5 Linear Copolymers
5.13 COMPLEX MOLECULAR ARCHITECTURES
5.14.2 C ONVERGENT G ROWTH
In an attempt to overcome some of the problems encountered in the divergent method, convergent synthetic routes were developed by Hawker and Frechet. In this approach, the synthesis starts at the outside of the molecule and progresses inward, toward the core. The method is best described with reference to the original Frechet synthesis, shown below.
The two building blocks are 3,5-dihydroxybenzyl alcohol (13) and a benzylic bromide such as (14), which is referred to as the first-generation molecule. In the first of a two-step synthesis, compound (13) reacts with two molecules of (14) to produce a second-generation wedge with four terminal phenyl units and a passive primary alcohol as the focal point. The latter is activated in the second step by bromination, and this product can be isolated and purified before proceeding to the next stage. The two synthetic steps are then repeated using (14) to link two of the second-generation wedges to form a third-generation wedge with eight terminal units and one focal point, which is brominated, separated, purified, and is ready for the next stage. Repetition of the process can produce dendritic wedge-shaped molecules up to the sixth generation. The dendrimer is assembled finally by reaction with a multifunctional core molecule, in this case 1,1,1,tris(4-hydroxyphenyl) ethane.
O
O Br
HO
HO
OH K2CO3
O
O
OH O
O
O
O
O
O
Br O
O
O
O +
1st generation
(18-Crown-16)
(13)
2nd generation Bromination
(14)
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This is a versatile approach that allows the formation of unsymmetrical structures in which wedges with different chemical compositions, i.e., different end groups, can be attached to the same core molecule.
5.14.3 DENDRIMER MOLECULAR WEIGHT
The molecular weight of the dendrimer, generated by a divergent approach, can be calculated theoretically, but there is a certain amount of confusion in the literature, partly brought about by differences in the numbering of the generations. Taking the synthesis of PAMAM (using NH3 as the core) as an illustration, structure (12) was originally labeled generation 1, but tables of molecular weights in the literature refer to this as generation 0. Tomalia proposed Equation 5.29 for the calculation of the molecular weight MW as
(5.29)
a function of the generation number G (leading to a G = 1, 2, 3,4, … series). Here, Mc, MR, and Mt are the molecular weights of the core, monomer repeat unit, and terminal group, respectively; Nc is the number of branches at the core unit (core multiplicity);
and NR is the number of branches on each monomer unit (repeat-unit multiplicity). This was later revised, and the term ( ) replaced , in which Nb is the multiplicity of the branch unit. Calculation of MW then requires differentiation between the core, the monomer repeat units, and the terminal group and can lead to confusion and miscalcu-lation. A simplified form of Equation 5.29 has been proposed by Majoros, which removed these ambiguities and makes the calculation of MW much easier.
The form proposed is
(5.30)
where Nc is the core multiplicity, Nr is the multiplicity of the repeat unit (MWm – MWE), g is the generation number, which, because a geometric series is used in the growth calculation, means the labeling starts at 0, i.e., g = 0, 1, 2, 3, … .
By calculating the monomer repeat unit as (MWm – MWE), in which MWm is the sum of the MW of each monomer unit and MWE is the MW of the molecule eliminated during the reaction (see Scheme 5.1 in Section 5.10), there is no need to identify the terminal unit. This will, of course, also depend on the type of reaction involved in activating the branch unit. For example, in the preparation of poly (propylene imine) dendrimers, a hydrogenation reaction is used to activate the branch unit, i.e., . So the monomer repeat unit would then be (MWm + 4H).
The use of Equation 5.30 can be illustrated using PAMAM as an example. If EDA is used as the core molecule, then MWcore = 60 and Nc = 4. The monomers in
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Linear Copolymers and Other Architectures 153
the branch unit are EDA and MA, and CH3OH is the eliminated molecule so the monomer repeat unit (EDA + MA – CH3OH) is (60 + 86 – 32) = 114 and Nr = 2.
Substitution in Equation 5.30 for g = 0 gives
(5.31)
and for g = 4
(5.32)
The calculations then yield the values shown in Table 5.4 for g up to 10.
If the degree of polymerization DP is required, then
(5.33)
and the number of terminal units Z is given by
(5.34) 5.14.4 PROPERTIESOF DENDRIMERS
In solution, dendrimers form close-packed, globular, or spherical shapes when the molecular weight is high enough, unlike the expanded flexible coil structures of linear polymers. This has a significant effect on the rheological properties, e.g., the
TABLE 5.4
PAMAM Using EDA Core
Generation Molar Mass Number of Terminal Groups
0 516 4
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viscosity of higher-molecular-weight dendrimers in solution is lower than that of an equivalent molecular weight linear polymer. This is molecular weight dependent as the intrinsic viscosity has been shown to pass through a maximum and then decline when measured as a function of molecular weight.
The nature of the groups on the surface also greatly affects the solubility behavior.
Thus, hydrophilic groups enhance solubility in polar liquids. If convergent-growth meth-ods are used, then it is possible to construct dendrimers in which segments with polar surface groups are combined with wedges having nonpolar surface groups (Figure 5.6).
These can form monolayers at aqueous–organic liquid interfaces.
5.14.5 APPLICATIONS OF DENDRIMERS
Although a high-molecular-weight dendrimer has a very tightly packed surface layer, there are fairly large internal cavities (see Figure 5.7). These have been used to trap
FIGURE 5.6 Schematic representation of dendritic copolymers with different polarities: (a) segment-block dendrimer and (b) layer-block dendrimer. (From Klajnert, B. and Bryszewska, M., Acta Biochimica Polonica, 48, 199, 2001. With permission.)
