First, let me suggest the paper in attachment for a proper classification of graphene-like materials. As stated therein: "One concern in graphene research is that the term graphene is used in a generic manner to describe many graphene-based materials (GBMs)."

Now, depending on each specific GBM, there might be different approaches to the preparation of carbon-based nanocomposites. Arguably,

a) the economically advantageous solution could be what you are suggesting: process micrometric flakes of graphite, partially exfoliated graphite (aka graphite microplates, few layer graphite, few layer graphene) or reduced graphene oxide (aka rGO, graphite nanoplates, graphene nanoplatelets) in an extrusion process to produce a nanocomposite masterbach, like the Nanocyl carbon nanotubes masterbatches. The advantage is that no solvents are required in an extrusion processes since the dispersion of the nanostructures in the polymer is induced by the mechanical work of the extrusion screw. Moreover, you can buy cheap rGO from many commercial sources (I have personally work on the rGO materials supplied by Graphenea and ACS Materials).

b) another option could be to prepare polymer nanocomposites by solution-based processes. Unfortunately, most of the GBMs discussed above have limited solubilities in common solvents. Therefore, the use of soluble graphene oxide or functionalized graphene-derivatives. The difference between the two is that graphene oxide derivatives have to be reduced after compatibiliazion in the polymer to obtain graphene-like materials, while functionalized graphene-derivatives do not require post-treatment usually.

c) the inconvenient and expensive approach would be to growth a perfect monolayer graphene by CVD and try to use it a filler in a nanocomposites.

Let me conclude suggesting a recent review on "graphene" nanocomposites by Vika Mittal (http://onlinelibrary.wiley.com/doi/10.1002/mame.201300394/abstract) and, if you are interested in functionalization processes, you might be interested in my work on the decoration of carbon nanostructures (including nanotubes and graphite derivatives) by a RAFT polymerization initator (https://www.researchgate.net/publication/254261135_Synthesis_and_characterisation_of_a_trithiocarbonate_for_the_decoration_of_carbon_nanostructures?ev=prf_pub).

Article Synthesis and characterisation of a trithiocarbonate for the...

Because it is not easy :) As shown above. Not even in solution, let alone in melt phase ...

Some time ago, I have already commented about graphene in (thermoplastic) polymers. For such a goal, one needs to consider what does it mean to prepare a (2-phase) compound in a polymer: the continuous polymer phase will adsorb on the surface of the dispersed phase, in your case: graphene.

However, you want to benefit from graphene's characteristic properties which are linked with the single nano-thin sheet, right? because as long as you have many graphene layers on top of each other, these layers can easily be moved against each other in "horizontal" direction = parallel to the layer length or width. In order to really benefit from graphene extraordinary properties, one would like to have the single sheet to become dispersed in the thermoplastic polymer, right?

But that is not possible (at least according to my experience), because the adorption of the continuous polymer phase happens in the way that a monolayer of polymer phase will be adsorbed on the dispersed phase as a layer which separates the dispersed from the continuous phase  (and links them together).

However, the thermoplastic polymer layer can only adsorb on the dispersed phase surface with a certain minimum bending angle, it can not bendaround a nm-thin sheet.

The best a polymer phase (having a normal molecular weight) it can bend will lead to about 100 nm small (big!) usually globular particles, which means: in your case, the graphene would simply only be dispsed in form of relatively loose (inside the particles) agglomerate of graphene sheets with a diameter of abput 100 nm (depending on molecular weight: the smaller the molecular weight, the smaller the particles can be).

