesis and Characterization of Maleic Anhydride Grafted Polypropylene with a Well-Defined Molecular Structure Min Zhang,† Ralph H. Colby,† Scott T. Milner,‡ and T. C. Mike Chung*,† †
Department of Materials Science and Engineering and ‡Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
Tianzi Huang and Willem deGroot The Dow Chemical Company, 2301 Brazosport Blvd., Freeport, Texas 77541, United States S Supporting Information *
ABSTRACT: Despite the commercial importance of maleic anhydride grafted polypropylene (PP-g-MAH), it has long been a scientific challenge to prepare this polymer with a wellcontrolled molecular structure. This paper discusses a new chemical route that can form PP-g-MAH with desirable MAH content, a single MAH incorporated unit, white color, high molecular weight, and narrow molecular weight and composition distributions. The chemistry involves a unique PP-co-p-BT copolymer as the “reactive intermediate” that can be effectively prepared by metallocene-mediated copolymerization of propylene and p-(3-butenyl)toluene (p-BT), with narrow molecular weight and composition distributions, high molecular weight, and a broad range of p-BT contents. The incorporated p-BT comonomer units provide the reactive sites for the subsequent free radical MAH graft reaction under a suspension condition at a low reaction temperature. The resulting PP-g-MAH polymers were carefully examined by a combination of NMR and GPC measurements, which shows almost no change in polymer molecular weight and distribution and a single MAH incorporation (no oligomerization). The incorporated MAH units increase with the increase of initiator concentration, p-BT content, and reaction time. Evidently, the combination of high reactivity of φCH3 moiety, a favorable mixing condition between the reactive sites and chemical reagents in the swollen amorphous phases, and low reaction temperature results in MAH grafting reaction selectively taking place at the φ-CH3 moieties without side reactions (i.e., chain degradation and MAH oligomerization). In addition, this suspension reaction process presents an economic method to prepare PP-g-MAH with high polymer content and easy product purification.
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INTRODUCTION Isotactic polypropylene (PP) represents a quarter of commercial polymers produced in the world and is one of the fastest growing thermoplastics due to its unique combination of properties, including high melting point, high tensile strength, stiffness, chemical resistance, excellent processability and recyclability, and low cost.1 Despite its commercial success, the functionalization of PP has long been a scientifically challenging and industrially important area.2−5 The constant interest is driven by the strong desire to improve PP’s poor interactive properties and broaden its applications to higher value products, especially in polymer blends and composites those in which adhesion and compatibility with other materials are paramount. Despite significant research efforts in the past decades by both direct and postpolymerization approaches, limited success has been yielded. By far, maleic anhydride modified polypropylene (PP-gMAH)6 is the most important commercial functionalized PP © XXXX American Chemical Society
polymer due to its unique combination of the low cost of maleic anhydride (MAH) reagent, high activity of the resulting succinic anhydride moiety, and good processability of the PP-gMAH polymer. Despite its low molecular weight (most of them 4.8) with a low molecular weight shoulder shown in all polymers, which are indicative of the starting PP-co-p-BT copolymer being prepared by the heterogeneous Ziegler−Natta catalyst. It is important to note that these polymers in set A-2 also exhibit a broad composition distribution. Most of the reactive p-BT groups and the subsequent incorporated MAH moieties are located in low molecular weight polymers that have highly undesirable molecular structures serving as the interfacial agents.
unit. In other words, there was no oligomerization of MAH monomers during this low-temperature free radical MAH grafting reaction of PP-co-p-BT copolymer to form white PP-gMAH product, which is quite different from the brown commercial PP-g-MAH polymers that also exhibit low polymer molecular weight. Figure 6 compares absolute GPC (LS) curves of two reaction sets A-2 and B-6 (Table 2), using light scattering (LS) detector to determine the absolute polymer molecular weight. Figure 6 (left) compares the starting PP-co-p-BT copolymer (with 1 mol % p-BT reactive comonomer units) prepared by the homogeneous metallocene catalyst and three corresponding PP-g-MAH polymers (with 0.29, 0.76, and 0.90 wt % MAH contents determined by 1H NMR). Figure 6 (right) compares the starting PP-co-p-BT copolymer (with 1.3 mol % p-BT reactive comonomer units) prepared by the heterogeneous Ziegler−Natta catalyst and three corresponding PP-g-MAH polymers (with 0.37, 1.0, and 1.1 wt % MAH contents determined by 1H NMR). The calculated polymer molecular weight information is summarized in Table 3. As discussed, before GPC measurements, the MAH groups in all PP-g-MAH polymers were also converted to MAH−CH2−Si(CH3)3 groups to prevent polymer chain interactions. All polymers in set B-6, including the starting PP-co-p-BT copolymer prepared by the homogeneous metallocene catalyst and three corresponding PP-g-MAH polymers with varying degrees of MAH grafting reactions, show very high molecular weights with narrow molecular weight distributions (PDI ∼
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CONCLUSION This paper addresses a long-standing scientific challenge and industrially important issue in the free radical maleic anhydride grafting reaction of polypropylene to prepare a desirable PP-gMAH molecular structure with high molecular weight, narrow molecular weight and composition distributions, single MAH grafting unit, and a controlled amount of MAH content. This new chemical strategy is started with the design of poly(propylene-co-p-(3-butenyl)toluene) copolymer that can be effectively prepared by metallocene-mediated propylene copolymerization to form a broad composition range of high molecular weight PP-co-p-BT copolymers with narrow molecular weight and composition distribution. The bulky pBT comonomer units containing the reactive φ-CH3 moieties are largely located in the amorphous domains with good side chain flexibility that facilitates the selective MAH grafting reaction at φ-CH3 moieties in suspension reaction conditions. Despite the proton concentrations of secondary CH2 and tertiary CH in the PP backbone, which are much higher than that of the fewer φ-CH3 in the PP-co-p-BT copolymer, we observed a remarkable selectivity of the free radical MAH
Table 3. Summary of Absolute GPC (LS) Results for Two Comparative Sets Shown in Figure 6 sample
Mn (g/mol)
Mw (g/mol)
Mz (g/mol)
B-6 B-6-MAH-1 B-6-MAH-2 B-6-MAH-3 A-2 A-2-MAH-1 A-2-MAH-2 A-2-MAH-3
262 900 255 300 245 100 226 000 60 100 64 900 57 100 59 700
559 400 531 400 526 700 466 200 369 200 357 900 336 300 291 400
946 300 885 800 878 500 773 700 1 030 300 1 005 800 916 900 738 000
Mw/Mn Mz/Mw 2.12 2.08 2.14 2.06 6.14 5.52 5.89 4.88
1.69 1.67 1.67 1.66 2.79 2.81 2.73 2.53 J
dx.doi.org/10.1021/ma4006632 | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
grafting reaction on φ-CH3 moieties. It appears that the combination of reactivity sequence of φ-CH3 > −CH2-φ ≫ CH (backbone) and CH2 (backbone), intact PP semicrystalline structure under suspension reaction condition, and low reaction temperature (