FIGURE 5.7 Schematic illustration of a fourth-generation dendrimer. (From Klajnert, B. and Bryszewska, M., Acta Biochimica Polonica, 48, 199, 2001. With permission.)
(a) (b)
End (terminal) groups Internal cavity Core
Branching units 9813_C005.fm Page 154 Friday, June 15, 2007 10:58 AM
Linear Copolymers and Other Architectures 155
small molecules, a feature first explored by Meier. Small molecules, such as Rose Bengal and p-nitrobenzoic acid, can be held in the cavities of a fourth-generation poly(propylene imine) dendrimer, with 64 surface branches, which form a “dendritic box.” The box can be “sealed” by reacting the surface amino groups with L-(phenyl alanine) and the molecules subsequently liberated by hydrolysis of this outer layer.
Similar systems are now being actively investigated for use in drug delivery in the human body.
Release systems have also been developed using azobenzene groups on the surface, which can be “opened” and “closed” via a photochemical-stimulated cis–trans isomeric change reaction. Other applications being explored for dendritic structures include light harvesting, use as contrast agents for magnetic resonance techniques, use for in vitro diagnostics, application in catalysis, and as carriers in gene therapy.
PROBLEMS
1. In a certain copolymer system, the reactivity ratios are r1 = 0.001 and r2
= 0.002. Sketch the copolymer composition diagram expected for this pair of monomers.
2. Mayo et al. (1948) have reported reactivity ratios for the copolymerization of vinyl acetate (reactivity ratio = r2) with eight representative monomers (corresponding reactivity ratio = r1).
a. In each case, calculate the composition of the copolymer that would be formed at low conversion from equimolar mixtures of the two monomers.
b. What is the comonomer feed composition required to form a copolymer with 50% vinyl acetate units?
c. Identify those pairs of monomers that display azeotropic conditions, and calculate the azeotropic copolymer composition.
d. Which system is closest to an ideal copolymerization?
3. Determine the composition of the copolymer obtained when an equimolar mixture of styrene (S) and methyl methacrylate (MMA) undergoes:
a. cationic copolymerization initiated by SnCl4 (the reactivity ratios are rS = 10.5 and rMMA = 0.1)
b. anionic copolymerization initiated by sodium in liquid ammonia (rS = 0.5 and rMMA = 0.44)
Monomer, M1 r1 r2
Acrylonitrile 4.05 0.61
Trichloroethylene 0.01 0.66
Styrene 55 0.01
Methyl acrylate 9 0.1
Vinyl chloride 1.68 0.23
Vinyl ethyl ether 0 3.0
Vinyl bromide 4.5 0.35
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4. Suggest conditions for the synthesis of:
a. random copolymers of styrene and 4-vinyl phenol
b. block copolymers of styrene and 4-vinyl phenol, i.e., polystyrene-b-poly(4-vinyl phenol) by ATRP
c. gradient copolymers of styrene and 4-vinyl phenol.
5. Poly(propylene imine) dendrimers are prepared by a divergent route, using 1,4-diamino butane as the core molecule and acrylonitrile (CH2=CHCN) as the monomer branch unit. Growth occurs via the two-stage process:
(a) a double Michael addition of acrylonitrile to the core followed by (b) hydrogenation under pressure to the nitrile group to a primary amine.
i. Draw the structure for generation 0.
ii. Calculate the molecular weight and number of end groups for gener-ation-three poly(propylene imine) dendrimer.
6. Calculate the molecular weight and number of end groups from generation 0 to generation 6 for a poly (amido amine) prepared using NH3 as core molecule and ethylene diamine and methyl acrylate as monomers.
REFERENCES
Chong, Y.K., Le, T.P.T., Moad, G., Rizzardo, E., and Thang, S.H., Macromolecules, 32, 2071, 1999.
Estes, G.M., Cooper, S.L., and Tobolsky, A.B., Block copolymers, Rev. Macromol. Chem., 5–2, 167, 1970.
Hawker, C.J., Bosman, A.W., and Harth, E., Chem. Rev., 101, 3661, 2001.
Klajnert, B. and Bryszewska, M., Acta Biochimica Polonica, 48, 199, 2001.
Matyjaszewski, K., Polym. Int., 52, 1559, 2003.
Matyjaszewski, K. and Xia, J., Chem. Rev., 101, 2921, 2001.
Mayo, F.R., Walling, C., Lewis, F.M., and Hulse, W.F., J. Am. Chem. Soc., 70, 1523, 1948.
Pepper, D.C., Q. Rev., 8, 88, 1954.
BIBLIOGRAPHY
Allen, G. and Bevington, J.C., Eds., Comprehensive Polymer Science, Vol. 3, 4, Pergamon Press, 1989.
Allport, D.G. and Janes, W.H., Eds., Block Copolymers, Applied Science Publishers, 1973.
Cowie, J.M.G., Alternating Copolymers, Plenum Press, 1985.
Folkes, M.J., Eds., Processing, Structure and Properties of Block Copolymers, Elsevier Applied Science Publishers, 1985.
Noshay, A. and McGrath, J.E., Block Copolymers: Overview and Critical Survey, Academic Press, 1977.
Odian, G., Principles of Polymerization, 4th ed., John Wiley and Sons, 2004.
Rempp, P. and Merrill, E.W., Polymer Synthesis, Hüthig and Wepf, 1986.
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