Please look here:

https://www.researchgate.net/publication/229816762_Electrical_conductivity_in_heterogeneous_polymer_systems._V_%281%29_Further_experimental_evidence_for_a_phase_transition_at_the_critical_volume_concentration?ev=srch_pub&_sg=c31zMPuCFqdg%2BiFR0fD0MyEf%2BPpNdZAOsKzCQpIXH%2FRJP%2BScXMl6y2sLV3KbG0TI_9PeXwaH02z%2Bf%2BBGRjsGJwwX%2FwVhhhnlSbsZNKQaDc6g6fBON9HMKKF2XXbrsMMFR_%2B30tNwqvwtmyQMIL3sfojaqvz%2FEMsUJ%2F0W38rooSdth1Q6HAbT1%2F7nuEj7XAScJy

and some related literature in my RG pages.

Article Electrical conductivity in heterogeneous polymer systems. V ...

any reports I have seen about graphene / polymer composites did not tell anything about particle size they had achieved, I only saw some mechanical test results, not interesting, tells us nothing if we don't see the inner phase structure. In my above cited publication, I have shown how one can look inside the polymer composite.

I have shown in other cases as well

Once in every 3-4 years, a new filler will be 'discovered', with all the usual hype in papers and portals.. ex: carbon silica dual phase filler, nano-clays, CNT, CNF, nano-graphite and now graphene... But in industry, carbon black and calcium carbonate were/are and will be the most dominant fillers for the foreseeable future. 

In-situ polymerization is better alternative to physical/chemo-mechanical mixing of graphene or GO into polymer matrix. 

Because neat graphene is relatively inert material with very low affinity to other materials. For this reason is very hard to disperse it homogeneously in polymer matrix. Also there is very high ability to re-stack to graphite structure.

I think it may be not easy to blend polymer with graphene.because the technology of mixture  is very high ability to inflence the property.

Zdenko and Chunling, please look at my comment above: there are principal dispersion (wetting) related reasons why such a mixing is not possible.

in literature, many reports available on CNTs by in situ free radical polymerization method. same will be the one possible way for graphene polymer composite preparation.

The main reason is usually compatibility; and this is true for most polymer nanocomposites. When the fillers (graphene or nanoparticles) and the polymer do not have compatible surface chemistry, they will aggregate. which can  be resolved by surface functionalization by an apprpriate molecule or ligand, which then allows for a uniform dispersion.

When mixed using a solvent, it is important to take care that the particles are compatible with both the solvent and the polymer used. Otherwise while the solution may look good in terms of dispersion, it will still aggregate once the solvent dries out.

Sumeet, I respectfully disagree. Please read my above explanation again, below I copy some part of it:

But that (to fully disperse graphene nanosheets) is not possible (at least according to my experience), because the adorption of the continuous polymer phase happens in the way that a monolayer of polymer phase will be adsorbed on the dispersed phase as a layer which separates the dispersed from the continuous phase (and links them together).

However, the thermoplastic polymer layer can only adsorb on the dispersed phase surface with a certain minimum bending angle, it can not bendaround a nm-thin sheet.

The best a polymer phase (having a normal molecular weight) it can bend will lead to about 100 nm small (big!) usually globular particles, which means: in your case, the graphene would simply only be dispsed in form of relatively loose (inside the particles) agglomerate of graphene sheets with a diameter of abput 100 nm (depending on molecular weight: the smaller the molecular weight, the smaller the particles can be).

Please look here:

https://www.researchgate.net/publication/229816762_Electrical_conductivity_in_heterogeneous_polymer_systems._V_%281%29_Further_experimental_evidence_for_a_phase_transition_at_the_critical_volume_concentration?ev=srch_pub&_sg=c31zMPuCFqdg%2BiFR0fD0MyEf%2BPpNdZAOsKzCQpIXH%2FRJP%2BScXMl6y2sLV3KbG0TI_9PeXwaH02z%2Bf%2BBGRjsGJwwX%2FwVhhhnlSbsZNKQaDc6g6fBON9HMKKF2XXbrsMMFR_%2B30tNwqvwtmyQMIL3sfojaqvz%2FEMsUJ%2F0W38rooSdth1Q6HAbT1%2F7nuEj7XAScJy

and some related literature in my RG pages.

Article Electrical conductivity in heterogeneous polymer systems. V ...